PRESERVATIVE TREATMENT OF WOOD

BY PRESSURE METHODS

Agriculture Handbook No. 40

* lllv-v ^t.,^..

UNITED STATES DEPARTMENT OF AGRICULTURE

Forest Service
ERRATA SHEET

for

Preservative Treatment of Wood by Pressure Methods

Agriculture Handbook No. 40

Page

9 -^ Line 2, "older and heartwood" should read "older the

heartwood".

17 ^ Group 4, second col. "(read heartwood)" should read

"(red heartwood)".

38 t. Line 21, reference italic "(50)" should be italic "(51)".

47i^Table 6, col. 4, change 200-200 to 200-210.

48 ^ Table 7, col. 4, first line, cha«ge 01.0 to 11.0.

72*^ Table 9, col. 5, beginning with second line, change 235

y to 135, 255 to 155, 276 to 176.

SO'-^Table 13, col. 9, last line, change 37.7 to 3.7.

105'^Table 17, col. 2, last line, change 2.067 to 2.167.

105 Table 17, col. 7 heading, change "per" to "both".

106 ^ 'Table 17, col. 3, 7th line, change 1.090 to 1.091.

106' Table 17, col. 7, 8th line, change 6.808 to 6.818.

108 i^Equation bottom of page should read "0.531W" not

, ".0531W".

110 ¿^ Example, last term in denominator should read "0.55"

not ".055".

121¿XTable 20, col. 1, 4th line under "Steamed Green Lumber

.and Timbers", insert figure "1".

135 ^Wd definition should read "oven-dry wood per unit" etc.

139 ^Table 22, Footnote 1, a\ should read a,, /^ should read /^

150 '-'First equation, delete " = (A)." insert "(A)" flush right.

150 ¿/line 17, "pp. 149-150" should read "pp. 150-151".

150 '-''Line 7 from bottom, "p. 150" should read "p.l49".

Forest Service

U. S. Department of Agricultuije

March 1953

6P0 »»-SM
PRESERVATIVE TREATMENT OF WOOD

BY PRESSURE METHODS!

By J. D. MACLEAN, 2 Engineer

Forest Products Laboratory, ^ Forest Service

Agriculture Handbook No. 40

U. S. Department of Agriculture December, 1952

For sale by the Superintendent of Documents, U. S. Government Printing Office

Washington 25, D. C.—Price 40 cents
1 This handbook supersedes the MANUAL ON PRESERVATIVE TREATMENT OF WOOD BY

PRESSURE, U. S. Dept. Agr. Misc. Pub. 224.

2 Acknowledgment is made to various members of the Forest Products Laboratory

who have been consulted on subjects relating to their fields of investigation and from

whom much helpful data and information have been obtained.

2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
CONTENTS

Page

Introduction __ _ i

Pressure processes 2

Full-cell processes o

Empty-cell processes __ _ 2

Wood preservatives ^_ _ _ 3

Coal-tar creosote _ _ _ _ 4

Water-gas tar and water-gas-tar creosote 4

W^ood-tar creosote ^

Coal tars _ 4

Petroleum oils 4

Creosote, coal-tar solutions 5

Coal-tar creosote and petroleum solutions ___ 5

Chemicals dissolved in solvents other than water 5

Water-borne preservatives 5

Proprietary preservatives G

Effect of wood structure on treatment 6

Differences in structure of hardwoods and softwood 6

Heartwood and sapwood g

Effect of tyloses on penetration 10

Bordered pits and simple pits 11

Resin passages 13

Density 13

Influence of structure on direction of penetration 14

Classification of species with respect to penetrability 16

Moisture content, fiber saturation point, shrinkage and swelling, specific

gravity, weight of wood, and air space in wood 18

Moisture content 18

Fiber saturation point 20

Shrinkage and swelling 25

Specific gravity of wood 26

Specific gravity of wood substance 28

Relation of specific gravity, moisture content, and air space in wood 28

Preparation of timber for treatment 30

Air-seasoning 32

Mechanical preparation 38

Conditioning processes 39

Rate of temperature change in wood and various factors affecting the results. _ 53

Heating medium 53

Moisture content 54

Rate of temperature change in transverse and longitudinal directions 54

Effect of specific gravity on the rat«' of temperature change in wood 55

Effect of snow or ice on rate of heating 55

Effect of vacuum on rate of heating 55

III
Page

Difference in temperatures obtained at tops and butts of long poles and

piles 56

Compensating for the time needed to bring the heating medium up to the

required temperature 56

Temperature computations 56

Temperatures required to sterilize wood 67

Inj ecting preservatives 67

Influence of viscosity and temperature of preservative 67

Relative penetration of preservative oils and water solutions 81

The use of vacuum 81

Preliminary air pressure 86

Preservative pressure 88

Pressure period 89

Kick-back 90

Final heating and vacuum, using steam or expansion bath 93

Absorption and penetration 94

Full-cell and empty-cell absorptions 95

Relation of dimensions of timber to absorption and penetration 96

Measurement of absorption 107

Effect of treatment on the strength and properties of the wood 110

Effect of preservatives 110

Variables affecting the strength properties HI

Effect of steaming on strength and physical condition of the wood 112

Effect of Boulton process on strength 113

Bleeding of treated wood 114

Treating conditions used in commercial practice 115

Specifications for treatment 123

General considerations 123

Avoiding injurious treating conditions 123

Selection of treating process 125

Unnecessary requirements 126

Retentions 127

Penetration 133

Framing and boring 134

Appendix 135

Symbols used in formulas relating to physical properties of wood 135

Formulas relating to physical properties of wood 136

Method of using figures when finding temperatures to be expected in timbers heated under any given conditions 138

Adjusting for different heating-medium temperatures 142

Temperatures in round timbers 143

Temperatures in sawed timbers 147

Symbols and formulas relating to temperature change in wood 149

Literature cited 152

Index 156

IV
INTRODUCTION

The principal wood-destroying agencies are the decay fungi, insects,

marine borers, and fire. Preservative treatment, which is used to retard

or prevent the operation of these agencies, has made wood an economical

material to use in many fields, whereas without such treatment its short

life and consequent high cost make its use prohibitive.

Pressure treatment with preservatives suitable for the particular

service requirements is now widely employed for material such as ties,

poles, piling, posts, bridge timbers, mine timbers, cross arms, conduits,

and lumber and timbers used in buildings where conditions are favorable

for attack by decay or insects. It is also employed in the impregnation

of wood with fire-retarding chemicals.

Pressure treatment when intelligently applied results in lower annual

cost, more satisfactory service, and a decreased drain upon the forest

resources. The effectiveness and economy of preservative treatment

depend on the qualities of the preservative and the thoroughness of the

treatment. Good treatment is economical, but poor treatment, whether

resulting from improper choice of preservative, improper specifications,

low preservative retentions, or insufficient penetration, is expensive and

may prove disastrous.

The problem of adequate treatment is complicated by many factors,

including the species, size, form, condition, and proposed use of the

timber; kind and amount of preservative to be injected; details of the

treating process employed; and the skill of the treating-plant operator.

The methods now in use for the injection of preservatives by pressure ^

are the result partly of technical research and partly of the accumulation of more than a century of world experience in wood preservation.

By the proper use of these methods it is practicable for purchasers to

obtain adequately treated timber that can be depended on for long life

under the most severe conditions of exposure.

The Forest Products Laboratory has done much work on the many

technical problems involved in the pressure treatment of wood, including

numerous experiments and observations at commercial treating plants.

The purpose of this publication is to discuss the application of the results of these experiments and observations, and to present general

information that will be of value to engineers, treating-plant operators,

inspectors, and others interested in pressure-treating processes and in

the preparation of specifications.

^ The Treatment of timber by open-tank ar^d other nonpressure methods is discussed in U. S. Department of Agriculture Farmers' Bulletin 2049, {5a).

Italic numbers in parentheses refer to Literature Cited, p. 152,
2 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

PRESSURE PROCESSES

The most effecti\T method of treating wood with preservatives is

by means of pressure.^ There are a number of pressure processes, all of

\\'hich employ the same general principle but differ in the details of

application. The timber to be treated is loaded on tramcars, which are

run into a large steel cylinder. After the cylinder door is closed and

bolted, preservative is admitted and pressure applied until the required

absorption has been obtained. Two principal types of pressure treatment, the full-cell (Bethell) and empty-cell (Lowry and Rueping), are in

common use. The essential characteristics of these processes are described below; their fields of usefulness are discussed on pages 95, 125,

and 126.

FULL-CELL PROCESSES

In making treatment« with the so-called '^full-cell'' or Bethell process,

a preliminary vacuum is first applied to remove as much air as practicable

from the wood cells. The preservative is then admitted into the treating

cylinder without admitting air. After the cylinder is filled with preservative, pressure is applied until the required absorption is obtained.

A final vacuum is commonly applied immediately after the cylinder has

been emptied of preservative to free the charge of dripping preservative.

When the timber is given a preliminary steaming-and-vacuum treatment (p. 39), the preservative is admitted at the end of the vacuum

period following steaming. In case the charge has received a preliminary

conditioning treatment by the Boulton or boiling-under-vacuum process

(p. 44), the unfilled space at the top of the cylinder is filled with preservative and pressure is applied as soon as this conditioning process has been

completed.

It is impossible to remove all of the air from the wood cells regardless

of the method of treatment employed. For this reason, even under the

most favorable conditions, there is some unfilled air space in the cell

cavities of the treated wood after impregnation by the full-cell process.

When the full-cell process is used for treatment with zinc-chloride

solution, it is commonly called the Burnett process.

EMPTY-CELL PROCESSES

Two empty-cell treatments, the Lowry and the Rueping, are commonly used, both of which depend upon compressed air in the wood to

force part of the absorbed preservative out of the cell cavities after

preservative pressure has been released.

Lowry Process

In the Lowry process, which is also designated as the "empty-cell

process without initial air,'^ the preservative is admitted to the treating

cylinder at atmospheric pressure. When the cylinder is filled, pressure

is apphed and the preservative is forced into the wood against the air

^ A discussion of pressure-treating-plant equipment and of various methods of

treating wood b:\^ both pressure and nonpressure methods is given in WOOD PRESERVATION {21, p. 329).
PRESERVATIVE TREATMENT OF WOOD 3

originally in the cell cavities. After the required absorption has been

obtained, pressure is released, a vacuum is drawn, and the air under

pressure in the wood forces out part of the preservative absorbed during

the pressure period. This makes it possible, with a limited net retention,

to inject a greater amount of preservative into the wood and to obtain

deeper penetration than when the same net retention is obtained with

the full-cell process. The Lowry process is convenient to use in any

pressure-treating plant, since no additional equipment is required.

Rueping Process

This process is also called ''empty-cell process with initial air.'' The

principal difference between the Lowry empty-cell process and the

Rueping process is that, in the latter, air is forced into the treating

cyHnder before the preservative is admitted. The air pressure is then

maintained while the cylinder is filled with preservative; thus, the wood

cells are left more or less impregnated with air under pressure. In

resistant woods this air pressure may penetrate only a short distance

from the surface, while in wood that is fairly pervious to the penetration

of air and liquids, such as the sapwood of many species, an air pressure

is built up in all of the penetrable portion.

In the application of this process the preservative is often admitted

from an equalizing tank (Rueping tank) and the air in the treating

cylinder interchanges with the preservative in this tank. In some

plants not equipped with a Rueping tank, the preservative is pumped

into the treating cylinder against the preliminary air pressure and

sufficient air is released during the filling period to keep the pressure

constant. Impregnation of the wood is obtained by applying a pressure

sufficiently high to force preservative into the timber against the air

pressure in the wood cells. Then, upon release of preservative pressure

and application of a vacuum, part of the preservative is forced out of the

wood by the expanding air. The amount of such recovery will usually

be greater with the Rueping than with the Lowry process, under comparable conditions.

WOOD PRESERVATIVES

Wood preservatives may be grouped into two broad classes: preservative oils and water-borne preservatives. Each of these classes

may be further subdivided in various ways. For example, preservative

oils include byproduct oils such as coal-tar creosote and other creosotes,

mixtures of coal-tar creosote with coal-tar, petroleum, or other oils,

solutions of toxic chemicals such as pentachlorophenol or copper naphthenate in selected petroleum oils or other solvents, and various mixtures

of these solutions with the byproduct oils and mixtures. The waterborne preservatives include solutions of single chemicals such as zinc

chloride or sodium ñuoride, which are not resistant to leaching, and

various formulations of two or more chemicals that react after impregnation and drying to form compounds with limited solubility and sometimes with high resistance to leaching, (i, ^,5).

Preservatives vary greatly in effectiveness and in suitability for

different purposes and use conditions. The effectiveness of any pre-
4 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

servative depends not only upon the materials of which it is composed,

but also upon the quantity injected into the wood, the depth of penetration, and the conditions to which the treated material is exposed in

service.

COAL-TAR CREOSOTE

Coal-tar creosote is defined by the American Wood-Preservers' Association as a preservative oil obtained by the distillation ''of coal tar

produced by high-temperature carbonization of bituminous coal; it

consists principally of liquid and solid aromatic hydrocarbons and

contains appreciable quantities of tar acids and tar bases; it is heavier

than water; and has a continuous boiling range of at least 125° C. beginning at about 200° C/'

Coal-tar creosote is highly effective and is the most important and

most extensively used wood preservative for general purposes.

WATER-GAS TAR AND WATER-GAS-TAR CREOSOTE

Water-gas tar is obtained from petroleum oil as a byproduct in the

manufacture of water gas.

Water-gas-tar creosote is produced by distillation from water-gas tar.

This creosote is defined as any and all distillate oils from such tars

boiling between 200° and 400° C. While water-gas tar and the creosote

produced from it are not considered so generally effective as coal-tar

creosote, service-test records indicate that they have good preservative

properties.

WOOD-TAR CREOSOTE

Wood-tar creosote is obtained from wood tar and distills mostly above

170° C. Since wood-tar creosotes have been produced in comparatively

small quantities and have usually sold at higher prices than coal-tar

creosote, they have not been extensively used as a wood preservative.

Good wood-tar creosotes have demonstrated high effectiveness, but they

seem to be somewhat less effective than coal-tar creosote.

COAL TARS

The various coal tars are, in general, unsuitable as wood preservatives

when used alone because their relatively high viscosity makes it difficult

to obtain satisfactory penetrations. They may not be quite so effective

as coal-tar creosote, but they possess good preservative properties.

Some of them have been used for the purpose and have given excellent

results when satisfactory retentions and penetrations were obtained.

PETROLEUM OILS

Petroleum oils, such as crude petroleum, topped petroleum, fuel oil,

and used crankcase oil, as a rule, do not possess toxic properties to make

them suitable as wood preservatives when used alone. Although in a

few cases good results have apparently been obtained with petroleum

oils used alone, in other cases complete failure has resulted. They are

used in preservatives merely as solvents of toxic chemicals or diluents

of preservative oils.
PRESEBVATIVE TREATMENT OF WOOD 5

CREOSOTE, COAL-TAR SOLUTIONS

Coal tar is extensively employed in solution with coal-tar creosote

for the treatment of ties and to some extent other classes of timber.

The coal-tar solutions are used principally in the Eastern and Southern

States. Mixtures of coal tar and creosote commonly contain about

20 to 50 percent of tar by volume.

COAL-TAR CREOSOTE AND PETROLEUM SOLUTIONS

Mixtures of coal-tar creosote and petroleum are widely used in the

Western States, principally for the treatment of ties, but also for the

treatment of lumber, timber, and land and fresh-water piling. In

general, their petroleum content ranges from 30 to 70 percent by volume,

but the content is usually about 50 percent.

Since the toxicity of the petroleum mixtures is furnished by the creosote,

it is important that the creosote be of high toxicity.

CHEMICALS DISSOLVED IN SOLVENTS OTHER THAN WATER

Preservatives composed of toxic chemicals carried in nonaqueous

solvents, such as petroleum-oil distillates, are now being used to an

increasing extent. These were originally devised for the purpose of

providing a clean treatment without causing swelling of the wood and

were originally applied by nonpressure methods.

A shortage of creosote that developed during World V/ar II created

an active interest in the use of these preservatives as a possible substitute for creosote, especially in the pressure treatment of poles. Particular attention was directed to the chlorinated phenols, which are

known to have a high degree of toxicity. Pentachlorophenol is the best

known and most widely used in this group.

Other preservatives of this type, which in the past have been largely

limited to use in surface treatments, are the metaUic naphthenates, such

as copper naphthenate. The latter has also been used to a limited

extent for pressure-treated poles.

Although some of these toxic chemicals, particularly pentachlorophenol,

have been giving excellent results over a considerable period of time,

service records are still inadequate to evaluate them completely in

comparison with coal-tar creosotes.

WATER-BORNE PRESERVATIVES

A variety of chemicals in water solution are used as wood preservatives.

These include zinc chloride, sodium fluoride, arsenic in various forms,

copper sulfate, and similar toxic chemicals. Most of these salts are

used in combination with one or more other chemicals, frequently including a chromium compound. Chromated zinc chloride, which is

composed of a mixture of zinc chloride and sodium dichromate, has

come into wide use in recent years. This preservative is now much

more extensively used than straight zinc chloride, which was formerly

the most widely used water-borne salt.

Sodium fluoride has moderate preservative properties, but it is seldom

used alone. It is an important ingredient in several proprietary pre-
6 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

servatives, some of which are finding considerable use in the treatment

of building lumber and structural timber.

Arsenic compounds have been used as preservatives for many years.

They are important ingredients of a number of proprietary preservatives,

some of which have demonstrated high effectiveness and are extensively

used.

Copper sulfate, although extensively used in Europe for many years

and demonstrated to be moderately effective in retarding decay, has

found little use for wood preservation in the United States except in

certain proprietary preservatives, in which it is combined with other

chemicals. Several of these preservatives are of high effectiveness and

extensively used. Copper sulfate is corrosive to iron and steel and,

therefore, cannot be used alone in ordinary treating equipment.

PROPRIETARY PRESERVATIVES

Various patented or proprietary preservatives are sold under trade

names for pressure treatment. For the most part they are composed

of various water-borne salts and are injected in water solutions. Others

employ a volatile solvent to carry the toxic substance into the wood.

Some of the water-borne preservatives contain chemicals that are intended to react after injection into the wood and to form substances

that are of low solubility and resistant to leaching.

EFFECT OF WOOD STRUCTURE ON TREATMENT

Wood varies greatly in its structure. Hardwoods differ from softwoods, and in each of these groups there are differences among individual

species. In fact, there are differences even in the same tree, since the

heartwood, although its gross structure is the same, commonly contains

certain substances not abundant in the sapwood. All these differences

have their influence upon the penetrability of the wood by preservatives.

An acquaintance with the following features of the structure of wood

is needed for a full understanding and appreciation of this publication.

DIFFERENCES IN STRUCTURE OF HARDWOODS AND

SOFTWOODS

Hardwood or broadleaf trees contain specialized cells that serve as

sap conductors. These cells, set end to end, are known as pores or

vessels, and form more or less continuous passages. Mechanical support

is furnished by wood fibers that surround the vessels.

The softwoods, generally known as conifers,^ do not have the specialized sap-conducting cells found in the hardwoods, but instead have

elongated cells, called tracheids or fibers, that have closed ends. These

tracheids serve both for sap conduction and for mechanical support

in the living tree. Liquids pass from one tracheid to another through

bordered pits, which are most numerous near the tracheid ends and may

contain minute perforations.

The terms hardwood and softwood are somewhat misleading, as there

are some softwoods that are even harder, from the mechanical standpoint, than some hardwoods. Although in general most of the hard6 Native conifers include the pines, Douglas-fir, the true firs, the spruces, hemlocks,

junipers, larches, and cedars, and cypress, redwood, and yew.
PRESERVATIVE TREATMENT OF WOOD

woods are actually ver}^ hard, some woods of this group are soft, such as

basswood, willow, and cottonwood. Similarly, though most of the

softwoods are relatively soft, some of them, such as yew, Douglas-fir

and longleaf pine, are very hard. ^ '

Figures 1 and 2 show the general structure of a hardwood and of a

softwood, respectively.

Hardwoods, such as gum, beech, birch, and basswood, in which the

pores or vessels are of fairly uniform size and distribution, are called

Ml 7069 F

FIGURE 1.—Drawing of a higlily magnified block of hardwood measuring about onefortieth of an inch vertically: /¿, End surface; rr, radial surface; tg, tangential surface;

y, vessel or pore; wf, wood fibers; ivvj wood rays; ar, annual ring.
8 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

tr fwr vrd

M17070F

FIGURE 2.—Drawing of a highly magnified block of softwood measuring about onefortieth of an inch vertically: tt, End surface; rr, radial surface; tg, tangential surface; ar, annual ring; sm, summerwood; sp, springwood; ¿r, tracheids, or fibers; hrd,

horizontal resin duct; vrdj vertical resin duct; fwr, fusiform wood ray or ray having

horizontal resin ducts; wr, wood rays; bp, bordered pits.

diffuse-porous woods; whereas those that have alternate layers of small

and large pores, such as oak, elm, hickory, and chestnut, are called ringporous woods. The penetrability of the hardwoods depends to a large

extent on the open or closed condition of these pores or vessels. On the

other hand, penetration in the softwoods is largely dependent on the

permeability of the cell walls.

HEARTWOOD AND SAPWOOD

In general, the most universal cause of the difference in the penetration of preservatives in both hardwoods and softwoods is the differ-
PRESERVATIVE TREATMENT OF WOOD 9

ence between heartwood and sapwood. Young trees are nearly all sapwood, but as they grow older and heartwoodVolume at the center gradually increases while new sapwood forms on the outside. The sapwood

is living and takes an active part in the growth of the tree, whereas the

heartwood is dead or inactive. During this transition from sapwood to

heartwood, changes generally occur in the pores of hardwoods, which

may become partially or completely closed with pithlike growths, called

tyloses, or with gum; while in the conifers the openings in the tracheid

walls may undergo changes that make them highly resistant to the passage of liquids.

The exact nature and the quantity of the various substances that

accumulate in the heartwood are not well understood, but it is known

that their character differs materially in different species and that their

quantity differs among different trees of the same species and even in

different parts of the same tree. They are sometimes referred to under

the all-inclusive term, ''gums, resins, and infiltrated substances." Those

that can be dissolved out of the wood by water or other solvents are

collectively called ''extractives." An infinite amount of painstaking

chemical work would be required to identify all of the compounds included in these substances.

Although the nature of the change from sapwood to heartwood is not

fully understood, some of the major effects of the change are clearly

evident. In most species there is a pronounced change to a darker color

and the line of demarcation between heartwood and sapwood is distinct.

However, in some species, such as the hemlocks and the spruces, there is

little or no change in color. In some of the pines also it is not always

easy to distinguish between heartwood and sapwood. A method for

showing the demarcation in the pines is given on page 134.

In a large number of species there is a marked superiority in the decay

resistance of the heartwood in comparison with that of the sapwood.

The sapwood of all species has low decay resistance, whereas the heartwood of many is highly durable. The degree of durability in the heartwood depends mainly upon the amount and character of the substances

that accumulate in it.

In most species the change from sapwood to heartwood greatly increases the resistance to penetration by preservatives, but this, again,

is not universally true. In some species, eastern hemlock for example,

both the sapwood and heartwood are resistant and the sapwood offers

nearly the same resistance to penetration as the heartwood. In ponderosa

pine and bristlecone pine the heartwood is more resistant to decay than

the sapwood but still not difficult to penetrate. In white oak, sweetgum,

and beech with red heartwood, the heartwood is almost impenetrable by

ordinary methods, although the sapwood is easy to treat. These variations in penetrability affect the operation of treating plants, the selection

of wood, and the preparation of specifications by purchasers.

A common fallacy is that heartwood is materially stronger than sapwood. The results of thousands of tests at the Forest Products Laboratory fail to show that this is true in any degree that is significant to

the engineer interested in the preservative treatment of wood. The

greater penetrability of the sapwood makes it superior from the standpoint of preservative treatment, and it should usually be preferred rather

than discriminated against in wood that is to be treated.
10 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

EFFECT OF XYLOSES ON PENETRATION

In some of the hardwoods, white oak, chestnut, and black locust

for example, the pores of the heartwood are filled with tyloses. Such

species are usually resistant or almost impervious to treatment in the

heartwood. In the sapwood, tyloses usually occur in the inner region

near the heartwood where the vessels are beginning to lose their ability

to conduct sap.

The influence of tyloses on penetration in the heartwood is illustrated by a comparison of the penetration obtained in the white oak

group and in the red oak group. In most cases the heartwood of the

white oaks is only slightly penetrable; that of the red oaks is, in general,

readily penetrable. These two groups of species have practically the

same wood structure, with the exception that the vessels in white oak

heartwood are plugged with tyloses, whereas those of woods in the red

oak group are usually free from tyloses. The other wood elements or

fibers of these two groups of species are resistant to the penetration of

liquids. The treatment of such species as red oak naturally depends to

a large extent on penetration from the end surfaces. A discussion of

penetration in the side surfaces is given on page 14. There are exceptional species, however, in both groups. For example, chestnut oak

{Quercus montana) is a white oak that has relatively few tyloses. On the

other hand, the red oak commonly called blackjack or jack oak (Q.

marilandica) has pores that are closed by tyloses, and the species in this

respect is similar to a white oak. Under certain growth conditions the

other red oak species sometimes have a sporadic development of tyloses

that may be sufficient to impede penetration.

Experiments on various hardwoods show that when the wood fibers

are resistant to penetration the presence or absence of tyloses or other

obstructions, such as gums, in the vessels largely determines whether

the wood can be impregnated with preservatives (8, 59), Tyloses may

merely impede penetration in species in which they do not completely

fill the pores. This is true in some of the ashes.

In some of the diffuse-porous woods, such as asi)en and willow, tyloses

occur more or less irregularly. Their influence in such species is, in

general, proportional to their distribution. Diffuse-porous woods that

are resistant to treatment appear to exhibit more erratic penetrations

than do the ring-porous woods, possibly on account of the irregular

distribution of tyloses or gums. Among such species are the maples,

sycamore, and bigtooth aspen.

When the vessels or pores of a hardwood are filled with tyloses or

gums so that the movement of liquids through them is completely

obstructed, penetration must be obtained through the other vertical

elements surrounding the vessels. The cells of these tissues have closed

ends, so that liquids in passing from one cell wall to another must be

absorbed through the wall or some portion of it. Very likely the penetration takes place through the pits or through checks in the fiber walls

(61). Hickory is a good example of a wood in which these wood elements can be penetrated to a greater or less extent, and the oaks of

wood in which these elements cannot be penetrated. Although the

vessels of hickory are closed by tyloses as in the white oak, the other

wood cells of hickory are much more permeable than those in oak.
PRESERVATIVE TREATMENT OF WOOD 11

BORDERED PITS AND SIMPLE PITS

A relatively unthickened portion of a cell wall, the thin membrane of

which permits liquids to pass from one cell to another, is called a pit.

A bordered pit (see bp, figure 2), has an overarching rim that is not

present in a simple pit, is usually more or less circular, and is partitioned

by the pit membrane. The central portion of this membrane, which is

thickened, is known as the torus. Figure 3 shows an enlarged crosssectional drawing of a bordered pit, and figure 4 shows both a face and a

profile view of bordered pits as seen through a microscope. At A in

figure 3 and a in figure 4 the pit membrane is shown centrally located.

In many of the refractory woods, particularly in the heartwood, the

membrane is often permanently displaced, as indicated at B in figure 3

and b in figure 4. In this position they may retard or prevent the penetration of liquids. When the tori are not displaced, or are not ^Vell

seated,^' it is possible that liquids pass from one tracheid to another

through the pit membranes or minute openings in them.

In conifers the cells formed in the spring are relatively large with

thin walls, while those formed later are smaller and have thicker walls.

The wood in the annual ring composed of cells with thin walls is called

springwood and the portion containing the more dense wood is called

summerwood. In some hardwoods, for example the oaks, the springwood is differentiated primarily by the larger pores that it contains. In

others, such as maple and sweetgum, there is no such marked distinction

between springwood and summerwood.

The summerwood of most of the conifers is more easily penetrated

than the springwood by both preservative oils and water solutions. In

ß

M22224F

FIGURE 3.—Cross-sectional sketch of a bordered pit: A, Pit membrane centrally

located; B, pit membrane permanently displaced.
12 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AtiUIClLTURE

M22747F

FIGURE 4.—Phototíraiili taken tlinuifíli iniscroscdi»- shdwirif; greatly magnified bordered pits in l(jngitu(linal sections: .1, Face view of l)iirciered pits on eell wall: /,

torus in bordered iiit; hp, ixirdered pits; w, one of several wliite areas where section

did not cut through torus. B, I'ldfile view of bordered pits: a, tori centrally located;

¿1, tori displaced; bp, bordered ¡jits.

the summerwood more of the tori m;iy be cení rally localcil in tlic Ijijrdered

pit.s than in the sprino;wood, and it is ¡¡ossible tliat thi.s i« one of the

reason« for the better penetration« obtained in the summerwood rings

(3). Microscojjical examinations of specimens that had proved very

resistant to treatment in the summerwood showed that a large ]X)rtion

of the summerwood tori had been dis])laced (//, 13, 53). Some investigators, however, have indicated that ]ienetration is not aiijtreciably

affected by the position of the tori in the bordered pits. Since these

microscopical studies have been made nn only a few species, much more

work nuist be done to establish the relative importance of the position

of the t(jri as a factor affecting penetration.

.Simi)k' pits are relativelj' unthickened pcjrtions in the cell walls that

are limited by a thin membrane without such thickening as a torus
PRESERVATIVE TREATMENT OF WOOD 13

and that do not have an overhanging border. These pits are found in

the wood parenchyma and ray cells of softwoods and hardwoods and in

some of the fibers of the hardwoods. Little is known about their inñuence on penetration. In some of the hardwoods the simple pits and

the small bordered pits present may materially assist i:>enetration when

the vessels are closed by tyloses. They may also be of material assistance

in distributing i)reservative through the wood from the open vessels.

RESIN PASSAGES

A number of the softwoods contain resin passages or ducts of indeterminate length. Resin ]:)assages are largest and most numerous in

the pines. The longitudinal resin ducts are sometimes intersected by

smaller radial resin passages within the wood rays, which, joarticularly

in a number of the pine species, assist in the penetration of liquids.

Species that contain normal resin passages to a greater or less degree

are the pines, spruces, tamaracks or larches, and Douglas-ñr. The other

softwoods, which include the true firs, hemlock, cypress, redwood, cedar,

and yew, do not normally have resin passages. In some such woods as

Engelmann spruce, white spruce, and the larches, the resin passages are

very small, comparatively scarce, and open only for short distances.

Species with resin ducts of this kind are usually resistant to treatment

in the heartwood.

DENSITY

A comparison of the results obtained in the treatment of different

species shows that there is no correlation between the density or specific

gravity of the wood and the penetration of preservatives. Some of the

woods having high specific gravities, such as the open-pored red oaks, are

fairly easily penetrated; and some, like white oak, are very resistant.

Similarly, some woods of low density, like ponderosa pine, are easy to

penetrate; and some, like corkbark fir, are very resistant to treatment.

Other factors, such as the presence and condition of pores and resin

ducts and the difference between heartwood and sapwood, have so much

greater influence that, if density has any effect at all, it is completely

obscured.

Within the same species, however, density may under certain conditions appear to have some influence on penetration in the softwoods.

Timbers of the conifers having a large proportion of springwood in

comparison with the summerwood are of lower density and are usually

more difficult to treat or take more erratic penetration than the denser

timber of the same species. This is not really an effect of density, however, but of the difference in penetrability of springwood and summerwood, which is very marked in many of the conifers. The important

factor here is the percentage and distribution of summerwood, which

affect both the density and the penetrability. Differences in the relative

penetrability of the summerw (j(jd and springwood are not particularly

noteworthy in the hardwoods, except in species such as the red oaks, in

which the springwood vessels are open and are larger than in the summerwood.

Softwood timbers of rapid growth often have both wide springwood

and wide summerwood bands, and the density of material of this kind

may be as high as that of slower-growth wood, although the springwood

207777°—53 2
14 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

grows much more rapidly. Frequently the softwood timbers of rapid

growth will show good penetration in the wide summerwood and little

or no penetration in the wide springwood rings. The preservative in

such timbers may follow the summerwood from the outside surface of

sawed timbers or may enter through checks or resin ducts and then

penetrate the summerwood in a tangential direction. In most cases

the less heavily treated springwood is sufficiently protected against

decay and insect attack by the adjacent covering of well-treated summerwood and by the preservative that may later diffuse into it from the

summerwood. This, however, is not equally true for treated wood exposed to marine-borer attack. Complete penetration of both springwood

and summerwood is desirable for the severe conditions of marine service.

Although density does not determine the penetrability of wood, it

does influence the maximum amount of preservative that may be absorbed

in seasoned material. The preservative enters the air spaces in the wood,

and when a large part of the air space is occupied no more preservative

can enter. Although water and water solutions are absorbed by the

cell walls as well as in the cell cavities, preservative oils such as coal-tar

creosote are absorbed chiefly in the cavities and only to a slight extent

in the cell walls. Pure hydrocarbon oils such as petroleum are not

absorbed by wood substance and therefore enter only the air spaces.

The denser the wood the less air space it contains, and consequently

the less preservative it can hold. Wood is seldom filled with preservative

to complete saturation, however, since the various obstacles to penetration usually prevent complete penetration in timber of structural size.

Some of the significant aspects of air space and density are discussed on

page 28.

INFLUENCE OF STRUCTURE ON DIRECTION OF

PENETRATION

Penetration may take place in wood in three directions, namely:

Longitudinally, which is in the direction of the length of the tree trunk;

radially, which is in the direction of the radius of the tree; and tangentially

or circumferentially, which is in the direction of the annual rings.

Practically all species are most easily penetrated longitudinally, since

liquids can follow in the direction of the vessels in the hardwoods and

through the full length of the cellular space of the tracheids before passing

through transverse cell walls in the softwoods. Liquids passing in a

transverse direction, radial or tangential, must generally pass through

many cell walls in moving a relatively short distance.

In some of the pines penetration is much better in the radial than

in the tangential direction because it is assisted radially by the resin

ducts of the wood rays (fig. 5). The heartwood of bristlecone, pinyon,

and ponderosa pine is relatively easy to penetrate through the radial

resin ducts, but the heartwood of species like red pine, longleaf, shortleaf,

loblolly, and lodgepole pine are in general much more resistant to penetration. The penetration that is obtained in the latter woods, however,

appears to be more or less dependent on the open or closed condition of

the radial resin ducts.

Other conifers with radial resin ducts are Douglas-fir, the larches,

and the spruces; but the radial resin ducts in these woods tend to be

smaller than in the pines, are not so numerous, and are often closed,

and they apparently have little if any influence on radial penetration.
PRESERVATIVE TREATMENT OF WOOD 15

7^s?>.

/

.^^^'

M16143F

FIGURE 5.—Cross section of trratcd liristlcconi' pine Tlir il: radiiil stri'aks in

the heartwood show that the pencti'atioii of the preservative was assisted by th(

wood rays.

In most of the conifers, except the pines, tangential penetration is

usually better than the radial, although in some resistant species the

difference is slight. Various hardwood s]iecies, suc^h as birch, maple,

and elm, also a])pcar to take better j^encfralion in the tangential than

in the radial direction. In many of the cdnifers tangential ix'nctratioii

is assisted by the fact that summerwood bands are usually more permeable than either the wood rays or the alternate layers of springwood.

Radial penetration is largely ]irevented when both the wood rays and

the springwood bands are resistant.

It might seem that transverse or side ¡¡enetration into the faces of a

sawed or hewed timber must de])end entirely on the penetrability in a

direction across the fibers or cell walls, but this is not necessarily true.

A certain amount of ]ienetration in the direction of the fibers or vessels

(longitudinal i)enetration) is obtained through practically all sawed or

hewed surfaces, since many of the vessels of the hardwoods or tracheids

of the softwoods lie at an angle with the cut faces and heli^ conduct

liquids to varying depths. When the angle made by the fibers is large,

such as in timbers having a marked cross-grained structure, penetration

is usually deeper than in timbers that are fairly straight-grained.

Variability in penetration at different ])oints along the surface of a

sawed timber is apparently influenced to a considerable extent by the

number of cells that assist in longitudinal penetration from the side

surfaces. It is natural that at some points most of the vessels or tracheids

may lie nearly parallel with the surface, while at other points a variable

number will lie at an angle with it. Even longitudinal penetration is
16 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

slight in the heartwood of some of the more resistant species, such as

corkbark fir and sweetgum, and it is practically impossible to obtain

appreciable penetrations in the radial and tangential directions in the

heartwood of such species.

CLASSIFICATION OF SPECIES WITH RESPECT

TO PENETRABILITY

It is impossible to arrange the different species in exact order of their

relative resistance to penetration by wood preservatives, because of

the many variables that must be considered. The following rough

classification, however, should be helpful: (1) Heartwood easily treated,

(2) heartwood moderately difficult to treat, (3) heartwood difficult to

treat, and (4) heartwood very difficult to treat. This grouping is based

both on the results of laboratory experiments and on observations made

under commercial treating conditions. Although it is recognized that

there may be considerable variation in the penetrability of the heartwood

of species within the same group, nevertheless, if the classification is

taken as a whole, there is a definite difference in the relative resistance to

treatment of the woods in one group as compared with those in another.

With a few exceptions, the sapwood of most of the softwood species

in the different groups is not particularly difficult to impregnate under

pressure. The more important exceptions are the hemlocks, the true firs,

and the spruces. These woods have sapwood that is nearly as resistant as

the heartwood. The sapwood of these species is also very difficult to

distinguish from the heartwood. Although the sapwood of the other softwood species will in most cases take satisfactory treatment under pressure,

there is, nevertheless, a marked difference in the relative ease with which

the sapwood of some species takes treatment in comparison with that

of other species. For example, the sapwood of the cedars is much more

resistant than that of the pines and Douglas-fir. The sapwood of

Douglas-fir, in turn, is considerably more resistant than the sapwood of

most pine species. Even species within the same genus may vary over a

considerable range in the relative penetrability of the sapwood. This is

notable in the treatment of round timbers of different pine species.

The sapwood near the circumferential surface is generally less resistant than that near the heartwood. Apparently during the gradual

transition of the sapwood into heartwood, the process of change affects

the sapwood permeability to an increasing extent as the heartwood is

approached. This variability, however, is more conspicuous in some

species than in others.

In treating-plant operation, easily treated woods and resistant woods

should not be included in the same charge. The easily treated material

under such conditions would absorb more than the average and the

resistant material less than the average amount of preservative per unit

volume calculated for the charge. Differences in average absorption

between the two groups might be several hundred percent.

In round material, such as poles, posts, and piles, only the treatability

of the sapwood ordinarily needs to be considered because the heartwood

is so deeply covered with sapwood that it cannot be penetrated except

at end surfaces. In ties, or other sawed material, much sapwood and

also considerable heartwood area is sometimes exposed. In such material the treating-plant operator cannot avoid having easily treated and
PRESERVATIVE TREATMENT OF WOOD 17

GROUP 1.—Heartwood easily penetrated

SOFTWOODS

Bristlecone pine (Pinus aristata).

Pinyon (P. edulis).

Ponderosa pine (P. ponderosa).

Redwood {Sequoia sempervirens).

HARDWOODS

American basswood {Tilia americana).

Beech (white heartwood) {Fagus grandifolia).

Black túpelo (blackgum) {Nyssa sylvatica).

Green ash {Fraxinus pennsylvanica var.

lanceolata).

Pin cherry {Prunus pensylvanica).

River birch {Betula nigra).

Red oaks {Quercus spp.).

Slippery elm {Ulmus fulva).

Sweet birch {Betula lenta).

Water túpelo {Nyssa aquatica).

White ash {Fraxinus americana).

GROUP 2.—Heartwood moderately difficult to penetrate

SOFTWOODS

Baldcypress {Taxodium distichum).

California red fir {Abies magnifica).

Douglas-fir (coast) {Pseudotsuga taxifolia).

Eastern white pine {Pinus sirobus).

Jack pine (P. banksiana).

Loblolly pine (P. taeda).

Longleaf pine (P. palv^tris).

Red pine (P. resinosa).

Shortleaf pine (P. echinata).

Sugar pine (P. lambertiana).

Western hemlock {Tsuga heterophylla).

HARDWOODS

Black willow {Salix nigra).

Chestnut oak {Quercus montana).

Cottonwood {Populus sp.).

Bigtooth aspen (P. grandidentata).

Mockemut hickory {Garya tomentosa).

Silver maple {Acer saccharinum).

Sugar maple {A. saccharum).

Yellow birch {Betula lutea).

GROUP 3.—Heartwood difficult to penetrate

SOFTWOODS

Eastern hemlock {Tsuga canadensis).

Engelmann spruce {Picea engelmanni).

Grand fir {Abies grandis).

Lodgepole pine {Pinus contorta var. latifolia).

Noble fir {Abies procera).

Sitka spruce {Picea sitchensis).

Western larch {Larix occidentalis).

White fir {Abies concolor).

White spruce {Picea glauca).

HARDWOODS

American sycamore {Platanus occidentalis).

Hackberry {Celtis occidentalis).

Rock elm ( Ulmus thomasi).

Yellow-poplar {Liriodendron iulipifera).

GROUP 4.—Heartwood very difficult to penetrate

SOFTWOODS

Alpine fir {Ahies lasiocarpa).

Corkbark fir (A. lasiocarpa^ var. arizonica).

Douglas-fir (Rocky Mountain) {Pseudotsuga taxifolia).

Northern white-cedar {Thuja occidentalis).

Tamarack {Larix laricina).

Western redcedar {Thuja plicata).

HARDWOODS ^

American beech (read heartwood) (Pagi¿s

grandifolia).

American chestnut {Castanea dentata).

Black locust {Robinia pseudoacacia).

Blackjack oak {Quercus marilandica).

Sweetgum (redgum) {Liquidamhar styraciflua).

White oaks {Quercus spp.)-
18 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

resistant wood in the same charge. It is not enough in treating such

material merely to get some specified retention, because the preservative

may be absorbed almost entirely by the sapwood and the heartwood be

left with little protection. The operator should assure himself that as

much penetration as possible is obtained in the heartwood faces, and all

his technical skill may be called into play to bring this about. The

empty-cell processes are particularly useful in the treatment of such

material, because they permit a larger gross absorption for a given net

retention than iis })ossible when the full-cell method is employed.

MOISTURE CONTENT, FIBER SATURATION POINT,

SHRINKAGE AND SWELLING, SPECIFIC GRAVITY, WEIGHT OF WOOD, AND AIR SPACE IN

WOOD

Wood contains wood substance, moisture, and air or gas. Wood substance includes both the cellular structure and extractives and usually

has a variable quantity of water in the cell walls. When wood is green

or wet, the water also occupies a part or, in some cases, nearly all of the

cell cavities. In addition, there is usually more or less air or gas in the

wood cells, depending on the density of the wood and the moisture

content. In the present discussion the volume occupied by air or gas is

designated as air space.

MOISTURE CONTENT

The amount of water contained in a piece of wood is commonly called

the moisture content. In the case of green timbers this is sometimes

called the sap. Since, however, sap often contains various organic or

mineral substances in solution, it is more satisfactory to designate the

evaporable liquid as the water or the moisture content in the wood.

It is almost universal practice in this country to base the moisture

content on the weight of the wood when oven-dry. This procedure has

several advantages. Among the more important are the following:

(1) The oven-dry weight can be determined at any time and therefore affords a constant base.

(2) On the oven-dry-weight basis the moisture percentage shows

directly the number of parts of water by weight to the number of parts

of oven-dry wood. For example, if a piece of wood has a moisture content of 15 percent, there are 15 parts of water by weight to 100 parts of

wood substance.

(3) The moisture content in equilibrium with a given temperature

and relative humidity is a fairly definite figure regardless of the density

or specific gravity of the wood. There would be no concordant relation

in this respect if the moisture content were based on the total weight of

wood and water.

The amount of moisture content on the oven-dry-weight basis, may

be computed by using the formula M = -—^-^ lOOj in which M represents the percentage of moisture in the wood. Wo the original weight,

and Wd the oven-dry weight.
PRESERVATIVE TREATMENT OF WOOD 19

Pulp-and-paper chemists and others sometimes prefer to use the total

or original weight as the base. If 77i represents the percentage of moisture

determined on this basis, then m= ^—"~ ^^ 100, where Wo and Wd are

the original weight and oven-dry weight, respectively. If it is desired

to change from one base to the other, this is easily done, since

,, 100 m , 100 M

100—m 100+M

Moisture Content in Green Timbers

The sapwood of freshly cut timbers often contains a much higher

moisture content than the heartwood, and this is especially true for most

of the softwoods. Table 1 shows data on the average moisture content

of green sapwood and heartwood determined for various species. These

data are average values of moisture determinations from specimens

taken from a varying number of trees of each species and also from

different parts of the trees.

TABLE 1.—Average moisture content of green heartwood and sapwood for

various species of softwoods and hardwoods

Average moisture Average moisture

content content

Species Species

Sapwood Heartwood Sapwood Heartwood

Softwoods: Percent Percent Hardwoods: Percent Percent

Baldcypress 171 121 Ash, green. 58

Douglas-fir Ash, white 44 46

(coast) 115 37 Beech, AmeriFir, grand

Henüock,

136 91 can. _ _ 72 55

Birch, sweet. __ 70 75

eastern 119 97 Birch, yellow. __ 72 74

Hemlock, Chestnut,

western 170 85 American 120

Redcedar, Elm, American. 92 95

western 249 58 Ehn, cedar 61 66

Fir, white 160 98 Hickory, water. 62 97

Larch, western. 119 54 Maple, silver... 88 60

Pine, loblolly— 110 33 Maple, sugar... 72 65

Pine, lodgepole. 120 41 Oak, black 75 76

Pine, longleaf__ 106 31 Oak, northern

Pine, ponderosa

Pine, western

148 40 red 69 80

Oak, southwhite 148 62 ern red _ 75 83

Pine, red 134 32 Oak, swamp red 66 79

Pine, shortleaf- 122 32 Oak, water 81 81

Pine, sugar 219 98 Oak, white 78 64

Redwood (vir- Oak, willow 74 82

gin growth)-_ 210 86 Tupelo, black __ 115 87

Spruce, Engelmann

Tuoelo. water . 158

173 51 Sycamore,

Spruce, Sitka__ 142 41 American 130 114

Sweetgum 137 79

Yellow-poplar.. 106 83
20 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

FIBER SATURATION POINT

When seasoned wood absorbs water, the water will first be taken up

by the fibers or cell walls until they become saturated. The moisture

WEIGHT (POUNDS PER CUBIC FOOT)

¿O 30 40 50 60 70 80

20 30 40 50 60

WEIGHT (POUNDS PER CUBIC FOOT)

70 60

FKíURE 6.—Relation of moisture content, specific gravity, and weight of wood in

pounds per cubic foot. (Specific gravity based on weight when oven-dry and volume when green = Sg.)

content of the wood at the point when the fibers are saturated but there is

no free water in the cells is called the ^'fiber saturation point.'' The

water contained in the cell walls is known as hygroscopic or adsorbed

water. Up to the fiber saturation point, swelhng of the wood accompanies

the adsorption of w^ater. After the walls have become saturated, water

may be absorbed in the lumina or cell cavities. Water contained in these

spaces is called free water. Swelling is not affected by the free water

contained in the cell cavities, and swelling therefore does not continue

with increasing moisture content after the fiber saturation point is

reached.

The fiber saturation point is not determined with exactness and varies

for the different species, ^'arious methods have been employed at the

Forest Products Laboratory to determine the moisture content at which

the fiber walls are saturated (5^» ^5^ 56),
PRESERVATIVE TREATMENT OF WOOD 21

These studies show that among the more important factors affecting

the moisture content at the fiber saturation point are temperature of the

wood, chemical substances in the cell walls, and chemical composition of

the wood. The moisture content at the fiber saturation point usually

ranges from about 25 to 35 percent. Data obtained from experiments

indicate that for normal wood temperatures a satisfactory average value

for the moisture content at the fiber saturation point is about 30 percent,

based on the oven-dry weight. The effect of increasing temperature is

to reduce the moisture content at the fiber saturation point, roughly

about 1 percent per 10° C. (18"^ F.) rise in temperature.

The weighing of green timbers should be done when the moisture content of the wood near the surface is above, or at least not much below,

the fiber saturation point (about 30 percent moisture), so that the effect

of shrinkage will not be an important factor affecting the volume. The

data given in figure 6 are based on a fiber saturation point of 30 percent.

With this as a base, weights shown for moisture-content values below

the fiber saturation point were computed by assuming that the volumetric shrinkage is proportional to the change in moisture content that

occurs in seasoning. Equations 14 to 17, page 137, can be used for

computing the weight of wood at any given moisture content.

To illustrate the use of figure 6, assume that sample poles of green

Douglas-fir weigh about 43 pounds per cubic foot. Table 2 shows that

the average specific gravity (Sg) (based on weight when oven-dry and

volume when green) for this species is about 0.45. Directly above the

weight of 43 pounds on the horizontal scale, a point midway between

Sp = 0.44 and 0.46 (which corresponds to the value S, = 0.45) is opposite

a moisture-content value of about 53 percent. This figure represents

the average moisture content of the timber including sapwood and

heartwood.

Equation 18, page 137, or figure 7 can be used in finding the percentage

of sapwood in the total volume, when the average diameter and the

average sapwood thickness are known. In estimating the average

moisture content from the weight of the timbers, it should be understood

that knots, variations in the specific gravity of different timbers, and

similar factors, influence the results; hence the moisture content determined by this method will necessarily be a rough approximation.

Methods of Determining .Moisture Content in Timbers

Several methods are in use for deterniininíí the moisture content of

wood (52), They vary considerably in detciil and in their suitability

for use under different circumstances. .

The most common method is oven-drying, which consists in weighmg

a representative small sample of the wood, heating it in an oven at 100

to 105° C. (212° to 221° F.) until the weight remains constant, then

weighing again to determine the loss in weight and the dry weight. ^ The

method is reasonably accurate for most woods, but in highly resmous

wood the readings may be high if the resins are melted and drip out

during the drying process. The amount of volatile resin products that

evaporate at temperatures employed in oven-drying is small, and even in

very resinous woods whatever volatile oils might be driven off would

normally have little effect on the moisture content determined by the

oven-drying method. If, however, the heavier constituents are melted
22 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

RATIO OF SAPWOOD THICKNESS TO DIAMETER = %

IQQ 0.1 0.2 0.3 0.4 0.5

O 0.1 0.2 0.3 0.4 0.5

RATIO OF SAPWOOD THICKNESS TO DIAMETER = T/D

FIGURE 7.—Chart showing percentage of sapwood in the total volume for any ratio

of eapwood thickness to diameter.
PRESERVATIVE TREATMENT OF WOOD 23

TABLE 2.—Average shrinkage and specific gravity of various commercial

species grown in the United States

Species

Average shrinkage from green

to oven-dry condition (percent

of dimensions when green)

Radial

Ash, commercial white _ 4.6

Aspen, quaking 3.5

Basswood, American 6.6

Baldcypress 3.8

Beech, American 5.1

Birch, yellow 7.2

Cherry, black 3.7

Chestnut, Americaji 3.4

Cottonwood:

Eastern 3.9

Northern black 3.6

Douglas-fir:

Coast type 5.0

Intermediate type 4.1

Rocky Mountain type 3.6

Ekn:

American 4.2

Rock 4.8

Slippery 4.9

Fir:

Balsam 2.8

Commercial white ^ _ _ 3.2

Hackberry 4.8

Hemlock :

Eastern 3.0

Western 4.3

Hickory :

Pecan 2 4.9

True-^ 7.3

Honeylocust 4.2

Larch, western 4.2

Loc\ist, black 4.4

Maple:

Bigleaf 3.7

Black 4.8

Red 4.0

Silver 3.0

Sugar 4.9

Oak:

Red 4 4.3

White 5 5.4

Pine:

Loblolly 4.8

Lodgepole | 4.5

Longleaf ; 5.1

Eastern white 2.3

Red , -i.e

Ponderosa 3.9

Shortleaf I 4.4

See footnotes at end of table.

Tangential

7.5

6.7

9.3

6.2

11.0

9.2

7.1

6.7

9.2

8.6

7.8

7.6

6.2

9.5

8.1

8.9

6.6

7.1

8.9

6.8

7.9

8.9

11.4

6.6

8.1

6.9

7.1

9.3

8.2

7.2

9.5

9.0

9.3

7.4

6.7

7.5

6.0

7.2

6.3

7.7

Volumetric

12.8

11.5

15.8

10.5

16.3

16.7

11.5

11.6

14.1

12.4

11.8

10.9

10.6

14.6

14.1

13.8

10.8

9.8

13.8

9.7

11.9

13.6

17.9

10.8

13.2

9.8

11.6

14.0

13.1

12.0

14.9

14.8

16.0

12.3

11.5

12.2

S.2

11.5

9.6

12.3

Average Average

specific specific

gravity gravity

based on based on

oven-dry oven-dry

weight weight

and and

green oven-dry

volume volume

-s. =s.

0.55 0.64

.35 .41

.32 .40

.42 .48

.56 .67

.55 .66

.47 .53

.40 .45

.37 .43

.32 .37

.45 .51

.41 .47

.40 .45

.46 .55

.57 .66

.48 .57

.34 .41

.36 .41

.49 .56

.38 .43

.38 .44

.59 .64

.64

.60 .67

.50 .59

.66 .71

.44 .51

.52 .62

.49 .55

.44 .51

.56 .68

.57 .68

.60 .72

.47 .54

.38 .43

.54 .62

.34 .37

.41 .51

.38 .42

.46 .54
24 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

TABLE 2.—Average shrinkage and specific gravity of various commercial

species grown in the United States—Continued

Species

Average shrinkage from green

to oven-dry condition (percent

of dimensions when green)

Radial

Average

specific

gravity

based on

oven-dry

weight

and

green

volume

Tangential ¡Volumetric I =SQ

Average

specific

gravity

based on

oven-dry

weight

and

oven-dry

volume

Slash

Sugar

Western white

Redcedar:

Eastern

Western

Redwood

Spruce :

Eastern ^

Engelmann

Sitka

Sugarberrj^

Sweetgum

Sycamore, American

Tamarack

Tupelo :

Black

* Water

Walnut, black

White-cedar, northern _

Yellow-poplar

5.5

2.9

2.6

3.1

2.4

2.Ü

4.3

3.4

4.3

5.0

5.2

5.1

3.7

4.4

4.2

5.2

2.1

4.0

7.8

5.6

5.3

4.7

5.0

4.4

7.7

6.6

7.5

7.3

9.9

7.6

7.4

7.7

7.6

7.1

4.7

7.1

12.2

7.9

11.8

7.8

7.7

6.S

12.6

10.4

11.5

12.7

15.0

14.2

13.(')

13.9

12.5

11.3

7.0

12.3

.56

.35

.36

.44

.31

.38

.38

.32

.37

.47

.44

.46

.49

.46

.46

.51

.29

.38

.66

.38

.42

.49

.34

.42

.43

.35

.42

.54

.53

.54

.56

.55

.52

.56

.32

.43

NOTE: TO compute the shrinkage due to seasoning from green to any moisture

content below fiber saturation point, multiply figure from table 1 by( 1—— J, where

M is the moisture content (percent) to which wood is seasoned.

1 Average of grand fir and white fir.

2 Average of bittcrnut hickory, nutmeg hickory, water hickory, and pecan.

^Average of shellbark hickory, mockernut hickory, pignut hickory, and shagbark

hickory.

4 Average of black oak, laurel oak, pin oak, northern red oak, scarlet oak, southern

red oak, swamp red oak, water oak, and willow oak.

5 Average of bur oak, chestnut oak, post oak, swamp chestnut oak, swamp white

oak, and white oak.

ß Average of black spruce, red spruce, and white spruce.

out, the effect on the oven-dry weight would be more important, depending on the size of the moisture specimens and the amount of nonvolatile material melted out. The samples used must be thoroughly

representative of the timber being tested, and the weighing must be

done promptly after sampling; otherwise a false indication will be obtained. A serious disadvantage of the method is the necessity of cutting

a sample from the timber, and thus the probable destruction of its

usefulness.

Another method, which causes less damage to the timber, is to take

borings with a bit or increment borer. The wood removed from the
PRESERVATIVE TREATMENT OF WOOD 25

bored holes should be placed immediately in a container, such as a test

tube or small bottle, and the container should be tightly corked to

prevent loss of moisture until the sample can be weighed. After weighing,

the wood can be oven-dried and the moisture content can be computed

from the weights taken before and after oven-drying. If it is desired to

find the moisture content of the sapwood and heartwood separately, the

borings can be separated where the heartwood starts and the two portions

can be weighed and dried separately.

Electric moisture meters are now used extensively in connection with

dry-kiln operations and in the manufacture of wood products. They

are also used to a limited extent at some wood-preserving plants. These

meters were introduced in commercial work about 1930. Several

companies now manufacture instruments that measure the moisture

content by one of the following methods: (1) By determining the

electrical resistance of the wood by what is known as the resistance-type

meter, or (2) by determining the radio-frequency power loss by what is

known as the radio-frequency power-loss type of meter.

These instruments are generally used for moisture determinations in

wood that has a moisture content between 7 percent and about 25 percent,

or close to the fiber saturation point (p. 21). Because of the high resistance of the wood at low moisture values, the meters are not usually

calibrated for readings below 6 to 7 percent.

Some manufacturers also make instruments to determine the moisture

content below 6 percent, and some make instruments that read as high as

120 percent moisture. Greater accuracy, however, is obtained in the

moisture content range below the fiber saturation point (p. 20).

When it is inconvenient to make direct moisture determinations from

timbers that are seasoning, a rough estimate of the average moisture

content can be made by weighing representative timbers and then

consulting figure 6. The average specific gravity (Sg) for the species in

question can be found in table 2.

SHRINKAGE AND SWELLING

The amount of shrinkage or swelling that occurs in wood will depend

mainly on its density and the change in moisture content {24J ^^J 4^)-

When wood is drying, shrinkage does not start until the water in the

cell cavities or lumina has evaporated and the fiber saturation point is

reached. The wood near the surface of a timber may, however, season

considerably below the fiber saturation point while the average moisture

content of the piece is well above the fiber saturation point. ^ This is

especially true for timbers of fairly large cross-sectional dimensions.

Pure hydrocarbon oils are not absorbed by wood substance and therefore cause no swelling when timbers are impregnated with them. Preservative oils, like the creosotes, are absorbed only to a small extent, and

the amount of swelling as a result of treatment is so slight that for practical

purposes it can be overlooked. Water solutions will, of course, cause

swelling if the wood has been partially seasoned before treatment.

No change in moisture content can take place unless a moisture

gradient or a combined relative-humidity, moisture-content gradient

exists. Sometimes those not familiar with this fact claim they are able

to maintain a uniform moisture content throughout the timber when
26 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

seasoning by the use of heat and changing humidity. Under such

conditions a moisture gradient is just as essential for seasoning as a

difference in pressure is necessary to force a liquid or gas through a

pipe, or a difference in voltage is necessary to make an electric current

flow through a conductor. Both the rate of seasoning and the rate of

water absorption are functions of the moisture gradient, for the rate

decreases as the difference in moisture content between the surface and

interior is reduced.

Wood shrinks chiefly in the radial and tangential directions, that is,

at right angles and parallel to the annual rings. In normal wood,

longitudinal shrinkage is very slight (between 0.1 and 0.2 percent) and

for such material may be considered negligible. The ratio of radial to

tangential shrinkage is roughly about 1 to 2, but this may vary to quite

an extent, depending on the species. The volumetric shrinkage of

hardwoods ranges from about 12 to 18 percent in seasoning from the

green to the oven-dry condition, and the corresponding shrinkage of the

conifers or softwoods ranges from about 8 to 14 percent. Wood when

air-dry usually has a moisture content of 12 to 15 percent, and shrinkage

from the green to the air-dry condition is therefore about half as much

as that to the oven-dry or moisture-free value. Table 2 shows the

average radial, tangential, and volumetric shrinkage from the green to

the oven-dry condition for various commercial woods.

SPECIFIC GRAVITY OF WOOD

The specific gravity of wood is usually defined as the ratio of the

weight of a given volume of wood when oven-dry at the current moisture

content, to the weight of an equal volume of water at its maximum density

(4° C). If the specific gravity is represented by Ä, the oven-dry weight

per unit volume by Wdj and the weight of an equal volume of ^ater by W^

Wd then S =:^. In this expression Ww=l in the Centimeter-gram-second

.(C.G.S.) system and equals 0.0361 pound per cubic inch, or 62.4 pounds

per cubic foot.

For example, assuming that a cubic inch of wood at 15 percent moisture content weighs 0.023 pound, the oven-dry weight would be 0.023^

1.15 = 0.020 pound. The specific gravity would then be computed as

0.0361 "^•^^•

Since density equals the mass per unit volume, the numerical values for

specific gravity and density would be the same in the C.G.S. system. In

other systems of measurement the density is necessarily expressed in the

units employed. For example, in the EngUsh system, the density of

water is 62.4 pounds per cubic foot, although the specific gravity of

water is unity, as it is in the C.G.S. system.

For practical purposes the specific gravity, based on the weight of

the oven-dry wood and the volume at current moisture content, may be

assumed to increase in about direct proportion to the loss of moisture

content below the fiber saturation point. The specific-gravity range

will then be between the specific gravity based on the weight of the ovendry wood and the volume when green and that based on the weight of
PRESERVATIVE TREATMENT OF WOOD 27

the oven-dry wood and the volume when oven-dry. From this relation

the specific gravity at any moisture content M can be determined from

the foUowmg equation:

S.= [ä.-(S.-S„)MJ

where S„ is the specific gravity at the moisture content M, Sj is the

specific gravity based on the weight and volume when oven-dry S is

the specific gravity based on the weight when oven-dry and the volume

when green, and M is the moisture content under consideration below

the fiber saturation point. In this equation the fiber saturation point

is taken as 30 percent. Average values of Sä and S, for various species

/.30

U9

I.ZB

\"'

^ I.Z5

K

"^ 1.24

I

i.z¿

g/2/

^ hZO

I

is IJQ

^ —

k

I«

III

III

1.10

\

\

_\

V

—

\

— \

\

— \

— \

\

— \

—

\

\

— \

— \

—

\

\

— \

\,

— \

— \

— N

—

N

\

— \

N,

—

\

"V^

"1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

X

L 4- 6 d to /Z /4 fb /Ô ZO ZZ Z4 Zif

MOfSTURß COA/TEA/T (PERCEAfj)

Zd 30 3Z

FIGURE 8.—Specific gravity of water in cell walls of wood at different percentages of

moisture.
28 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

are given in table 2. Since the maximum volume is obtained at the

fiber saturation point, the specific gravity AS^ remains constant for all

moisture-content values above the fiber saturation point.

While changes in specific gravity normally occur because of shrinkages

as the moisture is reduced below the fiber saturation point, there is a

condition known as ^^collapse^^ that sometimes occurs during seasoning

or when the wood is subjected to treating pressures that are too high.

As the term collapse suggests, the cell walls are more or less broken

down, and this causes a decrease in the amount of air space in the lumena

or cell cavities. This condition tends to increase the amount of wood

substance per unit volume and, consequently, increases the specific

gravity.

SPECIFIC GRAVITY OF WOOD SUBSTANCE

Oven-dry wood normally contains a large amount of air space or voids

because of the cellular structure, and the volume occupied by wood

substance alone in a block of wood is considerably less than the total

volume of the block. A certain amount of compression occurs in the

water contained in the cell walls, and the density or specific gravity of

wood substance determined in water is therefore somewhat higher than

when measured in a substance like helium gas that is not compressed

by the force of adsorption. Studies made at the Forest Products Laboratory {55, 56) showed that the density of wood substance measured in

helium gas was about 1.46, while an average value of the density determined in water was about 1.53. From these results the true specific

volume of wood substance is y^^, or about 0.685, while the apparent

specific volume in water is y-^, or about 0.654, a difference of 0.031.

Figure 8 shows the relation of the density of water in the wood cell walls

to the moisture content up to the fiber saturation point.

Figure 9 shows the weight of water in wood of any specific gravity

and having various percentages of moisture up to the maximum content

when all the air space is filled, and figure 10 shows the percentage of

volume filled with water when the wood has various percentages of

moisture. It should be borne in mind that a given volume of seasoned

wood will swell when water is absorbed ; hence a unit volume of seasoned

wood when wet will have a capacity to absorb more water than if the

volume remained constant. In general, the volumetric shrinkage or

swelling is about proportional to the change in moisture content.

RELATION OF SPECIFIC GRAVITY, MOISTURE CONTENT,

AND AIR SPACE IN WOOD

A knowledge of the amount of air space in wood is useful in estimating

the maximum amount of unoccupied space that is available under the

moisture conditions at which the wood is treated. This information is

needed if one is interested in computing the maximum absorptions that

can be expected. Laboratory experiments show that even in small

specimens of easily penetrated wood given a full-cell treatment, about 5
PRESERVATIVE TREATMENT OF WOOD 29

THI5 UNE 5H0W3 MÂX/MUM AMOUNT

OF WATER WOOD WILL HOLD AND

MAX/MUM M0/5TURE CONTENT WHEN

-250 ^^^ AIR 5PACE 15 F/LLED

0 0.1 O.Z 0.5 OA 0.5 O.b OJ OS 0.9 10 I.I IZ 1.3 1.4 IS

SPECIFIC GRAVITY

(BASED ON WEIGHT WHEN OVEN-DRY AND VOLUME AT CURRENT

MOISTURE CONTENT)

FIGURE 9.—Relation of weight of \v;iter in wood, specific gravity, and percentage of

moisture.

to 10 percent of the air .space will be unfilled. It is usually not practical

to obtain net retentions (using the full-cell treatment) that will fill more

than about 80 to 85 percent of the available air space in the penetrated

portion of commercial-size timbers. Figure 11 shows the relation of

air space, moisture content, and specific gravity.

207777°—53 3
30 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

^^

I

I

I

-T///5 LINE. 5H0W5 MAX/MUM AMOUNT

OF WATER WOOD W/LL HOLD WITH

ALL AIR 5PACE FILLED

-3>

o

^

^

o

o

o

-o

o

0.4 0.5 Ô.0 0.7 0.Ô 0.9 1.0 l.l I.Z 1.3 /4

SPECIFIC GRAVITY

(BASED ON WEIGHT WHEN OVEN-DRY AND VOLUME AT CURRENT

MOISTURE CONTENT)

FIGURE 10.—Percentage of volume filled with water when wood has various amounts

of moisture and any given specific gravity.

PREPARATION OF TIMBER FOR TREATMENT

Much of the success of treating operations depends upon the preparation of the timber to receive the preservative. Timbers of practically

all species require more or less seasoning or conditioning before they can

be satisfactorily impregnated. Air-seasoning is the method most generally employed, but for various reasons, such as unfavorable climatic

conditions, rush orders, or restricted storage space, it is often necessary

to use one of the artificial methods such as steaming and vacuum or

boiling in oil under vacuum in order to condition the wood for treatment.
PRESERVATIVE TREATMENT OP WOOD 31

S

'o

>

Ö

%

tí

o

Xi

>.

2 Ö O O

03 O

a«CO M

•S« 03 O

tí tí

(l/^30¿/3d) OOOM NI 30t^dr ¿//t/

tí

.2
32 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

AIR-SEASONING

Some of the more important considerations in air-seasoning include

species and size of timber, proportion of sapwood, time of cutting, peeling,

climatic conditions, locality in which the timber is seasoned, and method

of piling.

Species and Size of Timber

Even under the same conditions, timbers of similar size but of different species often vary considerably in the time required for air-seasoning.

This is partly on account of wide variations in the original moisture

content of the wood and partly because various woods differ greatly in

the rate at which they give off moisture. Table 1, p. 19, shows average

moisture-content values for green heartwood and sapwood of many of the

species commonly treated. It may be noted from this table that the

softwoods usually show a much higher moisture content in the sapwood

than in the heartwood and that this difference is less marked in the hardwoods. The values given in the table are averages for a varying number

of trees of each species. Individual timbers of a given species, or material obtained from a given locality, may differ to a greater or less extent from the averages given.

Large timbers season more slowly than those of small dimensions

because of the greater volume of wood in proportion to the surface area

and because the moisture at the interior must travel a greater distance

through the wood to reach the surface. It is advisable to season large

timbers slowly in order to avoid excessive damage from checking. Large

timbers have a higher average moisture content after a given period of

air-seasoning than timbers of smaller sizes, although the outer inch or

two of the larger timbers may be practically as dry as the smaller material.

Seasoning of Heartwood and Sapwood

The sapwood of most species seasons at a much faster rate than the

heartwood. On the other hand, the sapwood often contains a higher

percentage of moisture than the heartwood, and this tends to offset its

greater seasoning rate. In round timbers or sawed material in which

the heartwood is surrounded by sapwood, the faster seasoning rate of

the sapwood may encourage checking. Furthermore, the sapwood is

on the outside of the piece and, except at end surfaces, begins to season

sooner than the heartwood. As a result, the sapwood dries and begins

to shrink, while the heartwood still retains its original size. Checking

of the sapwood is inevitable in such cases, and the checks later may

extend into the heartwood as it dries. The size and severity of the

checks can be limited to a considerable extent, however, by controlling

the drying rate through piling methods and by the use of end coatings.

In material that is all sapwood there is less danger of checking than in

all-heartwood mat(TÍal.

Since sa]3wood has low resistance to decay, it should be seasoned

with as little delay as possible and under the most favorable drying

conditions. Rapid seasoning requires conditions contrary to those that

favor reduced checking. The danger of infection and the danger of

checking vary considerably with locality and species, however, so that

each operator must work out as best he can the seasoning methods most

suitable for his species and locality.
PRESERVATIVE TREATMENT OF WOOD 33

Coatings Used to Reduce End Checking

Since wood dries more rapidly from the end than from the side grain,

end checking is a problem in the seasoning of some material, particularly

hardwoods. Two types of coatings are commonly used with hardwood

lumber to prevent end checking. In one type the coating is applied

cold, and in the other it is heated first {57).

Although end coatings have been used considerably in kiln-drying

operations, they have not been widely used for reducing end checking in

ties, poles, and other timbers being air-seasoned for treatment. The

added cost, difficulty of early application, possibility that the end coatings may contaminate the preservative and retard penetration or may

clog the pumps, valves, or piping of the treating equipment, would need

consideration. These are doubtless among the factors responsible for

the limited interest in the use of end coatings for ties and similar material.

Moisture-retardant coatings such as tar, rosin, or other compounds

that are soluble in preservative oils might be helpful, but careful study

would be required to determine their merits or whether the extra cost

would be justified.

Use of Antichecking Irons to Reduce End Checking

Because hardwood ties may give considerable trouble from end checking, especially when they contain boxed heart (enclosing the pith portion

of the tree), antichecking irons are often driven in the ends of the ties

when they are stacked for air-seasoning. These irons should be driven

before important checking can develop. The antichecking irons most

commonly used are the so-called S-iron and C-iron. These names indicate the shape of the irons used. The use of antichecking irons does

not always prevent serious end checking, and there is some disagreement regarding their value. Their continued use by various railroads,

however, indicates that some still consider them an important help in

reducing the danger of injury from end checking.

Spiral Dowels

Another method of preventing end checking in ties and sawed timbers

is to drill holes near the ends at approximately right angles to the direction of the expected checking and to drive in spiral-shaped metal dowels.

A tight fit is obtained by making the holes slightly smaller than the

dowels. A machine has been developed to bore and dowel hardwood

ties automatically. The same general method may be used with timbers

that have checked seriously in seasoning, by first clamping the end of

the timber to close the check and then boring and dowehng.

Peeling

All material should be peeled before seasoning because bark retards

the drying of the wood, harbors insects, and favors decay infection in

the sapwood. The peeling should be thoroughly done, even to the

removal of the inner bark. The inner bark of many species, particularly

the conifers, is highly impervious to liquids, and if strips of appreciable

size are left on the wood they may not only interfere with seasoning but

also subsequently prevent the penetration of preservatives.
34 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

As a rule, timber cut during the active growing months from May to

July peels more easily than that cut at other seasons of the year. The

peeling of timbers cut in fall and winter can be facilitated by boiling or

steaming them for 1 to 3 hours.

Peeling or shaving machines are now being widely used in the woodpreserving industry for peeling poles and posts. They vary widely in

character and are used both for removing bark and for removing knots

and other irregularities on the surface (7).

Time of Cutting

Although round timbers peel most easily when cut in spring or early

summer, the most favorable time for cutting timber from the standpoint of preventing damage while awaiting treatment is in the late fall

or winter. With proper care, however, most kinds of wood can be

seasoned satisfactorily for treatment if cut at any time during the year.

In cold weather wood seasons more slowly, and there is less danger of

severe checking than in warm, dry weather. Insects and decay fungi

are inactive during cold weather; hence there is less danger of winter-cut

wood becoming infected. By the time warm weather causes the revival

of insect and fungus activity, the timber is usually out of the woods and

partly seasoned so that its resistance to infection is increased.

The advantage from winter cutting depends partly upon the climatic

conditions and partly upon the susceptibility of the wood to damage.

Greatest care is required in the Southern States where the winter is short

and not very cold and where many of the species of wood cut for ties and

poles have a large percentage of sapwood and are very susceptible to

insect and fungus damage. Winter cutting is best in the Southern

States, as in colder climates, and the necessity of removing the wood

promptly from the woods to proper seasoning yards is greater there than

where long, cold winters prevail. This holds for ties, piling, and poles

that are manufactured in the woods and for logs that are later to be

sawed into lumber, ties, or timbers.

It is commonly believed that there is less sap in the trees in winter

than in summer, but experiments do not substantiate this belief. Experiments made both in North America {10, 22) and in Europe show that

trees cut in winter have fully as much if not more sap in them as trees

cut in the spring or summer. Possibly the fact that sap is moving

through the sapwood in the warmer period of the year has led to the

belief that there is more sap in the wood during the spring and summer

months.

Climatic Conditions and Locality

Climatic conditions influence air-seasoning. In parts of the South

where the rainfall is considerable and the temperature and humidity are

relatively high throughout a large part of the year, it is difficult and

sometimes practically impossible to thoroughly air-season ties and

round timbers of certain species before serious injury occurs from decay.

This difficulty is experienced particularly with material that is largely

sapwood or is low in decay resistance. When timber must be air-seasoned

under such unfavorable conditions, it is essential that every precaution

be taken to regulate the controllable factors so that the most desirable
PRESERVATIVE TREATMENT OF WOOD 35

seasoning conditions will be provided. Some plant operators pretreat

with chemicals that temporarily prevent decay and safely allow a longer

seasoning period. For example, some plants pretreat poles and piling

by the Rueping method with light retentions of creosote, such as 4

pounds per cubic foot, or with a water-borne preservative by the full-cell

method, to protect the timber against decay while seasoning or while

stored in the yard awaiting orders for treatment. Good pretreatment

should provide effective protection against decay for many months of

seasoning.

In dry or arid regions decay infection is seldom encountered during

seasoning except under the most careless handling. In such regions the

prevention of severe checking caused by too rapid drying may be the

most important consideration, and the methods of handling and piling

must be selected accordingly.

Drainage and IVIethod of Piling

Much of the annual loss of ties, poles, and timbers ruined or damaged

by decay while seasoning or in storage could be prevented through improvement in piling methods. The seasoning yard should be located on

dry, well-drained ground where there is good air circulation. Low or

filled ground favors the retention of water in the soil, and if surrounded

by higher ground it may act as a pocket for the collection of damp air

and fog. The location of wood-preserving plants, however, is determined most often by shipping facilities, cost of land, and the like, rather

than by the advantages of the site for seasoning. Under such conditions

the plant operator must make the best of what is available.

Good pile foundations are essential for efficient air-seasoning. They

should be of treated wood or of some other material that does not decay.

It is bad practice to place the bottom course of timber directly on the

ground because it does not season well and, in most localities, is almost

sure to become infected with decay. Well-treated reject ties and timbers

make excellent foundation material. The foundations should be high

enough to keep the bottom course of timbers Ij^ to 2 feet above the

ground, because air circulates downward through a seasoning pile and

the moisture-laden air must have freedom to flow out at the bottom.

The seasoning piles should be built openly and far enough from each

other to allow free circulation {21).

The foregoing precautions are especially necessary where there is

severe danger of decay infection. In extremely dry or arid regions,

however, the precautions should be modified by making the piles tighter

and the spacing closer in order to prevent too rapid drying and excessive

checking. Every treating-plant operator should study his own conditions

thoroughly, select the methods best adapted to these conditions, and

keep constantly on the alert for improvements.

Sanitation

The seasoning yard should be kept clean and free from holes where

water, waste wood, or debris of any kind may accumulate. Weeds and

brush should be kept down not only within the yard but around it.

Neglect in these respects retards seasoning, favors decay, and increases

the fire hazard.
36 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Length of Seasoning Period

Table 3 gives the air-seasoning periods employed by wood-preserving

plants in various parts of the United States for timbers of different

species. The periods are for ties, piles, and poles, since such timbers

comprise most of the material air-seasoned for treatment. The seasoning

periods used for lumber, bridge timbers, and similar material are more

variable than those for ties, poles, and piles on account of the greater

differences in cross-sectional dimensions. The lack of uniformity in

practices indicates the variety of opinions among plant operators as to

the amount of seasoning desirable, differences in details of yard location

and piling methods, and differences in treating methods and purchasers'

requirements.

Most of the seasoning that occurs in larger-size timbers, such as ties,

will be obtained within the first year (51). The rate of seasoning becomes

extremely slow afterward, and it is doubtful whether there is enough

additional moisture loss to justify longer seasoning periods. Severe

decay infection may occur when timbers are air-seasoned for more than

12 months, and in some parts of the country sapwood timbers cannot be

yard-seasoned longer than 2 to 3 months without danger of serious loss

from decay. In the Northern States only a limited amount of seasoning

occurs in the 6-month period from about the middle of September to

the middle of March.

TABLE 3.—Air-seasoning periods employed by individual wood-preserving

plants in different parts of the United States for ties, piling, and poles,

of various species, 1949-50

Species

Form

of

timber

Number

of

plants

Region in which

plants are

located i

Seasoning

period

(months)

Beech__ _ Ties

__„do

3

2

3

2

1

1

1

2

1

2

1

1

1

1

2

2

1

3

Interior eastern.

do

Interior western_

do

do

do

do

do

Pacific

6

Do 6-12

Douglas-fir (Rocky Mountain)

Do

-._do

_ do

4-12

9-12

Do _ do 6

Do _ do - IM

10-12

4- 5

Douglas-fir (coast)

Do _

-__do

_ do

Do _ do 10-12

Do - do do

do

Southern

do

do

do

do

Interior western.

do

do

do

Interior eastern.

Interior western.

do

Interior eastern_

do

Southern

6- 7

Do _ - _ do 6-12

Gum _ ______ _ do 3- 5

Do . do 5- 6

Do _ do 6- 8

Do - do 2- 3

Do do 4- 6

Do .__do 5- 6

Lodgepole pine _ _ _ __.do 3- 6

Do ___do 8-10

Do ___do 9-10

Maple _ _ _ _ _ do. __ 6-12

Ponderosa pine _ _ _ _ _ do 6

Do .. . do 4- 6

Red oak

Do

Do

__.do

.__do

-._do

12-14

10-12

11-12
PRESERVATIVE TREATMENT OF WOOD 37

TABLE 3.—Air-seasoning periods employed by individual wood-preserving

plants in different parts of the United States for ties, piling, and poles,

of various species^ 1949-50.—Continued

Species

Form

of

timber

Number

of

plants

Region in which

plants are

located i

Seasoning

period

(months)

Red Oak (Continued)

Do

Do

Ties

___do

_ _do

1

2

1

2

5

1

1

1

1

1

1

1

1

1

3

3

1

2

8

3

2

2

1

Southern

do

do

do

do

do

do

Interior eastern.

do

Interior western.

Interior eastern.

Atlantic

Interior eastern.

Pacific

2- 3

Do . do 10 1*^

Southern yellow pine

Do__:

Do

---do

---do

do

3

2- 3

3 4

Do do d 6

Western larch do 6 12

Do do 4-10

White oak do 12-15

Do do 10-12

Yellow birch. do 6-12

Douglas-fir (coast) Piling

do

12-24

Do -----do

Southern

do

do

Interior western.

Southern

do

do

do

do

r^-15

Southern yellow pine

Do

_._do

_._do

2- 3

3

Do _ do 2

Lodgepole pine_ Poles

-__do

-__do

.-_do

-__do

do

4- 6

Southern yellow pine

Do

Do

Do

Do

2- 3

2

3- 4

1- 2

3

^ The Atlantic region comprises Maine, New Hampshire, Vermont, Massachusetts,

Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Maryland, Delaware, West Virginia, and Virginia. The southern region comprises Kentucky, Tennessee, North Carolina, South Carolina, Georgia, Alabama, Mississippi, Florida,

Arkansas, Louisiana, Oklahoma, and Texas. The interior eastern region comprises

Michigan, Ohio, Indiana, Illinois, Wisconsin, Minnesota, Iowa, Missouri, and the

eastern parts of North Dakota, South Dakota, Nebraska, and Kansas. The interior

western region comprises Idaho, Montana, Wyoming, Nevada, Utah, Colorado, Arizona, New Mexico, and the western parts of North Dakota, South Dakota, Nebraska,

and Kansas. The Pacific region comprises Washington, Oregon, and California.

There is a limit to the amount of water that should be removed from

resistant wood if the best results are to be obtained in treatment. Timbers that have been seasoned too long, or perhaps too rapidly, sometimes develop an increased resistance to penetration at the surface that

is commonly called ''case-hardening.'' ^ The degree of seasoning for

optimum penetration is not definitely known, but results obtained in

the treatment of both round and sawed timbers of various kinds, having

different amounts of moisture, indicate that the moisture content of the

treatable portion of the timber should not be much below the fiber

saturation point. This applies particularly to the treatment of resistant

material.

7 A better term might be ''surface-hardening,'' since case-hardening is the name

applied in kiln-drying or air-drying when internal stresses are set up in which the

inner layers of the wood are in tension and the outer layers are in compression.
38 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

On the other hand, more thorough seasoning is beneficial from thé

standpoint of Hmiting checking after treatment. Fairly permeable wood,

such as the sapwood of the southern pines and various hardwoods, can

usually be penetrated without difficulty when well air-seasoned and also

when the moisture content is considerably above the fiber saturation

point, but this is not usually true of other species when they have resistant sapwood. The surfaces of easily peeled, rapidly seasoned round

material are sometimes made resistant to penetration by the accumulation

of resins or other exudates on the surface during the early part of the

seasoning period. This is not a matter of degree of seasoning, but of

surface coating, and it is mainly confined to softwoods.

Most species have a resistant heartwood, and this may be particularly

difficult to penetrate when the moisture content is too low. It must

be borne in mind that it is not the average moisture content of the

timber but the moisture content of the portion to be penetrated that is

of particular importance. For example, studies of the moisture content

of red oak cross ties seasoned for 143^ and 18 months in Illinois and

Ohio showed an average moisture content of 26 to 28 percent in the

first outer inch, an average around 46 percent in the next inch, and from

57 to 61 percent at the core, while the average for the whole section

was from 39 to 41 percent {50)í{ Much more study is needed to develop

the relations between the character and degree of seasoning and the

treatability of the wood with different preservatives and processes.

If the treated timbers are to be used in dry climates where objectionable

seasoning checks are likely to develop and untreated wood might be

exposed, it is important to make sure that the wood is well seasoned

before treatment.

MECHANICAL PREPARATION«

Incising

It is common practice to incise sawed timbers of resistant woods prior

to treatment, particularly Rocky Mountain and western species, by

passing the timbers through a machine equipped with cutting teeth

projecting from rollers. ^ As the timber passes through the machine,

the teeth are pressed into the wood to form a predetermined pattern of

incisions that penetrate to a specified depth, usually one-half, fiveeighths, or three-fourths of an inch. A machine of different design is

required for incising round timbers.

The benefit of incising is attributable to the fact that longitudinal

penetration is much greater in most species than either radial or tangential

penetration. The incising teeth cut, tear, or break the wood fibers so

that some end-grain wood is exposed to the preservative and penetration

takes place longitudinally in both directions from each incision. There

is also some side penetration, but this is usually slight. In order to get

fairly complete penetration of all the wood to the depth of the incisions,

these must be spaced properly along and across the surface so that the

preservative will meet as it spreads from them. The necessary spacing

s For a more extended discussion of this subject see reference {21).

9 The incising of ties and timbers was patented in the United States in 1918 by

O. P. M. Goss. This patent, No. 1,252,428, was purchased by the Pacific coast woodpreserving companies and was dedicated by them to the free use of the public.

//7 j-Ront^
PRESERVATIVE TREATMENT OF WOOD 39

is worked out empirically. On the other hand, the incisions should not

be more numerous than necessary, so that the strength of the wood will

not be greatly impaired.

Incising is least effective in resistant woods that cannot be penetrated

well longitudinally. In suitable woods, however, it is an effective means

of getting uniform depth of penetration in heartwood or in resistant

sapwood.

Framing, Adzing, and Boring

Whenever practicable, all framing, adzing, and boring of ties, poles,

or structural timbers should be done before treatment. Cutting into

timber after treatment exposes untreated wood that is difficult to protect properly by any surface treatment applied afterwards. The life

of the structure may be seriously affected or maintenance costs increased

when such cutting is permitted. When preframing is done at a properly

equipped plant, there is the additional advantage that it often costs less

than framing on the job. Preframing is widely practiced at the present

time (P, 62),

Machine adzing and boring ties before treatment is common practice

and is highly effective in prolonging their useful life. The adzing provides suitable seats for the tie plates, and boring the spike holes makes

it possible for the preservative to penetrate the wood around the spike

holes where decay often starts.

CONDITIONING PROCESSES

Purpose and Processes Used

Frequently timbers must be treated without waiting for them to airseason. This is necessary in some cases because of unfavorable climatic

conditions that make air-seasoning a decay hazard, or because in other

cases rush orders make it necessary to treat the timbers soon after they

are cut. When green material is to be treated, it is customary to condition the wood by a special heat treatment so that it can be penetrated

with the preservative. The conditioning treatment generally removes

a substantial amount of moisture from the timber and also heats the

wood to a more favorable treating temperature.

Two methods for accomplishing this conditioning have been extensively

used in this country for many years. One is the steaming-and-vacuum

method, which is used mainly for southern yellow pine and to a less extent

for other pines. The second is the boiling-under-vacuum or Boulton

process, which is used mainly with Douglas-fir and, to a limited extent,

for western hemlock, western larch, and some hardwoods. A third

method, called ^Vapor-drying,'' has recently come into limited use,

mainly for southern pine poles and hardwood ties.

Steaniing-and-Vacuum Treatment

In applying the steaming-and-vacuum process the charge of timber

is first steamed for several hours in the treating cylinder, the time depending on the size of the timber and the steam temperature. At the

end of the steaming period, the steam pressure is released and a vacuum

is applied as quickly as possible in order to utilize the maximum amount

of heat available for moisture removal.
40 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

During the st onmine; period, a temperature gradient is set up from

the surface to the interior of the limbers and the temperature at any

niven distance from the surface will depend upon a number of variables

such as steam temperature, length of steaming period, cross-sectional

dimensions, and density of tlu^ wood.

Experiments ha\'e shown (^0) that practically no reduction in the

moisture conti^ni of the wood occurs during the steaming. In fact,

some water is actually added by condensation in the early part of the

steaming period, when the wood is cold. The amount of water absorbed

as a result of condensation will depend to a considerable extent on the

si)ecics, initial moisture content of the wood, initial wood temperature,

st(^am tem])(Tature, amount of sapwood present, resistance of the wood

to water absorption, and cross-sectional dimensions of the timber.

When the steaming is disc(jntinued and a vacuum is applied, the

boiling point is lo\\ered and part of the water in the wood, especially

that near the surface, is forced out mechanically by the steam generated

in the \\ ood cells or is evai)orated during the vacuum period. Since

there is a limit to the amount of heat that can be stored in the wood, there

is also a limit to the amount of water that can be removed by the subsi^cjuent vacuum.

The resistance of the wood to moisture movement, the heat losses

from various causes, and the relatively slow rate at which heat is conducted through wood make it impossible to utilize all of the stored

heat that would theoretically be available in the region from which

moisture is removed. Calculations show that if the resistance of the

wood to moisture movement and all heat losses were neglected, the heat

stored in the w(^od and in the absorbed moisture could not evaporate

even the limited amount of water commonly taken out by the steamingand-vacuum treatment (38). It is therefore evident that a large part

of the water removed from steam-conditioned timbers is forced out by

the steam pressure de\'eloped in the wood when the outside pressure is

released and the vacuum is applied.

Exj^eriments made at the Forest Products Laboratory, in which green

specimens of different species were weighed during the steaming-andA^acuum periods, showed that a large part of the total amount of water

removed came out of the specimens during the first few minutes after

steam pressure was released and the vacuum was applied {^0). A

considerable jmrt of the water removed was actually found to come out

of the wood when the steam pressure was released and before the vacuum

was started. In subsequent experiments on both green and watersoaked specimens the pieces were steamed in a glass cylinder (about 53^

inches in diameter and 46}^ inches long), and it was observed that water

flowed from both the sides and ends of the specimens while the steam

pressure was being released and during the early part of the vacuum

period.

Most of the water remoA'cd by the vacuum after steaming is taken

out during the early part of the vacuum period when the wood is hottest

and evaporation is most rapid. Experiments on green, round southern

yellow pine timbers indicate that when a vacuum period of 5 to 6 hours

is employed, from 50 to 60 percent of the total water removed is taken

out during the first hour of the vacuum period and from 70 to 80 percent

in the first 2 hours. The effectiveness of the vacuum is greatly reduced

when the temperature of the wood surface is lowered below the boiling
PRESERVATIVE TREATMENT OF WOOD 41

point corresponding to the vacuum. From the standpoint of moisture

removal there seems to be Uttle need to continue the vacuum much

longer than 2 hours.

The average amount of water removed by the steaming-and-vacuum

process from green southern pine poles and piling, which naturally have

a high moisture content in the sapwood, is usually not more than 5 to 6

pounds per cubic foot. This.would reduce the average moisture content by about 15 to 20 percent. Practically all this water comes from

the sapwood, which comprises from 85 to 95 percent of the total volume

of the round timber. Much less water is removed from green sawed

southern pine timbers with exposed heartwood faces because the moisture

content of the heartwood is only about 30 to 35 percent as much as the

moisture content of the green sapwood. In addition, the heartwood is

very resistant to moisture movement, which is not a characteristic

property of the sapwood of southern pine.

Seasoned or partially seasoned wood often shows some increase in

moisture content after the steaming-and-vacuum treatment is completed

(20). In such cases the benefit derived from the conditioning treatment

arises from the preheating of the wood that makes it easier to penetrate.

The effect of the wood temperature on treatment is discussed in more

detail on page 79.

When green, round southern pine timbers with deep sapwood are

steam-conditioned, the free water in the cell cavities is often forced into

the inner part of the sapwood zone during the pressure treatment and

this region may show little or no oil penetration. In such cases, when

an increment boring is taken, the water may gush out through the bored

hole because pressure on some of the confined water is released. Prolonged steaming, which is sometimes used in an effort to remove more of

this water when the vacuum is applied, is not effective.

Undue attention is sometimes given to the amount of water left in the

wood after the timber has been treated. The best criterion as to how

well the wood is impregnated is the retention of preservative and the

uniformity and depth of penetration—not the amount of water removed

by the steaming-and-vacuum treatment. The free water found in the

wood after treatment is not necessarily evidence of insufficient steaming,

since experiments show that, except on the surface, even the most effective

steam treatment rarely, if ever, brings the moisture content of green

wood close to the fiber saturation point. Moreover, free water remaining

in the wood after treatment, except when it prevents acceptable penetration, has no influence on the ultimate checking or durability, since

checking does not start until the water in the cell walls is below the fiber

saturation point.

Large timbers, which have a smaller ratio of surface area to volume,

will normally lose less water per unit volume during steaming-andvacuum treatment than timbers of smaller cross-sectional dimensions. In

some cases an attempt is made to judge the amount of water left in the

sapwood of steam-conditioned timbers by squeezing the wood in the core

of an increment boring. This is a very unreliable method of estimatmg

the moisture condition of the timber, because too many variables must

be taken into consideration and the conclusions depend too much on

guessw^ork. The objection that creosoted wood containing free water

will not season after treatment is not valid, because the preservative

will merely retard, not prevent, the loss of moisture. In one respect
42 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

this is an advantage, since the slower rate of seasoning helps to reduce

the amount of checking that normally occurs in more rapid seasoning.

The practice of keeping the steam coils heated during the vacuum

period does not aid appreciably in reducing the moisture content of the

wood because heat does not circulate freely in a vacuum. While some

radiated heat may reach timber surfaces facing the hot metal surfaces,

most of the timbers in a cylinder charge will be heated very little by

radiation.

Experiments show that alternate steaming-and-vacuum treatments

are considerably more effective in reducing the moisture content of

wood than are a continuous steaming and a final vacuum period of the

same total duration (28). In such treatments, however, the initial

steaming period must be sufficient to allow a reasonable time to heat

the wood, and this period will depend upon the size of the timber and

the steam temperature. Since the rate of temperature change decreases

rapidly as the distance from the surface increases, as may be noted from

the time-temperature curves, the use of alternate steaming and vacuum

treatments should be more effective than a long-continued steaming

followed by vacuum.

It should be understood that the steaming-and-vacuum process will

not reduce the moisture content of green material sufficiently if particularly heavy retentions of preservative are needed. This is an important consideration in the case of ]3Íles and other timbers that will be

exposed to marine-borer attack, since timbers used in marine structures

should be treated with maximum retentions. For example, even if the

timbers were nearly all sapwood, calculations show that it is not possible

to get retentions of 18 pounds or more of creosote per cubic foot in green

southern pine piles conditioned by the steaming process. Such material

should be well air-seasoned before treatment or should be seasoned by

some other means that will remove enough water to allow retentions that

meet the service requirements.

Steam pressure and steaming period.—The maximum steam temperature

and steaming period that may be used with safety cannot be stated with

technical accuracy because there is insufficient knowledge of the effect

of steaming on the strength and other properties of wood. Undoubtedly

the effects vary considerably with the size, moisture content, and species

of the wood. In commercial specifications, however, certain limits

have been set that are based on observation and experience in the treatment of different species and in the uses and serviceability of the wood.

Temperatures ordinarily used in steaming wood are, of course, much

lower than those that would disintegrate or char wood in the heating

periods employed, but they are high enough to injure the wood seriously

unless care is exercised in selecting them and in determining the length of

the steaming period. The common practice of steaming southern pine

timbers at 20 pounds' gage pressure as a maximum (approximately 259°

F.) may be considered satisfactory until further experiments show otherwise. The use of more severe steaming conditions, such as 30 pounds'

gage pressure (about 274*^ F.) or more, or the application of particularly

long steaming periods does not appear to be justified, and the danger of

injury is considerably increased thereby. Steaming temperatures of

259° F. are common also with jack, red, and lodgepole pine, but with

most other species 240° is usually the maximum permitted.
PRESERVATIVE TREATMENT OF WOOD 43

Although there is a definite steam temperature for each steam pressure,

a thermometer as well as a pressure gage should be used in determining

the temperature of the steam within the heating cylinder, because

pockets of air are frequently left in the cylinder after the steam is admitted. Where air pockets exist, the required pressures may be indicated

on the gage when the steam temperature is actually lower than that

which should be obtained for the specified pressure. The use of both a

thermometer and pressure gage also serve as a check in case one instrument is in error.

Table 4 summarizes the steaming and vacuum conditions employed

at various plants treating green southern pine timbers in 1950. It may

be noted that all but one plant steamed at 20 pounds per square inch.

TABLE 4.—Steam-and-vacuiim treatments used at various plants treating

green southern pine timbersj 1950

Plant

No. Material Steam

pressure

Steaming

period

Vacuum

period

after

steaming

9-.

10.

11_

12_

13_

14_

15_

16_

17_

Poles

Poles

Lumber and timber

'Poles

Piling

Lumber and

Poles

Lumber and

Posts

Piling

^Poles

__.do.___

Pounds

square

timber,

timber.

Piling

Posts

Poles

PiUng

Lumber and timber.

Posts

Poles

IPiUng

Poles

..._do

Poles...

Posts..-

Poles...

..do.

Piling

Poles

Lumber and timber

Poles

Cross arms

Lumber and

Poles

Lumber and

Poles

timber,

timber.

per

inch

20

20

20

20

20

20

20

30

30

30

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

Hours

10-12

8-16

8

12-18

20

10

10-14

12-14

8

16

12-15

10-15

12-16

6- 7

8-14

12-15

5- 6

6

10-14

12-14

8-12

12-13

9-10

6

5-12

12-15

15-18

10-12

8-10

10-12

8-10

8-10

8-10

8- 9

10-14

Hours

2

2

2

2

2

2

2

1

1

3

2^2-3

2^2-3

2^2-3

21..-3

-21 2

-21^.

-21 9

2

2

-21.2

-2^2

2

3

2

2

2}4

2^2

21.>

2^2

2

2

2

2

2
44 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

TABLE 4.—Stcam-and-vacuum treatments used at various plants treating

grean southern pine timbers, 1950—Continued

Plant

No. Material Steam

pressure

Steaming

period

Vacuum

period

after

steaming

18_

19.

20.

21.

22

23'

24 _

25.

26.

27.

Poles

Lumber and timber.

Polos

Cross arms

Lumber and timber.

Piling

'Poles

Piling

I Cross arms

Poles

[Sawed timber

Poles

.do

Poles

I Sawed timber.

Piling

Poles

Posts

[Sawed timber_

Poles

__do

Pounds per

square inch

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

20

Hours

8-10

8-10

8-12

10-12

11-12

14

10-12

14

8

8-12

8-10

8-12

10-12

8-15

7-10

13

10-14

7

10-12

8-14

8-12

Hours

2

2

2

2

3

1

2H

2

2

3

21

21

21

21

21

21

3

2

The plant in which there was an exception to this rule used a steam

pressure of 30 pounds, which is contrary to standard specifications and is

considered poor practice.

Advantages and disadvantages of steaming.—Among the principal advantages of steaming are: Steam heats faster than any of the other

heating mediums; it is easily applied and requires no special equipment,

either on or attached to the treating cylinder; the temperature can be

easily controlled, both in the rate at which it is raised and after the

desired temperature is reached; and the wood is left clean after steaming

is completed.

The principal disadvantages are: No moisture is evaporated during

the time steam pressure is applied, hence the wood surfaces are exposed

to the actual steam temperature during the entire steaming period;

only a limited amount of moisture can be removed during the subsequent

vacuum, even when the w^ood is heated throughout to the temperature

of the steam; and it is generally necessary to use considerably higher

temperatures than would be needed, for example, in the use of the

Boulton process.

The Boulton Process

The original process for seasoning or conditioning timber by boiling

in creosote under vacuum was patented by S. B. Boulton in England in

1879 and in the United States in ISSl {30). The Boulton process as

applied at the present time is essentially as follows: The treating
PRESERVATIVE TREATMENT OF WOOD 45

cylinder is filled with hot preservative oil so that all timbers are covered,

although some unfilled space may be left above the oil surface. The

preservative is then kept heated while a vacuum is applied. In this

case the oil serves to keep the wood hot while the vacuum lowers the

boiling point of the water in the wood and causes part of the water to

evaporate. The evaporated moisture and some of the accompanying

vapors from the oil pass through a condenser, and the rate of accumulation of condensed moisture is a measure of the progress of the conditioning treatment.

In conditioning sawed timbers of Douglas-fir by the Boulton process,

the temperature is customarily maintained at 180° to 200° F., although

specifications permit 210°. For poles and piling, temperatures of about

200° to 220° are generally employed. A low vacuum is often used at the

start and gradually increased as the moisture evaporation progresses.

Some plants, however, apply the vacuum as rapidly as possible, while

others first heat the wood in the preservative for a few hours at atmospheric pressure before starting the vacuum.

Since the Boulton process requires time at the start to heat the wood

so that the water will evaporate from it, the condensate comes off somewhat slowly at first, gradually increases to a maximum, and then gradually

decreases as the heating and vacuum are continued. Increasing the

vacuum too rapidly may cause oil to surge over into the vacuum system

in plants not specially designed to prevent it. In plants treating Douglasfir, the maximum vacuum towards the end of the conditioning period

commonly reaches 24 inches of mercury or more. The condensate can

be weighed or measured to determine how much water has been removed

from the charge, and the volatile oils evaporated from the creosote and

condensed with the water may be separated from the water and returned

to the preservative tank.

When an empty-cell treatment is specified, the cylinder is emptied of

preservative after the conditioning period and air at atmospheric pressure or higher is admitted as desired. The preservative treatment is

then applied as for air-seasoned material. In treating by the full-cell

process, the cylinder is filled with preservative after the conditioning is

completed and i^ressure is applied at once. Some preservative is absorbed during the boiling-under-vacuum period, depending on the kind

of timber treated, amount of sapwood and heartwood, and other variables

Although the Boulton process or a modification of it has been widely

and successfully employed on the Pacific coast for a long time, it is only

within fairly recent years that the process has been used in other parts

of the United States for species other than Douglas-fir. Unseasoned

red oak, which is severely checked by the steam-and-vacuum treatment,

has shown but little checking when conditioned by the Boulton method,

and the in-service results reported for the treated wood appear to be

satisfactory. Green beech, gum, and southern yellow pine have also

been conditioned successfully by the Boulton process.

Conditions ctnploijcd for the Boulton /^mv.s.s.—Table 5 giv(\s data on the

temperature and conditioning periods employed in 1940-50 at various

commercial plants using the Boulton process, mostly in the treatment of

coast-type Dou<;his-fir. Considerable variation in the treating conditions

is evident, due in i)art, perhaps, to variations in local conditions or in

moisture content of timbers, and to differences of opinion as to the best

conditions to use.

207777°—53 34
46 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

TABLE 5.—Coiiditions employed at various plants using the Boulton process^

chiefly for coast-type Douglas-fir timbers^ 194-9-50

Plant

No.

Kind of material

conditioned ^

Boiling-imder-vacuum

conditions

Boiling

period

Temperature of

preservative

during boiling

period

1

3_

4.

5.

6_

7_

8-

9_

10_

11.

12.

Poles and land piling

Poles

Piling

Ties

Lumber and timber..

Poles

Ties

Poles and piling

Poles

Piling

Ties

Lumber and timber..

Poles

Ties

Piling

Ties

Lumber and timber..

Poles

Piling

Piling

Ties and lumber

Ties

Gum ties

Southern pine ties...

Ties

Lodgepole pine ties..

Hours

30-36

30-40

36-45

14-15

15-18

20-25

8-12

20-25

18-19

40

14^16

12-14

18-36

15

35-40

12-14

12-16

20-30

20-40

20-40

8-15

10-12

16-18

8

8

9-10

215

220

220

190

190

190-200

190-200

210-220

200

210

190

190

200-220

200

220

195

190-195

220

220

190-200

190-200

185-190

210

190-200

210

200-205

1 All coast-type Douglas-fir except as otherwise indicated.

Amount of moisture removed by the Boulton process.—Table 6 gives the

average amount of moisture removed from timbers conditioned at some

commercial plants using the Boulton process.

Treating-plant operators sometimes overlook the fact that the high

moisture content of the sapv^ood that may be present on some parts of

sawed Douglas-fir timbers, and evaporation of water from the end surfaces,

may be responsible for most of the water removed in the Boulton treatment, and that the heartwood as a whole loses only a small amount of

moisture. Since the moisture content of green Douglas-fir heartwood

usually averages less than 40 percent, the green heartwood has about 55

percent of air space, so that no additional seasoning is necessary to

obtain the preservative retentions usually specified. Even if the moisture

content were reduced to 15 percent, the air space would not be greatly

increased. At 15 percent moisture content the air space in heartwood

would average about 60 percent in comparison with 55 percent air

space in green wood with a moisture content of about 35 percent.
PRESERVATIVE TREATMENT OF WOOD 47

TABLE 6.—Amounts of water collected from green timber of various species

conditioned by the Boulton process in commercial treatments

Species Charges

treated Kind of material

Average

temperature of

preservative dui'-

ing conditioning

Average

time in

hot bath

before

starting

vacuum

Average

length

of

boiling

period

length

of

Average

amount

water

collected

during

boiling

period

Red oak__

Beech

Southern

pine.

Douglasfir

(coast

type).

Do____

Number

11

7

7

25

52

Ties and bridge

timbers.

Switch ties

Ties and timbers. _ _

Sawed timbers

PiUng.

ISO

180

180

180-190

n

200-200

11ours

4

6

Hours

24.5

13.5

11.1

10.8

43.0

Pounds

per

cubic

foot

8.7

5.0

4.6

1.9

13.4

There is apparently a high resistance to the movement of moisture in

the heartwood of Douglas-fir, and it is probable that much of the water

removed during the Boulton treatment of this species is from the wood

within the first inch or two from the surface. Some plants using the

Boulton process for Douglas-fir timbers continue the boiling period

until the average moisture content within 1 to 13^ inches from the

surface has been reduced below the fiber saturation point. Where the

timbers are short and therefore have a considerable amount of endsurface area exposed, more water will be removed than when the timbers

are long and moisture movement is largely from the sid(i surfaces.

More moisture can be removed in a given time from incised heartwood

timbers of coast Douglas-fir than from unincised material. Some plant

operators say that they boil sawed timbers under vacuum for sufficient

time to reduce the average moisture content of the incised wood well

below the fiber saturation point. Unless the timbers are of fairly small

cross section, however, little, if any, seasoning can be expected below

the depth of the incisions.

Round Douglas-fir piles and poles contain more moisture than sawed

material because they have more sapwood and commonly are also kept

in the water in log booms until just before treatment. Their average

moisture content after treatment is also high. Table 6 shows that about

13 pounds of water per cubic foot were removed from the round timbers

in comparison with about 2 pounds from tlu^ sawed material. This

difference is to be expected, considering the appreciably higher moisture

content of the sapwood, higher oil temj^^eratures, the longer seasoning

periods employed for the piles, and the smaller resistance of the sapwood

to moisture movement.

^ ;ZO0 -A/o
48 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

TABLE 7.—Amounts of water collected in Boulton treatment of green southern yellow pine pole specimens 10 feet long

Specimens

in group

(number)

Average

diameter of

specimens ^

Temperature

of creosote

during

boiling

period 2

Length of

boiling

period

Average

amount of

water

collected

during

boiling

period 3

3

Inches

8.2

9.8

9.0

10.8

11.2

10.1

9.8

10.3

11.0

11.3

180

200

200

200

200

210

220

220

220

235

Hours i(

Oi.O10.0

11.0

12.0

13.0

12.0

6.6

11.0

13.0

13.0

Pounds per

cubic foot

9.2

6 9.3

6 9.9

3 11.8

12 10.5

3 9.0

6 6.5

3 8.3

6 13.3

3 16.2

1 Average depth of sapwood about 3.25 inches.

2 Temperature of preservative maintained constant during entire boiling period, except that in some treatments from 15 to 20 minutes were required to reach the desired temperature.

3 Average initial moisture content of sapwood varied from about 90 to 100 percent.

Table 7 is a summary of data obtained in Boulton treatments on

specimens of green southern yellow pine poles. These experiments were

made at the Forest Products Laboratory to study the feasibility of

using the Boulton process instead of the steaming-and-vacuum method

for conditioning green southern yellow pine poles preparatory to treatment (36),

Although appreciable quantities of water may be removed in the

Boulton process, the wood, as in the steaming-and-vacuum process, may

be but partially seasoned and may still have a high average moisture

content at the end of the conditioning treatment. Both the heartwood

and sapwood of red oak have a high moisture content when green, and in

some individual charges treated after conditioning by the Boulton process

as much as 9 or 10 pounds of water have been removed per cubic foot of

wood. Even then, however, the wood was left very wet. To illustrate,

by taking the average moisture content of green red oak heartwood as

85 percent and the average specific gravity (based on weight when ovendry and volume when green) as 0.56, the original weight of water is

found from figure 9, p. 29, to be nearly 30 pounds per cubic foot of wood.

By assuming that 10 pounds of water per cubic foot are removed by the

Boulton treatment, figure 9 shows that the moisture content of the

wood still remains more than 56 percent, which is much above the fiber

saturation point. Nevertheless, creosote can be made to penetrate it

satisfactorily at this high average moisture content.

Green Douglas-fir heartwood has an average moisture content of

^ /A o
PRESERVATIVE TREATMENT OF WOOD 49

only about 36 percent, which during the Boulton treatment may be

reduced appreciably below the fiber saturation point in the outer inch

or so. However, the interior of all but thin material will have little

moisture change, and the average moisture content may still be considerably above the fiber saturation point. By using the specific gravity of

0.45 given in table 2, p. 23, it is found from the data given in figure 9

that with 36 percent moisture content the heartwood contains more

than 10 pounds of water per cubic foot. If the water could be removed

as readily and in such large quantities as from red oak or southern pine

sapwood, the Douglas-fir heartwood could be made very dry by the

Boulton process, but this is not the case.

Experiments made at the Forest Products Laboratory on green Douglasfir heartwood specimens, which were boiled under vacuum at 200° F.

for periods of 10 to 20 hours, showed average moisture reductions varying

from 8 to 15 percent (37), The final average moisture content, however,

was usually at or above the fiber saturation point even in specimens as

small as 4 by 4 inches in cross section boiled under vacuum for more

than 20 hours. The specimens used in these experiments ranged from

about 4 by 4 inches to about 6 by 12 inches in cross section and were

from 4 to 7 feet long. Evaporation from the ends was prevented by steel

plates with gaskets, which were bolted over the end faces.

Similar experiments were made with green, round Douglas-fir specimens boiled under vacuum for 21 to 23 hours with the preservative held

at 190° F. (37). Some of these specimens contained sapwood that

averaged from ^ to 1^4 inches in thickness for the different pieces.

Other specimens were turned in a lathe to remove all the sapwood, so

that moisture movement through the heartwood was in a radial direction.

These various specimens averaged about 10 inches in diameter. The

average moisture content of the sapwood before boiling under vacuum

was about 100 percent, and that of the heartwood about 36 percent.

When the boihng period w^as completed, a disk was cut from the midsection of each specimen and moisture samples were taken of the sapwood

alone and of the heartwood at intervals of 1 to Ij^ inches from near the

surface to the center.

The specimens were removed as soon as the boiling period was completed, and only very slight absorption of the preservative occurred even

in the sapwood. Moisture determinations were made of the unpenetrated

wood only. The final average moisture content of the sapwood varied

from about 18 to 42 percent, depending on the original depth. When the

average moisture content of the sapwood was brought below the fiber

saturation point, the average moisture content of the first inch of heartwood directly under the sapwood was usually 5 to 6 percent higher than

that of the sapwood. Otherwise there was very little change in the

moisture content of the heartwood covered with sapwood.

With the specimens that were turned on the lathe to remove the sapwood, the average moisture content after the Boulton treatment was

about 26 percent for the first 13^ inches from the surface, with practically

no change beyond that distance. These specimens were unincised.

Incising would probably have increased the moisture evaporation appreciably.

In these experiments a vacuum of 25 to 26 inches was usually reached

within 2 to 3 hours after the preservative was admitted to the cylinder.
50 AGKICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Temperature Changes in Wood When Boiled Under Vacuum

and When Heated in Creosote at Atmospheric Pressure

A study of the rate of temperature change in green Douglas-fir heartwood timbers boiled in creosote under vacuum at temperatures of 190°

to 210° F. showed that the rate of temperature change was, in general,

about the same as when the wood was heated without vacuum until

within 5° or 10° of the creosote temperature. For example, with the

creosote temperature at 200° and the maximum vacuum about 25 to 26

inches of mercury, the rate of temperature change in these specimens

was about the same as without vacuum until a temperature of 190° to

195° was reached. The temperature would then remain fairly constant,

as long as the vacuum was not broken, until the rate of evaporation

became sufficiently slow to permit an increase in the wood temperature.

In these tests the temperature of the creosote was constant during the

boiling period, and the maximum vacuum of about 26 inches was reached

within an hour.

Experiments on green, round coast-type Douglas-fir specimens boiled

under vacuum showed that when the timber contained a considerable

amount of sapwood and a high moisture content the rate of temperature

change was about the same as when the wood was heated at atmospheric

pressure, until a temperature about 20° to 25° F. lower than the creosote

temperature was reached. Beyond this limit the rate of temperature

change was usually much slower than when the wood was heated at

atmospheric pressure, because further temperature increase was retarded

by the evaporation of moisture. The diameter of these specimens

varied from about 9 to 12 inches, and the sapwood depth varied from

^ to IH inches. The moisture content of the sapwood averaged about

100 percent for the different specimens, and that of the heartwood about

36 percent. In these experiments on round specimens the preservative

temperature was kept constant during the boiling period, and the maximum vacuum of 25 to 26 inches was usually reached within 2 or 3 hours

after the test was started. The total boiling period was from 21 to 23

hours.

In green, round southern pine timbers boiled under vacuum the effect

of vacuum on the rate of temperature change was more pronounced than

that noted for round Douglas-fir timbers, because of the lower resistance

and greater depth of sapwood in the pine and the consequently higher

rate of evaporation. The rate of temperature change in the round pine

specimens boiled under vacuum was about the same as when heated

without vacuum until the wood reached a temperature about 35° or 40° F.

below the creosote temperature. The wood temperature then changed

but little until a considerable amount of water had been removed from

the sapwood.

The increase in temperature above the boihng point corresponding

to the vacuum conditions used in the Boulton process is largely because

of the resistance of the wood to air and moisture movement. In other

words, the vacuum within the timber is less than that surrounding it and

may decrease rapidly from the surface to the interior. As a matter of

fact, pressure measurements in heartwood timbers have shown that an

appreciable pressure can be built up and remain for hours in the interior

of the heated wood with a high vacuum surrounding it. As might be

expected, the vacuum will affect the rate of temperature change within
PRESERVATIVE TREATMENT OF WOOD 51

the timber to a greater extent in wood that has a large proportion of

sapwood than it will in wood that is largely heartwood and therefore

very resistant to the movement of liquids and vapors. No temperature

measurements were made in red oak while boiled under vacuum, but the

effect of the vacuum on temperature changes would undoubtedly be very

pronounced in this species, on account of its open-porous structure.

The effect of the resistance of the wood on interior temperatures has

been discussed. A certain increase in wood temperature above the

boiling point corresponding to the vacuum is also necessary to overcome the pressure due to the weight of the preservative above the timber.

When the wood begins to season below the fiber saturation point in the

surface portion, the resistance to moisture movement increases and a

correspondingly higher wood temperature is necessary to overcome this

resistance. As the wood dries below the fiber saturation point, the rate

of temperature rise also tends to decrease, since seasoned wood heats

more slowly than green material. If the boiling period is continued a

sufficient length of time, the temperature throughout the timber will

eventually become about the same as that of the heating medium.

Except in the case of timbers of small cross-sectional dimensions, however,

the wood is rarely heated throughout to the temperature of the heating

medium because of the extremely long time that would be required and

because little advantage would be accomplished by heating the interior

to the maximum temperature at the surface.

In the case of round timbers, however, such as poles and piling that

have been heated under vacuum until the rate of water evaporation is

fairly rapid, the rate of temperature change will be considerably slower

that when the wood is heated at atmospheric pressure. Factors affecting

the results to a greater or less extent will be diameter of timber, moisture

content of sapwood, depth of sapwood, temperature of the preservative,

rate of applying vacuum, and maximum vacuum applied. For general

conditions, however, it may be assumed that in boiling under vacuum

the rate of temperature change at any particular point in a round timber

will be about the same, until a temperature about 25° F. lower than the

preservative temperature is reached, as would be obtained if no vacuum

were used. The temperature would then probably change very slowly

or remain fairly constant until seasoning had progressed sufficiently to

require a higher temperature for further seasoning.

If the timbers had a large amount of sapwood so that the preservative

could penetrate a considerable distance from the surface, the temperature

would, of course, be higher than when no penetration of preservative

occurs, since more or less heat would be carried in by the preservative.

In the case of heartwood timbers and those with resistant sapwood,

penetration is usually very limited and the heating effect of the creosote

that penetrates the timber does not have such an important influence,

especially when the cross-sectional dimensions are fairly large.

The temperature changes in seasoned wood heated in oil would be little

influenced by vacuum because heat consumption due to moisture evaporation would be slight.

Advantages and Disadvantages of the Boulton Process

The chief advantage of the Boulton process, in comparison with the

steaming-and-vacuum method for preparing timber for treatment, lies m
52 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

the milder temperatures that can be employed to make possible good

treatment with mininmm effect on the strength and on the physical

condition of the wood. This effect is the determining factor in the

choice of a conditioning process for woods that are sensitive to high

temperatures.

Other advantages are: The Boulton treatment never increases the

moisture content of the wood or of the oil, and a greater moisture reduction can be obtained than is possible with the steaming process.

With the Boulton process it is possible to reduce the moisture content of

green round timbers below the fiber saturation point in the sapwood.

Because of the higher resistance of the heartwood of most species, even

long boiling-under-vacuum periods will not often season the wood in

heartwood material more than a short distance from the surface.

The Boulton process is sometimes employed for seasoned or partially

seasoned wood; but, in general, there is little advantage to be gained by

using it under such conditicjns, except when it is desired to remove air

from the w(jod so that higher retentions can be obtained in material

treated by the full-cell process. Such removal is an important consideration in treating timbers for structures that will be exposed to marineborer attack.

Because of the important advantages of this conditioning process, it

has found widespread use in the past, and at the present time it is considered the standard method for conditioning green Douglas-fir timbers

at the western treating plants.

The chief disadvantages of the Boulton process are that it is suitable

for oils only, it often costs more than air-seasoning, it heats the wood

more slowly than steaming or boiling at high temperatures without

vacuum, and it usually requires a considerably longer time than the

steaming-and-vacuum process. Nevertheless, it is much more effective

than the latter process in removing moisture from wood.

Vapor-Drying Process

Recently a conditioning treatment called the ^Vapor-drying process^'

has been developed and patented (patent No. 2,273,039, Feb. 17, 1942,

and others), which utilizes the heat of vapors from boiling organic liquids

at or below atmospheric pressure. The drying agents generally used are

coal-tar or petroleum fractions with initial boiling points in the range of

212° to 400° F. This process has been employed largely for conditioning

green cross ties and poles prior to pressure treatment {16, 17). Evaporation of water from the green wood during the heating period is said to

keep its temperature down sufficiently to prevent detrimental effects

from the vapor temperatures employed. Up to the present time green

red oak ties and green southern pine poles have made up a large part of

the tie and pole material conditioned by this process.

The following advantages are claimed for the process: The time required to condition the timbers for treatment is relatively short because

the vapors can be maintained at a fairly high temperature while the

water is being removed; deep checking is said to be considerably less

than in air-seasoned wood, especially in species like red oak that have a

tendency to develop large checks; it is also said to give good results

both in removing moisture and in heating the wood to a favorable treating temperature; and it is finding increasing commercial use.
PRESERVATIVE TREATMENT OF WOOD 53

RATE OF TEMPERATURE CHANGE IN WOOD AND

VARIOUS FACTORS AFFECTING THE RESULTS

The temperature of the wood is one of the most important factors

affecting treatment and is also an essential consideration when conditioning material for treatment. It is therefore desirable, when round

or sawed timbers are being heated in any given heating medium, to

know the approximate rate of temperature change ^^ that will occur at

different distances from the surface. Such information is useful in

selecting heating periods for timbers of different dimensions when the

wood is conditioned by the steaming-and-vacuum or by the Boulton

process; in finding the temperature that can be expected at any point in

the timber when the wood is heated in the preservative ; for determining

the time at which heating should be discontinued because of the slower

rate at which temperature changes take place as the temperature of the

heating medium is approached; for comparing the advantage of using

different heating-medium temperatures; and in determining the time

required to sterilize the timber.

A large amount of experimental work has been done at the Forest

Products Laboratory to determine the rate of temperature change in both

round and sawed timbers and also to study the factors that affect the

rate of heat transfer under different heating conditions. These experiments were made by using steam, water, preservative oils, hot plates,

and air at different humidities, as heating mediums. Formulas based on

the results of these experiments were developed for computing the

approximate temperatures at any particular point in a timber. The

methods of conducting the tests, the development of formulas employed,

and the data obtained are discussed in other publications (18, 34, 35,

37, 38, 40, 42, 43, 44. 45, 47, 48, 64, 65).

The following were found to be the principal factors affecting the rate

of temperature change, apart from the timber dimensions: (1) The

heating medium; (2) the moisture content of the wood; (3) the

direction in which heat movement takes place with respect to the grain

of the wood, that is, whether radially (at right angles to the annual

rings), tangentially (parallel to the annual rings), or longitudinally

(along the fibers) ; and (4) the density or specific gravity.

HEATING MEDIUM

Results of these experiments showed that steam heats wood faster

than liquids, liquids heat it faster than hot plates, and hot plates heat

it faster than dry air, if the heating temperature and heating period are

the same in each case. Water heated wood faster than preservative

oils, but not so fast as steam. In general, the rate of heating in water

was about 5 to 10 percent slower than in steam. Although the rate of

heating was slowest in dry air, the rate of heating in air, of course, mcreased as the humidity was increased.

Variations in the rate at which wood heats at the same temperature

10 Although wood is not a homogeneous, isotropic substance, it has sufficient uniformity of structure to permit the appUcati(jn of the mathematical theoi}^ ot heat

conduction in soUds to determine the approximate rate of tempcratui'r change in

timbers of different sizes and shapes. The resuhs are sufficiently accurate tor practical

purposes.
54 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

in different mediums are caused by variables such as type of surface

contact, which affects the surface coefficient of heat transfer; the rate

of circulation of the heating medium, and some limited surface penetration of the heating medium when liquids and gases are used; specific

heat of the heating substance ; and heat of vaporization, as in the case of

steam. Timbers that have checks extending to various depths as a

result of seasoning and sapwood that is well seasoned and easily penetrated, may absorb some of the heating medium during the early part

of the heating period. This may help hasten the rate of temperature

change to a limited extent. Reliance, however, should not be placed on

such factors, especially if the wood is not particularly permeable, if it

has a fairly high moisture content, or if it does not show a considerable

amount of deep checking over a large part of the surface.

The use of a vacuum during the heating period will naturally affect

the rate of heating when moisture is evaporated, as mentioned eariier

in the discussion of the Boulton process.

MOISTURE CONTENT

Experiments showed that seasoned wood heated somewhat more

slowly than green wood in steam and in liquids. The rate of heating

increased to some extent with increasing moisture content up to the

fiber saturation point of the wood, beyond which no significant change

was found. Green material will therefore heat at about the same rate,

regardless of differences in the moisture content, above the fiber saturation point. In making calculations of the rate of temperature change, it

should be sufficient for practical purposes to assume that timbers with

an average moisture content at or below 20 percent are seasoned and that

those in which most of the wood has a moisture content above 20 percent

are green. Seasoned material will usually heat about 10 to 20 percent

slower than green wood under the same conditions, depending on the

moisture content and on the moisture distribution from the surface to

the interior of the seasoned timbers.

RATE OF TEMPERATURE CHANGE IN TRANSVERSE

AND LONGITUDINAL DIRECTIONS

For normal wood there is apparently no significant difference in the

rate of heating in the radial and tangential directions, but the rate is

about 2)4: to 2% times faster in the longitudinal direction (along the

fibers) than in the transverse direction. Longitudinal heating, however,

does not need to be taken into consideration unless the timbers are

exceptionally short and the cross section is large in proportion to the

length. End heating will have no effect or will be a negligible factor, in

heating the wood at the midportion of round timbers if the length is five

to six times the diameter or more, even if the heating period is sufficiently

long to heat all of the wood to the temperature of the heating medium.

Since most round timbers are only partially heated to the temperature of

the heating medium in ordinary treating operations, lengths considerably

less than six times the diameter can be considered as heated in the radial

direction only. ^^

^1 A discussion of temperature changes in short-length round timbers, and curves

showing the temperature distribution from the end surfaces to the midlength, are

given in a pubUcation on this subject (^7).
PRESERVATIVE TREATMENT OF WOOD 55

End heating will have no effect on sawed timbers the surfaces of which

are at the same temperature, and may be disregarded as a factor affecting

the temperature toward the midsection, if the length of the sawed timber

is about 10 times the shortest cross-sectional dimension.

EFFECT OF SPECIFIC GRAVITY ON THE RATE OF TEMPERATURE CHANGE IN WOOD

The rate of temperature change in any solid depends upon the diffusivity, which, like conductivity, may be considered a constant over normal

ranges of temperature. Diffusivity is a measure of the rate of temperature

change, and it may be defined as the change in temperature produced

in a unit volume of the substance by the quantity of heat that passes

in unit time through unit area of a layer of unit thickness and having

unit difference of temperature between the faces.

The equation showing the relation of diffusivity, conductivity, specific

heat, and specific gravity is given in the appendix. (See list of symbols

used in formulas, page 140).

In the case of wood, the product of specific heat and density that

appears in the formula represents the heat capacity of both the wood

substance and the water in the wood.

Although the heat conductivity of wood increases with increase in

specific gravity, the diffusivity decreases with increase in specific gravity.

That is, the lighter woods will reach a given temperature more rapidly

than the heavier woods, although the latter are better heat conductors

In determining the required heating period to be used in conditioning

wood for treatment, the following items should be taken into consideration.

EFFECT OF SNOW OR ICE ON RATE OF HEATING

When wood is covered with ice or snow or when moisture is frozen

in the timber, a somewhat longer heating period will be required to melt

the ice and heat the wood than when the water is not frozen. In heating

round or sawed timber in steam, of which there is an ample supply and

good circulation, not more than about 20 to 30 minutes' increase in the

normal heating period would probably be required to provide for the

extra time needed to melt the ice or snow. The initial wood temperature

would, of course, be taken as the temperature under the frozen conditions,

which might be well below 32° F. in winter.

EFFECT OF VACUUM ON RATE OF HEATING

Since the evaporation of water during the heating period will tend to

lower the wood temperature, timbers heated under vacuum will heat

more slowly after the temperature has been raised somewhat above

the boiling point corresponding to the vacuum applied. This is discussed under the subject heading Temperature Changes in Wood AVhen

Boiled under Vacuum and When Heated in Creosote at Atmospheric

Pressure, p. 50.
56 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

DIFFERENCE IN TEMPERATURES OBTAINED AT TOPS AND

BUTTS OF LONG POLES AND PILES

Long poles and piles are often much smaller at the top than at the

butt because of the natural taper of the timber. For this reason the

top portion will be heated to a higher temperature than the butt when

the heating period is completed. This fact should be considered in

selecting the heating temperature and heating period, especially when

steaming, to avoid conditions that may subject the top region to temperatures that will unnecessarily impair the strength properties. On the

other hand, it is particularly important that the heating period be

sufficient to bring the butt part to a favorable treating temperature.

COMPENSATING FOR THE TIME NEEDED TO BRING THE

HEATING MEDIUM UP TO THE REQUIRED TEMPERATURE

Usually it is not practicable to maintain a constant temperature from

the time heat is first applied; in the early part of the heating period the

temperature of the timber and that of the treating cylinder are generally

close to the atmospheric temperature and a certain amount of time is

needed for the heating medium to reach the desired maximum temperature. Some plant operators also feel that it is desirable to raise the

temperature gradually, and very commonly from 1 to 2 hours is taken

to reach the maximum.

It can generally be assumed that the rate of temperature rise of the

heating medium is reasonably uniform. Therefore, it is sufficient to

count the time required to reach the maximum temperature as the time

equivalent to heating at the maximum temperature for half the time

needed to reach the maximum. If, for example, 2 hours were taken to

reach the maximum, this would count as heating for 1 hour at the maximum temperature in order to adjust for the lower temperature at the

start.

If the temperature conditions are not reasonably uniform on all

surfaces of the timbers, or if the heating-medium temperature is not

fairly constant after the maximum temperature is obtained, the length

of the heating period should be increased to compensate for the temperature difference. The plant operator must depend on his judgment in

estimating the additional time needed to compensate for the variable

conditions.

TEMPERATURE COMPUTATIONS

In addition to the effect of the variables mentioned, the diffusivity

determined experimentally was found also to be influenced by the heating

medium and other variables; hence the diffusivity factors thus determined may be considered as ^^apparent'' rather than actual since the

influence of these variables is included. In the following discussion,

however, they will be called diffusivity factors for the sake of simplicity.

Figure 12 shows diffusivity factors obtained from green and from

seasoned wood of different specific gravity when heated in steam and

in creosote. Average diffusivity factors for green and for seasoned

wood of different species heated in creosote or steam are shown in table

22, p. 139.

Even when the diffusivity factor is known, temperature computations

require a large amount of work because of the involved equations that

are unsuitable for convenient numerical computations.
PRESERVATIVE TREATMENT OF WOOD 57

0.00038 N ^•\

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Â.-CREEN WOOD HEATED IN COAL-TAR CREOSOTE

m-AIR SEASONED mOD HEATED IN COAL-TAR CREOSOTE

NUMBERS OPPOSITE POINTS INDICATE NUMBER OF

SPECIMENS IN AVERAGE.

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If, however, computations are made for any assumed timber dimensions, diffusivity, initial wood temperature, and heating-medium temperature, there are simple time-temperature relations that make it

possible to use computed data when these data are tabulated or plotted

in the form of curves. Such computed data can be used for finding the

temperature to be expected at a given point in a timber when any or all

of the three variables, diffusivity, initial wood temperature, and heatingmedium temperature, are different from those used m making the original
58 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

calculations. Figures 13 to 22 show the computed rate of temperature

change at different distances from the surface of timbers of various crosssectional dimensions. The method of using the plotted data for any

particular type of timber heated under any given temperature condition

is explained in the appendix, pp. 138-151. Examples are also given

in the appendix showing the procedure to be followed in finding the

temperature in either round or sawed material.

0 0.1 0.2

PROPORTIONAL

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PROPORTIONAL RADIAL DISTANCE {E/R}

ZIÍ60

FIGURE 13.—Temperatures between surface and center of lO-inch-diameter timber

after various heating jx-riods. Distance from surface expressed as proportion of

radius—" (Diffusivit}'=0.00025; initial wood temperature = 60° F.; heating-meR

dium temperature = 200° F.) (See appendix pp. 143-147 for discussion of method of

using figure 13 and for illustrative examples.)
PRESERVATIVE TREATMENT OF WOOD 59

LUMBER I INCH THICK

MINUTES7 9 II

WIDTH (INCHES)

13

tJO.

-25\

=20

'15

-lois 17

FIGURE 14.—Temperatures at a distance of ^ o inch from surface of sawed lumber 1

inch thick after various heating periods. (Diffusivity =0.00025; initial wood

temperature =60° F.; heating-medium temperature = 200" F.)

LUMBER 2 INCHES THICK

210

i

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170

150

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12 14 16

FIGURE 15.—Temperatures at a distance of 1 inch from surface of sawed lumber 2

inches thick after various heating periods. (Diffusivity =0.00025; initial wood

temperature = 60" F.; hfating-medium temperature = 200" F.)
60 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

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PRESERVATIVE TREATMENT OF WOOD 67

TEMPERATURES REQUIRED TO STERILIZE WOOD

Laboratory studies (ö, 15) have shown that while heating conditions

required to kill wood-destroying fungi depend both on the temperature

and on the duration of heating, the required heating period decreases

rapidly with increase in temperature. Results of these experiments

indicate that it is not practical to sterilize wood at temperatures appreciably lower than 150° F., since the most resistant fungus tested was

not killed even when heated for 12 hours at 140°. Following are the

temperatures and times of application recommended to make sure that

wood is effectively sterilized {6) :

Temperature Time

(°f' ) (minutes)

150 75

170 30

180 20

200 10

212 5

When the temperature of the wood at the interior of a timber is lower

than that of the heating medium, it will continue to rise in the region

near the center to a variable extent after heating is discontinued and

cooling starts, since heat flows from the region of higher to the region of

lower temperature. After cooling starts, heat will flow both toward

the surface and toward the center as long as the temperature of the

wood between the surface and center is higher than that at the center.

The amount of temperature rise that occurs at the central portion of

the timber will depend upon such factors as the surface temperature of

the wood while heating, the temperature to which the center has been

heated before cooling starts, the distance of the point under consideration

from the surface, the cooling temperature, and the conditions of cooling.

This subject is discussed more fully in another publication (48).

When the heating-medium temperature is 200° F. during the heating

period, the temperature at the center may rise from 10 to 15 degrees or

more if the center has been heated to a temperature around 130° to 135°

before cooling starts. On the other hand, if the heating-medium temperature is 260° during the heating period and the center temperature

is around 130° to 135° when cooling starts, the temperature may rise

25 to 30 degrees higher, roughly twice as much as when the heatingmedium temperature is 200°.

INJECTING PRESERVATIVES

The character and viscosity of the preservative, and the preservative

temperature, temperature of the wood, vacuum, air pressure, preservative

pressure, and length of pressure periods all have a marked effect on

treatment and are so closely interwoven that the relative influence of

an individual factor is often unrecognized. Their importance is greatest

in wood that is resistant to treatment or in treating mixtures of easily

treated and resistant wood.

INFLLTENCE OF VISCOSITY AND TEMPERATURE OF

PRESERVATIVE

Preservatives differ considerably in the ease with which they can be

made to penetrate resistant wood. One of the principal properties
68 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

involved is viscosity W, 25, 26, 27, 29). Other factors, such as surface

tension or ability to wet the wood, are probably also of importance,

but the extent to which their influence is different from that of viscosity

has not been established.

The viscosity of a preservative varies with its temperature, and the

two cannot be discussed independently.

Figure 23 illustrates variations in viscosity of different preservative

oils. Tne figure gives temperature-viscosity curves for a representative

coal-tar creosote, a petroleum oil, and mixtures of the creosote and

petroleum oil. The petroleum and creosote were those used in making

the treatments for which the data are given in table 8. The curve for 3

percent zinc-chloride solution has been added for comparison. The

marked differences in viscosity between the creosote, the petroleum,

and the mixtures stand out clearly. The low viscosity of the water

solution, in comparison with the viscosities of the oils, is also noteworthy.

The differences decrease greatly, however, as the temperature increases.

Liquids that are mobile or of low viscosity penetrate better, other

things being equal, than more viscous liquids. Since temperature has

such a pronounced effect on the viscosity of preservative oils and solutions, it affects their ease of penetration into wood. High preservative

temperatures give better absorptions and penetrations than low temperatures.

r

\

\

\

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\, o

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SOLUTION

1

100 no IZO 150 140 ISO ISO no too I90 zoo 2to ZZO 230 Z40

TEMPERATURE {"F)

FIGURE 23.—Temperature-viscosity curves for a representative coal-tar creosote,

petroleum oil, mixtures of the petroleum oil and creosote, and for a 3 percent zincchloride solution.
PRESERVATIVE TREATMENT OF WOOD 69

Coal-Tar Creosote and Mixtures of Petroleum

or Tar with Creosote

Table 8 shows the effect of temperature on treatments of air-seasoned

eastern hemlock ties with 100 percent coal-tar creosote, a 50-50 percent

creosote-petroleum mixture, and a 90-percent petroleum and 10-percent

creosote mixture. The preliminary vacuum, length of pressure period

and final vacuum were the same in all charges treated. With each

pressure and preservative the only factor varied was the temperature

of the preservative during the pressure period, which in turn varied the

viscosity. Both the sapwood and heartwood of eastern hemlock are

sufficiently resistant to prevent complete penetration under the conditions

used.

In the treatments with creosote, increasing the temperature (lowering

the viscosity) invariably increased the penetration and, in most cases,

increased both the net and gross absorptions. Table 8 shows that a

temperature of 180° F., which is a temperature that in the past was

widely used^ in commercial creosoting, gives much better results than

140° or 160°. Still better results were obtained, however, at the higher

temperatures. In the treatments with the petroleum mixtures, the

penetration again was invariably greater at the higher temperatures,

but the absorptions were not so consistent. The absorptions and penetrations of the creosote were much greater than those for the more

viscous petroleum mixtures.

In order to compare the relative effect of different viscosities of the

same preservative, which are a function of temperature, the side penetrations and absorptions obtained with creosote at each temperature

(table 8) have been averaged for the three pressures. These averages

are given in table 9 and show the marked influence that temperature and

viscosity changes have on penetration and absorption.

Since the treatments with coal-tar creosote, covered by table 8, were

made at the same preservative temperatures with each of the three

pressures, the general effect of pressure alone is indicated by averaging

the results of the six treatments made at different temperatures for each

pressure. These averages are also shown in table 8. This gives an

average based on 140 to 149 ties for each pressure. A comparison of

the results thus obtained shows that the average of side penetration

and gross absorption varied approximately in proportion to the increase

in gage pressure.

Figure 24, which is based on data given in table 8, shows the penetrations obtained in the treatments made at 150 pounds' pressure when using

the petroleum-creosote mixtures and creosote alone.

Figure 25 shows the effect of varying the viscosity without changing

the temperature in a series of treatments of small, air-seasoned Douglas-fir

heartwood specimens with creosote and creosote mixtures. The specimens

were matched in quality and four were used in each treatment. No

preliminary or final vacuum was used, the treating temperature was kept

at about 185° F., the treating pressure was about 125 pounds gage, and

the pressure period was 2 hours. The results shown in the figure are

averages. It is clearly evident that a distinct relation exists between the

penetration and the viscosity.

Table 10 shows the effect of changing the temperature without changing

the viscosity. The results, which were obtained by varying the tempera-
70 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

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PRESEEVATIVE TREATMENT OF WOOD 71

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72 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

TABLE 9.—Average} penetration and absorption of coal-tar creosote at six

temperatures in air-seasoned eastern hemlock ties ^

Absolute

Ties in Preser- viscosity

at temperature

used

Average net Average gross

vative Average side retention per absorption per

average

(number) temperature

penetration square foot

of surface

square foot

of surface in treatment

Cettiiy. potses Inch Percent Pounds Percent Pounds Percent

74 140

160

180

3.80

2.81

2.22

0.29

.39

.45

100

*235

;f255

1.36

1.78

2.01

100

131

148

1.43

1.89

2.15

100

73 132

73 150

72 200 1.80 .51 ie276 2.27 167 2.54 178

72 220 1.50 .()1 210 2.58 190 3.18 222

73 240 1.22 .07 231 2.()1 192 3.85 269

^ Data from table 8 averaged for pressures of 125, 150, and 175 pounds per square

inch.

2 A\'ood not heated before ai)plying treatment.

3 Percentages based on results obtained with lowest temperatures.

LEGEND

90'PERCENT PETROLEUM AND

10 PERCENT C/?EOSOTE

O-50-PERCENT PETROLEUM AND

00-PERCENT CREOSOTE

U-COAL-Tf^R CREOSOTE

0.2 0.3 0.4 0.5 0,6

AVERAGE S¡DE PENETRAT/ON (INCHES)

FIGURE 24.—Relation of absolute viscosity of preservative oils and penetration in airseasoned eastern hemlock ties.
PRESEKVATIVE TREATMENT OF WOOD 73

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6â

64

60

56

52

48

44

44

36

32

28

24

20

16

12

6

à i

LEGEND

%-COAL-TAR CREOSOTE

O-PETROLEUM OIL A AND

MIXTURES OF OIL A AND

COAL-TAR CREOSOTE

^-MIXTURES OF PETROLEUM

OIL B AND COAL-TAR

CREOSOTE

A- MIXTURES OF PETROLEUM

OIL C AND COAL-TAR

CREOSOTE

"

1

\

V

\

V

A

^

(k^

\J^

~¡u

0 0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.8 0.9

AVERAGE SIDE PENETRATION (iNCNES)

FIGURE 25.—Effect of varying viscosity (without changing temperature) upon the

penetration of preservative oils in air-seasoned Douglas-fir heartwood specimens.
74 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

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PRESERVATIVE TREATMENT OF WOOD 75

ture of different preservative oils so as to obtain the same absolute

viscosity while the other treating conditions were held constant (26, 27),

indicate that temperature alone may afïect treatment, but that its effect

IS much more significant when the viscosity of the preservative is reduced.

Mixtures of coal tar and coal-tar creosote give results similar to those

obtained with petroleum-creosote mixtures {60), The addition of coal

tar to coal-tar creosote increases the resistance to penetration, depending

both on the amount and the kind of tar added.

Water Solutions

The viscosity of water solutions, such as treating solutions of zinc

chloride, is much lower than that of creosote and creosote mixtures, and

it changes less with given changes in temperature. Nevertheless, experiments with zinc-chloride solution have shown that the small changes in

viscosity that take place with increases in temperature have a marked

effect on its penetration into wood. Although most of the water-borne

preservatives in use today cannot safely be heated to the higher temperatures used with zinc chloride in these experiments, the data indicate the

im.portance of using temperatures as high as permissible with water

solutions.

Table 11 and figure 26 show the effect of viscosity and temperature

of zinc-chloride solution on penetration in sawed specimens of coasttype Douglas-fir. The viscosity of the solution was varied by changing

the temperature, and results were compared at four different pressures.

TABLE 11.—Efect of viscosity and temperature of S-percent zinc-chloride

solution on penetration in sawed air-seasoned coast-type Douglas-fir

heartwood ^

Pressure

(pounds per

sgiLare inch)

Temperature of

solution

Average side

penetration

Average longitudinal

penetration

Specimens

in

average 2

y. Inches Percent s Inches Percent » Number

f 86 0.35 100 14.7 100 20

1 120 .48 137 16.1 110 20

100 160 .60 171 17.0 116 20

200 .89 254 20.3 138 20

f 88 .36 100 16.4 100 13

1 120 .52 144 21.3 130 13

125 160 .67 186 19.8 121 13

1 200 .97 269 22.5 137 13

f 79 .36 100 15.7 100 12

1 120 .52 144 17.4 111 12

150 160 .79 219 21.1 134 12

200 1.30 361 26.8 171 12

83 .39 100 16.8 100 18

120 .65 167 17.1 102 18

175 160 .99 254 22.1 132 18

200 1.48 379 24.1 143 18

^ All treatments had a preliminary vacuum of 30 minutes and a pressure period of 3

hours. Wood not heated before applying treatment.

2 Each treatment had seven to eight specimens 5 by 10 inches by 8 feet in size.

Other specimens were 4 by 4 inches by 4 feet in size.

3 Percentages based on results obtained at lowest temperature used at each pressure.
76 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

1.0

0.9

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LEGEND

V^VISCOSITY

X^PENETRATION

)

^

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^^

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AVERAGE PENETRATION (iNCHEs)

1.6 I.Ô 2.0 2.2

FIGURE 26.—^Relation between absolute viscosity of zinc-chloride solution and penetration in air-seasoned coast-type Douglas-fir heartwood specimens.

Each charge contained seven or eight pieces 5 by 10 inches by 8 feet in

size, and the remainder were 4 by 4 inches by 4 feet. At each pressure

the same number and size of specimens were used, and they were matched

as well as possible against each other.

Table 11 shows a consistent increase in penetration as the temperature

was increased; the greatest penetration was obtained at the highest

temperature. In other words, the 40-degree temperature change from

160° to 200° F. was more effective than a similar change between any two

lower temperatures.

The relation between the absolute viscosity and the penetration obtained is expressed very closely by hyperbolic functions slightly different

for each pressure. The empirical formulas with their respective curves

are shown in figure 26. When plotted, the actual values obtained in

experiments coincided closely with the mathematical curves, as may be

noted by plotting the average penetration values given in table 11.

Figure 27 shows the effect of different pressures and temperatures

on the gross and net absorptions of zinc-chloride solution into coast-type

Douglas-fir. An increase in temperature always increased the gross

absorption, but at the two higher pressures the kick-back, upon release

of the pressure, was high in the charges treated at 200° F. This resulted

in the net retentions in these two charges being lower than in their

companion 160° charges. As would be expected, however, the greater
PRESERVATIVE TREATMENT OF WOOD 77

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PRESSURE

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0 2 4 6 6 10 12 14 16 IB ¿0 ¿Z 24 ¿6 20 JO 32 34

ABSORPTION ROUNDS PER CUBIC FOOí)

FIGURE 27.—Effect of different pressures and temperatures on gross and net absorptions of zinc-chloride solution in air-seasoned coast-type Douglas-fir heartwood.

Full-cell treatment with 3-hour pressure period used in each run.

gross absorptions gave deeper penetrations despite the lower net absorptions.

Table 12 shows the results obtained in treatments of air-seasoned

Rocky Mountain-type Douglas-fir ties using different solution temperatures in combination with different treating pressures. All ties were

sawed transversely at the middle and lengthwise through the center for

measurements of penetration. Most of the material was heartwood,

although a few of the ties had a small amount of sapwood. Penetration

measurements were made on heartwood only. Although this wood is the

same species botanically as the coast-type Douglas-fir, it is far more

difficult to penetrate than the latter and is one of the most resistant of the

native conifers.

The 3-hour pressure period used for the treatments outlined in table

12 was the same as that used in the treatments of coast-type Douglas-fir

(table 11). On account of the marked resistance of the Rocky Mountaintype Douglas-fir, however, this pressure period was too short to obtain

appreciable penetrations except when the higher solution temperatures

were used. As in the treatments made on coast-type Douglas-fir, the

results show that the solution temperature and viscosity had a very

pronounced effect on absorption and penetration at all pressures, and

that the effectiveness of the treating pressures was considerably increased at the higher temperatures. None of the ties showed collapse

or checking except those in the treatment made at 200° F. and 250 pounds'

pressure.

207777^—53 6
78 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

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PRESERVATIVE TREATMENT OF WOOD 79

Similar data for yellow birch and eastern hemlock are shown in table

13. The effect of temperature and viscosity on penetration is conspicuous in the results with each species, although not so consistent in

every case as the results in table 11. A small amount of sapwood was

present on some of the yellow birch ties, and in making measurements of

penetration an effort was made to eliminate all penetrations that might

be in the sapwood.

From table 13 it may be noted that the absorptions in eastern hemlock

ties treated with zinc-chloride solution were somewhat higher in proportion to the penetration than in those treated with preservative oils

(table 8). This is probably on account of the deeper end penetration

that occurs when water solutions are used, and also on account of water

solution being absorbed by the cell walls in the treated portion.

Relation of Wood Temperature to Treatment

The data in tables 8 to 13, inclusive, show that the temperature of the

preservative (and wood) has a most important effect on both the absorption and the penetration, and this is especially true in the treatment

of resistant material. Except when a charge of timber is conditioned

by the steaming or Boulton process, heating of the wood will depend

on the preservative temperature and the time the wood is in contact

with the preservative.

The question that naturally arises is: To what temperature should

the wood be heated to obtain the most satisfactory conditions for pressure

treatment? There is no direct answer. One consideration is the relative

resistance of the wood to treatment, since the wood temperature is of

less importance in the treatment of such fairly permeable material as

the sapwood of various species. Since incipient decay may be present

in some timbers, such as air-seasoned ties, without being visible, it may

be desired to heat the center of all pieces to a temperature that will be

sufficient to sterilize the wood. The time that a given temperature

should be maintained to sterilize wood is given on page 67. It may be

noted that the lowest temperature listed is 150° F., which should be

held for about 75 minutes.

Small-dimension timbers that heat quickly and cool quickly should

be heated to a temperature somewhat higher than 150° F. to allow for the

more rapid cooling that takes place. Round timbers about 6 inches or

less in diameter and sawed material about 6 inches or less in thickness

could be placed in the latter category.

In view of the heating temperatures and length of conditioning periods

commonly employed under commercial treating conditions, all material

that is given a conditioning treatment would probably be well sterilized

and only air-seasoned material that depends on the length of the pressure

period for heating would need to be considered. For example, the timetemperature chart, figure 22, shows that a 7- by 9-inch air-seasoned tie

should be in the preservative at 200° F. for about 4^ to 5 hours to

steriUze the wood at the center, A somewhat longer heating period

should be used when the wood is very cold before treatment, as in the

winter months.

Preservative temperatures of 190° to 200° F. can be used satisfactorily,

with preservatives that are not damaged thereby, even for woods that

are susceptible to injury during treatment. Temperatures somewhat
80 AGRICULTURE HANDBOOK 40, U. Ö. DEPT. OF AGRICULTURE

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PRESERVATIVE TREATMENT OF WOOD 81

higher than 200° can be used to advantage for species like southern

yellow pine, provided such temperatures are not applied too long and

that the pressure is kept low enough to avoid checking or collapse.

Where there is a tendency for severe checking or collapse to occur

with a given temperature-pressure combination, in wood that is resistant

to penetration, the preservative temperature should be maintained at

190° to 200° F. and the pressure should be lowered until satisfactory

results are obtained {29^ 33), With wood that is not resistant to treatment, high preservative temperatures are not so necessary. High temperatures may also prove undesirable when injecting water solutions of

preservatives into resinous wood, because such temperatures may bring

resin to the surface where it will interfere with painting or finishing if

the wood is not resurfaced after treatment. It is a question in such cases

whether it is better to accept the poorer penetrations and cleaner surfaces

resulting from cold solutions or the resin exudation and the much better

penetrations resulting from hot solutions. This difficulty does not

arise with woods that do not exude resin. Preservative oils injected at

the higher temperatures leave the wood cleaner after treatment than

when lower treating temperatures are used.

High temperatures must not be used with some proprietary preservatives that are injected in water solution, for the heat will precipitate the

preservative from its solution before it enters the wood. It is best to

follow the recommendations of the manufacturers as to temperature

conditions permissible with such preservatives,

RELATIVE PENETRATION OF PRESERVATIVE OILS

AND WATER SOLUTIONS

Table 14 gives the relative penetrations and absorptions of zincchloride solution and coal-tar creosote obtained in two of the very

refractory conifers. Similar results obtained in experiments with

various other species show in a marked manner that resistant woods are

considerably more difficult to penetrate with creosote and other oils

than with water solutions A still greater difference in the relative

penetration of water solutions and preservative oils would have been

shown with longer pressure periods than those used with the refractory

species. Water solutions are absorbed within the cell walls, while

petroleum oils are not absorbed. Creosote oils are absorbed only

shghtly in the cell walls under normal treating conditions. This may

account for the superiority of penetration by water solutions as compared

with oils of the same viscosity.

THE USE OF VACUIM

In both the steaming-and-vacuum and the Boulton or boiling-undervacuum processes the function of the vacuum is to help remove moisture

from green wood (pp. 40, 41, 45). When seasoned timber is to be treated

by a full-cell process, a preliminary vacuum is used to remove as much

air as practicable from the wood before admitting the preservative.

With full-cell oil treatments a final vacuum is usually employed to recover

some of the surplus oil from and near the surface of the wood m order to

reduce the amount of dripping after the charge is removed from the treating

cylinder. A final vacuum is also used with water solutions for the same
82 AGRICULTURE HANDBOOK 40, U. S. DEPÏ. OF AGRICULTURE

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PRESERVATIVE TREATMENT OF WOOD 83

purpose; but when the solutions are used at reasonably high treating

temperatures the final vacuum is of little value, for the surface of the

wood dries quickly when the hot wood is withdrawn from the treating

cylinder, and, as a result, there is very little drip. With cold or cool

solutions the drip after treatment is greater and the final vacuum is

more useful. In the empty-cell treating processes a final vacuum is

employed, after the release of pressure and withdrawal of preservative,

for the purpose of hastening the expulsion of the surplus preservative

by the expansion of the air imprisoned and compressed in the wood.

In employing a preliminary vacuum in full-cell treatments to remove

air from the wood, care should be exercised not to readmit the air by

releasing the vacuum before the cylinder is full of preservative. Since

the vacuum is not perfect, there is always some air left in the cylinder.

The resistance of the wood to the outward movement of the confined air

also makes it impossible to remove more than a limited amount, which

will depend on the species, the size of timber, whether sapwood or heartwood, the moisture content, and other factors. As hot preservative is

admitted to the bottom of the treating cylinder, it warms the wood and

drives out more air, and this air, with that left in the space surrounding

the wood, collects above the rising preservative. As the cylinder becomes nearly full of preservative, the amount of air collected may be

sufficient to release the vacuum entirely in the space above the preservative and thus a moderate pressure may be produced. The amount of

air that thus accumulates will, of course, be least with a high original

vacuum.

This accumulation of air can be prevented by keeping the vacuum

pump running during the filling process. Suction of preservative into

the vacuum system during this period can be avoided by drawing the

vacuum through a cylinder or drum of suitable size between the treating

cylinder and the vacuum pump. Whether there is any advantage in

continuing to operate the vacuum pump during the filling process has

not been established by any known experimental evidence. If a high

vacuum is first drawn, it would seem that so little air could collect above

the preservative that running the pump during the filling process would

have a negligible effect, but when a low initial vacuum is applied, there

might be a noticeable effect on the air pressure in the upper timbers

in the charge. The temperature of the preservative when admitted to

the cyhnder will also affect the results. At higher temperatures more

air will come out of the wood during the filling process and more vapors

will be given off by the preservative. Certainly, however, if the surging

of preservative into the vacuum system is prevented, there can be no

objection to keeping the pump running during the filling period.

Some specifications require that when air-seasoned wood is given a

full-cell treatment the preliminary vacuum shall be held for a definite

period after the maximum is reached. This seems to be an unnecessary

requirement, for after the maximum vacuum is reached it is probable

that little if any additional air is removed. This is indicated by the fact

that when there are no air leaks in the treating equipment the vacuum

can usually be held without appreciable drop when the vacuum pump is

stopped. In general, it should be sufficient to specify that the preliminary vacuum be applied until the maximum is obtained.
84 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

When the vacuum is used after steaming or when the material is

conditioned by the boiling-under-vacuum process (Boulton method), the

air is removed more completely than when a vacuum is applied to cold,

air-seasoned material. This is because the air in the cylinder can be

more completely removed by the hot steam or vapors and because the

expansion of the air in the wood cells by the heat makes it possible to

evacuate the cells more completely than when the timber is cold. In

addition, air does not have an opportunity to fill the space originally

occupied by the water that is removed under the vacuum. In airseasoned material the space originally filled with water, except as reduced by shrinkage, is full of air at the time of treatment.

Treating specifications sometimes specify the minimum vacuum requirements for sea level and allow a correction to be made by plants

at higher elevations. This is commonly 22 inches of mercury, based on

sea-level conditions. Figure 28 shows the vacuum correction in inches

of mercury for different altitudes. The correction shown for any altitude

must be subtracted from the minimum vacuum specified for sea level

in order to give the corresponding vacuum for that altitude. Figure 28,

which is based on the standard barometric pressures for the different

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VACUUM CORRECTIONS (^INCHES OF MERCUR^

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FIGURE 28.—Vacuum corrections corresponding with different altitudes. (Data from

Smithsonian meteorological tables.)
PRESERVATIVE TREATMENT OF WOOD 85

altitudes, does not take into account the daily variations in barometric

pressure due to weather changes. Such a degree of refinement, however,

is unnecessary in injecting preservatives into wood.

In using the steaming or the boiling-under-vacuum process at any

plant above sea level, it may be desirable to know the boiling points of

water corresponding to different vacuums at that location. Figure 29

shows the boiling points of water under different vacuums at sea level

and the corresponding saturated-vapor or absolute pressure. The

vacuum required at a given altitude for a specified boiling point is equal

to the vacuum required for that boiling point at sea level minus the

vacuum correction for the altitude under consideration.

Example of the Use of Figures 28 and 29,—Gmen timber is conditioned

by the Boulton process at a plant where the altitude is 5,000 feet. The

temperature of the creosote is 185° F. What minimum vacuum will be

required to reach the boiling point of water?

Figure 29 shows that at sea level a vacuum of about 13 inches is required to boil water at 185° F. The vacuum correction for an altitude

of 5,000 feet (fig. 28) is approximately 5 inches of mercury. Therefore,

the vacuum required at a 5,000-foot altitude to boil water at 185° is

13 minus 5, or 8 inches of mercury. The right-hand scale of figure 29

shows that the saturated-vapor pressure is about 8.4 pounds per square

inch when the boiling point of water is 185°.

The temperature at which water boils under atmospheric pressure at a

5,000-foot altitude is the same as that at sea level when the vacuum is

approximately 5 inches. Figure 29 shows this temperature to be about

203° F. The right-hand scale of this figure shows that the corresponding

absolute or saturated-vapor pressure is about 12.3 pounds per square

inch.

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BOILING POINT OF WATER CF)

FIGURE 29.—Boiling points of water corresponding with different vacuums at sea level.
86 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

A curve, similar to figure 29 but corrected for the altitude of the individual plant, would answer directly the question answered in the

computation shown.

In applying the Boulton process, preservative temperatures higher

than the boiling temperature of water corresponding with the maximum

vacuum obtained should be used in order to overcome the resistance of

the wood to moisture movement, to heat the wood more rapidly, and to

compensate for the reduction in vacuum caused by the hydrostatic pressure of the preservative above the timbers.

Little experimental work has been done to determine the effect of the

preliminary vacuum on the net retention or the effect of the final vacuum

on the recovery of preservative after the cylinder has been emptied of

preservative.

Experiments have shown, however, that when air-seasoned timbers

are to be treated by the full-cell process the vacuum is much more effective in removing air from the wood if the timbers are first heated in

steam or in the hot preservative. For example, if air-seasoned Douglasfir piling is to be treated to refusal for marine use, heavier retentions will

be obtained if the charge is boiled under vacuum a sufficient length of

time to heat the wood and expand the air so that a greater amount of

the air can be withdrawn by the vacuum. When the vacuum is applied

to a charge of cold timber, it will generally remove little air from the

wood cells, particularly if the material is fairly resistant.

In commercial practice preliminary vacuums are commonly applied

for 15 to 30 minutes when air-seasoned material is given a full-cell treatment. The longer period is more commonly used because specifications

frequently call for it. Final vacuums used with coast-type Douglas-fir

vary from about 3^2 ^o 3 hours, but in most cases are about 1 to 2 hours.

Final vacuums used on air-seasoned and steamed material of other

species vary from about 3^ to 2 hours, with an average of about 1 hour.

The maximum vacuums usually obtained are about 22 to 26 inches of

mercury, based on sea level.

PRELIIVIINARY AIR PRESSURE

As previously stated, the purpose in using a preliminary air pressure

is to take advantage of the compressed air to force out a part of the

gross absorption when the preservative pressure is released. Within

certain limits this favors improved penetrations, as compared with fullcell treatment, because of the greater amount of preservative that can

be injected without leaving an undesirably high retention of preservative

in the timber. The use of preliminary air pressure is more effective in

the treatment of timbers that have a considerable amount of sapwood

that is not distinctly resistant to penetration, or in the treatment of

heartwood timbers that are reasonably permeable. When there is the

possibility of undesirably heavy end penetration in heartwood material,

the use of preliminary air pressure may be desirable. Preliminary air

pressure probably also causes greater reduction of gross absorption in

the easily treated pieces than in the more resistant pieces and thus helps

to reduce the differences in net retention between the different pieces.

Preliminary air pressure, however, is much less effective in the pressure

treatment of heartwood or sapwood that is difficult to penetrate.
PRESERVATIVE TREATMENT OF WOOD 87

For this reason it cannot be assumed that the same retention will give

equally good penetrations in the sapwood of round timbers of one species

as m those of another species, even when the more resistant timbers

have^ less sapwood based on the total volume. In fact, considerably

heavier net retentions may be required to obtain satisfactory penetrations m material having a fairly resistant sapwood, although the proportion of sapwood IS much less than in more easily treated poles or piling

of another species. Increasing the preliminary air pressure will not

remedy this difficulty, since increasing the air pressure in the wood cells

increases the resistance to penetration and this added resistance may

result m erratic penetration in refractory material. The optimum conditions for penetration in very resistant material would naturally be

obtained by removing as much air as possible from the wood before

applying the preservative pressure, as is done when the so-called full-cell

treatment is employed.

If the retention must be limited, however, a full-cell treatment is impractical when the wood is not particularly resistant and a large proportion of the total volume can be penetrated.

A further discussion of the effect of preliminary air pressure is given

later under the heading ''Kick-back,'' p. 90.

The Lowry and Rueping processes are widely used in the treatment of

railway ties and poles and, to a somewhat less extent, in sawed timbers

and land piling. They are not used for treating marine piling or for

other timbers where heavy net retentions are required. Vñien the

Boulton process is employed to condition the timber before treatment by

an empty-cell process, it is necessary to drain the cylinder after the

vacuum period is completed. Air is then admitted at atmospheric pressure or at a higher pressure, depending upon whether the Lowry or

Rueping method is used.

Most plants using the Rueping process for Douglas-fir apply preliminary air pressures of 20 to 60 pounds, which are held for ^ to 1

hour. Plants using the Rueping process for Rocky Mountain species

generally apply preliminary air pressures of 50 to 80 pounds, which are

held for about J4 to 3^ hour. Most of the plants employing this process

for southern yellow pine poles and piling and for hardwood ties, such

as oak, gum, maple, birch, and beech, use air pressure of 50 to 75 pounds.

Preliminary air pressures used in the Rueping process for southern

yellow pine ties vary from about 65 to 80 pounds, although a few plants

use lower pressures of 30 to 60 pounds. Most of the plants treating

southern yellow pine and hardwood timbers run the preservative into the

cylinder as soon as the required air pressure is reached. The wide

variations in the use of preliminary air pressure result in part from differences in the wood being treated and in part from lack of evidence as to

what are the most satisfactory air pressures and air-pressure periods for

different requirements.

At the present time comparatively little is known about the rate at

which air diffuses in the wood of different species or what the effect of

moisture or intensity of pressure is on the transmission of air in the

timber. Some preliminary laboratory experiments on the rate of air

transmission across the grain were made on heartwood specimens of

green Douglas-fir. With an air pressure of 100 pounds in the treating

cylinder, a pressure period of more than 5 hours was required to obtain

between 4 and 5 pounds' gage pressure in a hole three-eighths of an inch
88 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

in diameter, bored longitudinally, at a distance of about three-quarters

of an inch from the surface. Under the same air-pressure conditions a

pressure of less than 23>^2 pounds gage was obtained in 5 hours at a distance

of about 1% inches from the surface. Changes in air pressure occur

more slowly and to a lesser depth in green than in seasoned timbers,

provided checks do not affect the results.

Very little experimental work has been done to study the relation of

preliminary air pressure to preservative pressure. It has been a commonly accepted view that in the empty-cell treatment the preservative

pressure should be increased somewhat in proportion to the preliminary

air pressure used. As an illustration, with timber that would be treated

at 150 pounds' pressure by the full-cell process, the plant operator

would probably apply 200 pounds' preservative pressure after using a

preliminary air pressure of 50 pounds, or 175 pounds' preservative pressure after a 25-pound preliminary air pressure.

The difference between the preservative pressure and the preliminary

air pressure, however, does not necessarily represent the pressure effective

in forcing the preservative into the wood. Except for material that is

very easily penetrated, such as the sapwood of various species or timbers

of small dimension, the air pressure is not uniform throughout the

timber. In resistant woods subjected to preliminary air pressures, the

intensity of air pressure in the wood may decrease rapidly and the air

may penetrate only a short distance from the surface. Experiments on

timbers of various refractory species showed that although a given

preservative pressure may cause no checking or collapse when used in

full-cell treatments or when no preliminary air or vacuum is employed,

severe checking and collapse may occur if a preliminary air pressure is

used and the preservative pressure is increased by an amount equal to

the preliminary air pressure. This general practice, therefore, is not

safe to follow; rather, the amount of increase in preservative pressure

needed should be worked out by experience and observation at each plant.

PRESERVATIVE PRESSURE

At some plants the maximum preservative pressure is applied as soon

as it can be reached, whereas at others it is applied gradually, reaching

the maximum in from 3^ to 2 hours, depending on the kind of material

and species treated. When pressures of 200 pounds or greater are used,

they are generally approached gradually during the entire pressure

period so that the average pressure is considerably less than the maximum.

No study has been made, so far as is known, to determine the relative

merits of applying pressure gradually in comparison with a constant

pressure applied over the entire pressure period. Possibly plants using

pressures gradually raised to a relatively high maximum could obtain

similar results if the average pressure, or even one somewhat lower than

the average, were maintained during the entire pressure period.

Resistance of the wood to the transmission of liquids and gases necessarily makes the effectiveness of the preservative pressure decrease as

the depth of penetration increases. This effect may be noted in sawed

timbers or pole ties where both sapwood and heartwood faces are exposed.

Penetration is, of course, deeper in the exposed heartwood faces than in

the heartwood covered by the sapwood, since the resistance of the sapwood reduces the effect of pressure on the heartwood beneath it.
PRESERVATIVE TREATMENT OF WOOD 89

In the discussion relating to the effect of preservative temperature

attention has been directed to the fact that the treating pressure must

be carefully controlled when the timber shows evidence of checking or

collapse under the treating conditions employed. Higher treating pressures can be used on seasoned wood or cold wood than on wood that is

green or that has been heated for a long time.

PRESSURE PERIOD

The length of pressure period required to obtain a given absorption

depends largely upon the ease with which the timber can be impregnated

and upon the treating conditions employed. In the treatment of resistant material the pressure period should be long enough to obtain

the maximum benefit of the pressure employed and to allow sufficient

heating of the wood to produce favorable conditions for treatment.

Attempting to shorten the treating period by the use of high pressures

frequently results in erratic penetrations or unsatisfactory absorptions.

Treatment is sometimes discontinued when the rate of absorption becomes slow, since it is assumed nothing much can be gained by using a

longer pressure period. In some cases the rate of absorption is slow

because the wood has not had time to become heated enough for the best

penetration to be obtained.

Experiments have shown that better penetrations are usually obtained

with moderate treating pressures and moderately long pressure periods

than by very high pressures for short periods. On the other hand, little

is to be gained by holding the pressure after absorption has practically

ceased and, from the standpoint of treating costs, a reasonable balance

must be established between treating time and treating pressure.

A few plants treating resistant woods preheat the charge in the hot

preservative for a time before applying pressure. Whether this is as

effective as applying pressure from the start, and thereby obtaining a

longer pressure period without increasing the total time of treatment,

is a question on which there is some disagreement. When pressure is

applied, the preservative helps carry heat into the wood as it penetrates.

It therefore appears that an advantage would be gained by applying

pressure as soon as possible after the cyUnder is filled with the preservative,

in order to help increase the rate of heating and also to have the benefit

of a longer pressure period. So far as is known, however, no definite

data have been obtained on this subject.

The pressure periods required for Douglas-fir timbers that have been

conditioned by the Boulton process are, in general, considerably shorter

than are required to obtain a similar treatment in air-seasoned wood.

The Boulton-processed wood has a higher prehminary temperature at

the beginning of the pressure period than seasoned wood that has not been

heated, and this higher wood temperature is apparently of considerable

assistance in reducing the length of the pressure period.

It has been observed in experiments on air-seasoned wood m which

different pressure periods are employed that the tendency toward checking and collapse increases as the length of the pressure period increases,

other treating conditions being constant. In such cases the difficulty

has been overcome by lowering the treating pressure and, when necessary,

by using a somewhat longer pressure period to compensate for the

lower pressure.
90 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Results of treatments made on Rocky Mountain-type Douglas-fir

ties when pressure and treating period varied and solution temperature

was constant (table 15) show that the penetrations and absorptions were

very much better in 6- and 8-hour treatments than those obtained with

pressure periods of 2 and 4 hours. In general, an increase of 2 hours

in the pressure period gave better results than an increase of 50 pounds

in pressure with the treating period constant. A solution temperature

of 175° F. was used in these tests so that a wider range of pressure could

be used in the study of the relative effect of pressure and pressure period.

If a higher solution temperature had been used, it would have been

necessary to use lower pressures to avoid collapse and checking. All

ties excepting those in the 4- and 6-hour treatments at 250 pounds' pressure were apparently uninjured by the treating conditions used. About

half of those in the 4-hour treatment at 250 pounds pressure showed

considerable collapse and checking, and all of those in the 6-hour treatment at this pressure were badly injured by collapse and internal checking.

These results show that both the pressure and the length of the pressure

period affected the condition of the timber.

KICK-BACK

When the pressure is released at the end of the pressure period, some

of the preservative flows out of the wood and the treating cylinder

and returns to the working tank. The quantity of preservative that

flows back into the tank is called the ^'kick-back.'' The total quantity

of preservative injected into the wood during the treating operation

is called the ''gross absorption.'' In othçr words, the gross absorption

includes the amount of preservative absorbed by the wood while the

cylinder is being filled 12 as well as that injected during the pressure

period. The difference between the gross absorption and the kick-back,

plus the preservative recovered during the final vacuum, gives the net

absorption or net retention.

The most important factor causing kick-back under moderate pressure

conditions is the expansion of air compressed in the wood during the

pressure period. The outstanding characteristics of the empty-cell

processes, as distinguished from the full-cell, is that a larger part of the

gross absorption is expelled by the imprisoned air after the pressure

period. This expulsion usually permits reasonably large gross absorptions and deep penetrations with comparatively small net retentions.

The net retention, however, may vary widely for any particular gross

absorption, depending within certain limits on the preliminary air

pressure, the resistance of the species, whether sapwood or heartwood,

the moisture content, and other variables.

As the preservative is forced into the wood during the empty-cell

treatment, some of the confined air becomes mixed with the preservative ;

although, when penetration is not complete, some of the air must obviously occupy space in the untreated portion. It is probable that the

air in the wood cells that are partly filled with preservative is largely

responsible for the kick-back. A proof that small air bubbles must be

trapped in the preservative is shown in the empty-cell treatment of sapwood material and of other easily treated material that takes complete

12 The amount absorbed while the cylinder is being filled is called the initial absorption, which is discussed on page 110.
PRESERVATIVE TREATMENT OF WOOD 91

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92 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

penetration. If the air were merely compressed and forced farther into

the timber, it would not be practical to penetrate the wood completely in

the region occupied by the entrapped air.

In addition to the effect of air pressure on kick-back, if the timbers

have a high moisture content at the time of treatment, vaporization of

the water in the wood when the final vacuum is applied may also force

out more or less preservative. This may reduce the net retention,

especially in round timbers, even when the full-cell treatment is employed. In some cases this may be particularly objectionable when

high retentions are needed, as in the treatment of marine timbers. Much

of this difficulty can be avoided by more thorough seasoning of the

material before treatment.

Another factor that influences the kick-back, depending on the material and treating conditions employed, is the compression of the wood

during the pressure period and its expansion upon release of pressure.

This effect is greatest with woods that are resistant to treatment, especially if they are of low compressive strength and if the timber has

been conditioned by the steaming or Boulton process. When these

processes are used with green timber, the wood has a high moisture

content and is well heated, so that it is more pliable when pressure is

applied than colder or drier wood. The tendency of the wood to compress is naturally greater with high treating pressures and long treating

periods. In experiments conducted at the Forest Products Laboratory

on the ties of low density and high resistance to penetration, the ties

practically recovered their normal volume when pressure was released

immediately after treatment. Similar ties when held under a pressure

that gradually dropped from 150 to 125 pounds while the preservative

cooled, showed a permanent reduction of 6 to 7 percent of the original

volume. A combination of high temperature and high pressure is likely

to cause considerable permanent reduction in volume, accompanied by

checking and collapse.

The kick-back resulting from compression of the wood has often been

overlooked in treating practice. This compression of the wood will also

indicate a higher gross absorption than is actually obtained. A charge

of timber on which a high treating pressure is used may appear to have

a much higher gross absorption than another charge treated under a

lower pressure, yet the true gross absorption may be greater in the case

where the lower pressure is used. Wlien this is not recognized, the

treating-plant operator may believe that he is obtaining a high kickback and a correspondingly high gross absorption while the actual kickback of preservative from the wood cells may be much less than that

indicated.

Unfortunately, there is no way in which the kick-back caused by

compression of the wood can be determined in the normal treating

operation. Kick-back is less in timbers having a high moisture content,

as might be expected, since the amount of air in the wood decreases as

the amount of water increases. The relation of air volume and moisture

content is shown in figure 11. An approximate determination of the

maximum amount of preservative that can be absorbed in the available

air space of the treated wood (pp. 29 and 104) may help in finding whether

kick-back is partly caused by compression of the wood.

Increasing the preservative temperature, the treating pressure, or the

length of the pressure period also increases the amount of kick-back.
PERSERVATIVE TREATMENT OF WOOD 93

The effect of temperature and pressure on kick-back is shown in figure

¿1 and also by the data given in tables 8, 9, 12, and 13.

A comparison of the results obtained at each pressure given in table

15 shows that the general effect of increasing the pressure period was to

increase the kick-back. This is indicated by comparing the averages of

all the 2-, 4-, and 6-hour periods, respectively.

Naturally there is a large variation in the relative penetrability of

diiterent woods, and kick-back caused by expansion of compressed air

m the wood will be less from resistant timbers than from sapwood material or from species that are more pervious to the passage of air and

liquids. Thus, while in empty-cell treatments of air-seasoned southern

yellow pme timbers that are largely sapwood the kick-back often amounts

to a large part of the gross absorption, it is usually much less in material

that IS largely heartwood. The kick-back from resistant sapwood will

also be considerably less than that from easily penetrated sapwood.

Even when the full-cell process is employed, there is always a certain

amount of kick-back because more or less air is left in the wood after the

preliminary vacuum is applied. This kick-back will be greater from airseasoned timbers on which the preliminary vacuum is applied when the

charge is cold.

In experimental full-cell treatments on air-seasoned ties of different

species containing a considerable amount of sapwood, the kick-back

varied from about 4 to 10 percent, whereas for ties of resistant woods

the kick-back varied from about 6 to 25 percent of the apparent gross

absorption. This higher kick-back from the resistant woods was partly

caused by compression of the timber under the treating conditions employed. Preservative oils were used in these treatments. The kickback was somewhat higher for similar treating conditions when zincchloride solution was used, probably largely because of the greater

compression of the wood under pressure.

FINAL HEATING AND VACUUM, USING STEAM OR

EXPANSION BATH

Use of Final Steaming and Vacuum

A preliminary steaming-and-vacuum treatment is used to aid in removing moisture from the wood and to help heat the material to a favorable treating temperature. In addition a short final steaming-andvacuum treatment is often permitted for the purpose of cleaning the

wood surfaces of surplus preservative and to reduce the tendency of the

timbers to exude the preservative after treatment. A clean surface is

of particular interest in treating poles, since an oily surface is objectionable to the linemen, and often to the public when the poles are used on

city streets.

In treating some timbers—steam-conditioned poles, for example—

a final steaming has sometimes been applied for 3 to 4 hours, on the

assumption that the steam pressure will help force the absorbed preservative farther into the wood. It is questionable, however, whether this

final steaming accomplishes much from the standpoint of improving

the penetration, since the moisture in the wood develops a back pressure

that reduces the effective steam pressure, which is relatively low in

any case.

207777°—53 7
94 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Use of Expansion Bath and Vacuum

The preservative pressure is released at the end of the pressure period

in what is called the expansion bath, used largely in the treatment of

Douglas-fir poles, and with the charge of timber still submerged the

temperature of the preservative is raised from 10° to 20° F. above the

temperature maintained during the pressure period. A vacuum is

commonly applied during this heating period to assist in removing air and

some excess preservative. The duration of the expansion bath depends

upon the kind of material treated and similar factors and may vary from

about 2 to 6 hours. After the expansion bath, the preservative is withdrawn from the cylinder and a final vacuum is applied in the usual

manner.

Pressure from the expansion of air and formation of steam at the

lowered boiling temperature probably help force out some of the preservative during the expansion bath, but removal of air is probably an important factor affecting subsequent bleeding. If the vacuum is released

while the wood is submerged, a small reabsorption of preservative is

possible, but it is probably insignificant, since the atmospheric pressure is

low compared with the pressure applied during the treating period, and

the time during which the cylinder is being emptied is short.

ABSORPTION AND PENETRATION

The effectiveness of treatment depends on both the depth of penetration and the amount of preservative injected. The net amount of preservative retained by the wood (net retention) cannot be taken alone as a

measure of effectiveness, for there may be considerable differences in

penetration and distribution of preservative in different timbers or in

different charges having the same retention. Results are most nearly

comparable when the treating conditions are similar and the timbers

are of the same size, character, sapwood content, and degree of seasoning.

The following are some of the more important reasons why the net

retention alone does not necessarily indicate whether the wood has been

properly treated:

1. The treating conditions may be controlled, as in the empty-cell

processes, so that a large gross absorption and, consequently, a deep

penetration will be obtained with subsequent recovery of a considerable

proportion of the original or gross absorption. On the other hand,

treating conditions may be used where only a small amount of kick-back

occurs, such as in the full-cell process, and therefore a much heavier net

retention is required to obtain an equally good penetration. Kickback is much less when the wood is resistant to treatment, and in such

material there may be little gained by the use of an empty-cell treatment from the standpoint of penetration.

2. Some woods take a deep end penetration and very small side penetration, while in others the ratio of end to side penetration is not so

high. In the first case the absorption may be largely at the ends of the

timbers, particularly when they are fairly short and the end-surface

area is large in proportion to the side-surface area.

3. When the timbers in a charge have varying amounts of sapwood

and heartwood, or are of mixed species that vary widely in their relative

resistance to treatment, the more easily treated sapwood or more easily
PRESERVATIVE TREATMENT OF WOOD 95

treated species may absorb a disproportionate amount of the averaRc

retention mdicated for the charge.

4. Timbers that have not had fairly uniform seasoning may take

heavy absorptions and deep penetrations in some portions and have poor

treatment m other portions.

5. In timbers that vary considerably in size and in those that cannot

be completely penetrated, the penetration for a given retention is greatly

influenced by differences in ratio of surface area to volume (p. 96)

Penetration is more nearly a measure of effectiveness of treatment

than IS retention, but, with the same penetration, higher retentions

indicate that the treatment is better because it results in a greater concentration of preservative in the wood. The gross absorption is also a

better measure of treatment than net absorption (net retention) since

good gross absorptions are necessary to obtain satisfactory penetrations.

Unfortunately, the true gross absorption is difficult to determine accurately for reasons mentioned.

FULL-CELL AND EIVIPTY-CELL ABSORPTIONS

In the empty-cell treatment of poles of southern yellow pine, which

in the more commonly used sizes are largely sapwood, the net retention

of preservative varies from about 25 to 75 percent of the gross absorption,

depending on the air pressure in the wood, the character and condition

of the timber, and other factors. With an 8-pound empty-cell treatment

and an average net retention of 50 percent, the gross absorption would

be 16 pounds per cubic foot. If a full-cell treatment of 12 pounds were

applied, a kick-back of about 25 percent of the gross absorption, or 4

pounds per cubic foot, for the same gross absorption as that obtained

in an 8-pound empty-cell treatment, would be necessary. This kickback, however, is considerably higher than would ordinarily be obtained

with a full-cell treatment of such timbers. As deep penetrations

should therefore not be expected in southern pine poles with a full-cell

treatment of 12 pounds, as with an 8-pound empty-cell treatment.

¿6

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0.1 O.Z 0.3 0.4 0.5 0.6 0.7 0.Ô 0.9 1.0 I.I 1.2

AVeRAGE SIDE PENETRATION (INCHES)

1.3 1.4 1.5

FIGURE 30.—Relation observed between penetration and absorption of preservative

oils in air-seasoned eastern hemlock ties treated by full-cell process. (Numbers

opposite the points indicate the number of ties averaged in the group. The tics

were grouped for differences of 0.1-inch penetration.)
96 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Figure 30 shows the relation observed between penetration and net

retention in air-seasoned, sawed, eastern hemlock ties treated with

preservative oils by the full-cell process. In this figure the individual

ties in various treatments are grouped according to differences of 0.1-

inch penetration. All ties having from 0- to 0.1-inch penetration form

the first group. The average gross absorption obtained in the treatments was about 2 pounds greater than the net retention.

RELATION OF DIMENSIONS OF TIMBER TO ABSORPTION

AND PENETRATION

Heartwood Timbers

For many years preservative treatment was largely confined to cross

ties and larger material. Since ties do not vary widely in dimensions,

the volumetric absorptions necessary to give satisfactory results with

timbers of this kind have been fairly well established from experience.

In the application of preservative treatment to many different types of

timber having widely different cross-sectional dimensions, the tendency

of many purchasers is to specify the same retentions regardless of dimensions. If all of the wood could be penetrated, the dimensions would not

need consideration. Very few species, however, are sufficiently permeable in the heartwood to permit complete penetration, even when the

timbers have relatively small cross-sectional dimensions. In large

timbers the ratio of surface area to volume is much smaller than in

pieces of small dimensions. Since there is less surface area per cubic

foot in the large timbers than in small ones, a given quantity of preservative per cubic foot can penetrate deeper in the larger sizes.

The following example will illustrate the disparity in treatment that

may result if the same retention is specified for heartwood timbers having

the same volume but different surface areas. Assume a 7- by 9-inch by

8-foot heartwood tie is to be treated with a net retention of preservative

of 8 pounds per cubic foot. A tie of this size will have a volume of

33^ cubic feet and a total surface area of about 22.2 square feet, or about

6.35 square feet per cubic foot. If the same timber should-be cut into

four pieces 33^ by 43^ inches by 8 feet in size, the volume would still be

the same but the total surface area would be 43.54 square feet or about

12.4 square feet per cubic foot. This is an increase of about 96 percent

in surface area over that of the tie. Naturally, the 8 pounds of preservative will penetrate deeper when injected through 6.35 square feet

of surface than when the same amount of preservative is used for 12.4

square feet of surface.

If the average ratio of end to side penetration were determined, it

would be possible to compute the approximate amount or percentage of

treated wood in a timber of any given size. With this information it

would be a simple matter to specify volumetric absorptions that should

give similar average penetrations and concentrations of preservative in

heartwood timbers of different dimensions. This could be done as

follows :

Let V = Total volume of the timber in cubic feet

Fi^Volume of treated wood in cubic feet

Vs = Volume of wood treated by side penetration only

L = Length of the timber in feet
PRESERVATIVE TREATMENT OF WOOD 97

Li = Average depth of end-surface penetration in inches

A = Width of timber in inches

B = Thickness of timber in inches

p = Average depth of side-surface penetration in inches

7?p = Longitudinal penetration where 7i is the ratio of depth of

end to depth of side-surface penetration = Li

.li = Square inches of penetrated wood in cross-section beyond

average depth of end penetration

Figure 31 is a sketch showing the dimensions named.

^ Z \A Pi

CROSS SECTION BEYOND

END PENETRATION

FIGURE 31.—Sketch showing dimensions used in computing the volume of treated

wood in a sawed heart wood timber.

The volume of penetrated w^ood at the ends with the dimensions ABL^j

\BL where Li = np, is evidently -^.^V'ubic feet at each end, or for both ends

ABU

864 •

The area At^ or the number of square inches of treated wood in a cross

section where all penetration is from the side surfaces, is 2p (.1 +5—2p) ;

hence the volume of wood, in cubic feet, that is penetrated entirel}^ from

side penetration is

_, -r .^.+B-2v)\fL-2Li\_ (p)(A+B-2p)((;L-Li)

y s —

fsp (A+B-êp)\(L-2LI\

\ 144 )\ 12 / 432

The total volume of treated w^ood (F«), including that penetrated by both

end and side penetration, is therefore:
98 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

,, ÍABLi\ , / {v){A-^B-2v){6L-'Li) \

^'^\S^)^\ 432 )

If a timber such as a tie is taken as a standard and has Vi cubic feet

and the volume of treated wood in the tie is V2 cubic feet, for comparable

treatments the required retention in a timber having a total volume V

and treated volume Vt would be computed as

where Wj, is the absorption in pounds per cubic foot specified for the tie.

The average side and longitudinal penetrations have been measured

in a large number of air-seasoned heartwood specimens of several different woods, such as southern pine and Douglas-fir, which take only a

limited heartwood penetration {SI, S2), In general, the longitudinal

penetration of creosote was found to be from 10 to 20 times as deep as

the side penetration and that of zinc-chloride solution from 15 to 25

times as deep. Table 16 has been prepared to serve as a guide in specifying absorptions for heartwood timbers and shows the approximate

amount by which volumetric absorption should be increased or decreased to give a treatment equivalent to that obtained in a 7- by 9-inch

by 8-foot timber. This timber of tie size is taken as a standard for the

comparison of absorptions in timber of other dimensions, since the

necessary volumetric absorptions for ties are better known than those

for timbers of any other size.

Many measurements of the average side penetration in unincised

heartwood timbers of the commonly treated species indicate that a fair

average value for general conditions would be about 3^ inch. The

computations for table 16 were therefore made on the assumption that

the values for the ratio of end to side penetrations are 15 and 20, respectively, for preservative oils and water solutions, and that the value

for side penetration is 3^ inch. On this basis the average end penetration

of preservative oils would be roughly about 7 to 8 inches and that of

water solutions about 10 inches. These values should give results that

are reasonably close and will apply to treatment with either the full-cell

or empty-cell processes, since the same gross absorption obtained by

either process should give similar penetrations. As table 16 includes a

variety of sizes, a close estimate can be made for intermediate dimensions.

If the ratio of end to side penetration is taken as 15 for preservative

oils and 20 for water solutions, the volume of treated wood in a 7- by

9-inch by 8-foot tie would be 1.25 cubic feet for preservative oils and 1.4

cubic feet for water solutions. With the penetrations as assumed, about

36 percent of the total volume of a heartwood tie, having an average

side penetration of 3^ inch, would then be treated with preservative oils

and about 40 percent of the volume would be treated when water solutions were used. A somewhat deeper average penetration is obtained

when heartwood timbers are incised, and this would help increase the

percentage of treated wood.

Mixed Sizes in a Charge of Heartwood Timbers

If a charge of heartwood timber contains pieces of different dimensions

but of the same species and similar moisture content, the larger sizes
PRESERVATIVE TREATMENT OF WOOD 99

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PRESERVATIVE TREATMENT OF WOOD 101

^^ i!-i^^A"^^^^^ absorb less than the average volumetric absorption specified

while the smaller sizes will take a heavier absorption. Theoretically the

penetration would be the same in all timbers, assuming equal resistance

to treatment. If it is necessary to treat more than one size of timber in a

charge, the approximate proportional absorption should be determined

tor all timbers of each size. The total absorption for the charge will then

be the sum of the absorptions required for the material in each group.

Example of Computation of Absorption in Mixed Sizes in a Charge

of Heartwood Timbers

Suppose that a charge has approximately 1,500 cubic feet of timbers

4 by 8 inches by 16 feet and 1,000 cubic feet of timbers 10 by 12 inches by

16 feet m size. All are to be treated with a net retention of creosote

equivalent m penetration to an 8-pound-per-cubic-foot treatment in a

tie. What is the total absorption for the charge?

From table 16 it is found that the 4- by 8-inch by 16-foot timbers

should have about 111 percent of the absorption required for the tie, or

8.9 pounds per cubic foot. Similarly, the 10- by 12-inch by 16-foot

timbers should have 67 percent of the 8-pound absorption, or about 5.4

pounds per cubic foot. The total absorption to be specified for the

charge would then be (8.9X1,500) + (5.4X1,000) = 18,750 pounds, or an

average absorption of about 7.5 pounds per cubic foot. An 8-pound

absorption in such a mixed charge should be better than would be expected from an 8-pound treatment in ties. If the small-size pieces constituted a much higher proportion of the total volume, calculation would

show that it was desirable to specify an average absorption of somewhat

more than 8 pounds per cubic foot. Since, however, the smaller-dimension

timbers will be heated to a higher temperature, which is more favorable

for treatment, the larger pieces will usually be more difficult to penetrate

satisfactorily. It is therefore desirable, whenever possible, to avoid

treating mixed charges of timbers that vary widely in cross-sectional

dimensions.

Sapwood Timbers and Easily Treated Woods

Timbers that have fairly easily penetrated heartwood, that are practically all sapwood, or that contain a large proportion of sapwood, should

have heavier volumetric absorptions (retentions) than heartwood timbers

that take only a small penetration. If the proportion of treated wood in

a heartwood tie is assumed as approximately 36 percent when impregnated

with preservative oils and 40 percent when water solutions are used,

then in order to obtain the same concentration of preservative per unit

volume of treated wood in a timber that can be completely penetrated,

the net retention should be increased by an amount equal to 1/0.36, or

about 2.8 times, when treated with a preservative oil, and by 1/0.4, or

about 2.5 times, when treated with a water solution. In most cases,

however, the same concentration would not be necessary or justified,

although some increase would be desirable over the retention required

in heartwood timbers. There is evidence from service-test records to

show that a given retention that may be found satisfactory for heartwood timbers may be entirely inadequate for similar material that is

largely sapwood or that can be fairly easily penetrated in both the
102 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

heartwood and sapwood portion. For example, a large proportion of a

group of experimental ties of red oak, sapwood gum, and sapwood southern

yellow pine treated by the empty-eel] method with 5 to 6 pounds of

coal-tar creosote per cubic foot were badly decayed after 7 years of

service. Other ties in the same group and of the same species that had

a net retention of about 12 pounds of creosote per cubic foot showed no

signs of decay within the same period. These ties were installed in the

South, where conditions are particularly favorable to decay. Under

similar service conditions the lower retentions have been considered

satisfactory for heartwood ties.

A discussion of retentions recommended for sapwood timbers will be

found on pages 130, 132.

In round timbers such as poles and piling the end-surface area is so

small a percentage of the total-surface area that the influence of end

absorption is generally negligible, and only in exceptional cases is it

practical to penetrate much more than the sapwood.

Absorptions and Penetrations in Round Timbers

For any given species the average diameter and the average depth of

sapwood must be considered in computing equivalent treatments in

round timbers of different dimensions. An equivalent treatment is one

that will give a retention proportional to the amount of penetrated wood

in the total volume. This naturally assumes that the treated wood has

a similar concentration of preservative. It is not sufficient to consider

the surface area only, since the proportion of sapwood in the total volume

depends both on the average diameter and on the average depth of sapwood, and in timbers of most species used for poles or piling penetration

is usually limited by the depth of sapwood.

If Rs represents the ratio of surface area to volume and D is the average

diameter of the timber, then ß,=^. When D is in inches, Rs is the

number of square inches per cubic inch; and when D is in feet, Rs is the

number of square feet of surface area per cubic foot. If Pa represents

the percent of sapwood in the total volume, T the average sapwood

thickness, and D the diameter, then

If, from this equation, T is determined in terms of P« and D, it will

be found that

T=DI 0.5-0.05 VIOO-P, \

or

— =0.5-0.05 VlOO-Ps.

Figure 7, which has been produced by using the outlined equation,

shows the percentage of sapwood, based on the total volume, plotted

against the sapwood thickness divided by the diameter, that is, the

ratio T/D. In order to determine from figure 7 the percent of sapwood

in a timber of any diameter, divide the sapwood thickness by the diam-
PRESERVATIVE TREATMENT OF WOOD 103

eter and, above this ratio, found on the horizontal scale, read the percent

of sapwood, based on the total volume, on the vertical scale. For

example, assume that a timber has an average diameter of 11 inches and

that the sapwood thickness averages 1.3 inches. In this case the ratio

T/D = 1.S/11, which is approximately 0.118. The corresponding percent

of sapwood for this value of T/D is found on the left-hand scale to be

about 42.

Figure 7 will be found convenient for comparing the relative sapwood

depth in timbers of different diameters for any ratio T/D, or for finding

the required sapwood depth in timbers of any diameter to give the same

percentage of sapwood Ps. If, for example, the proportion of sapwood is

assumed as 65 percent, the chart shows that the corresponding value of

T/D is 0.205. The average sapwood thickness that would give 65 percent

sapwood based on the total volume would then be ^=0.2051), where T

is the sapwood thickness and D is the diameter.

The ratio of the surface area per unit volume of a smaller-diameter

timber to one of a larger diameter is always greater than the ratio of the

percent of sapwood Ps in the smaller timber to the percent of sapwood

in the larger timber when both have the same sapwood thickness. As

the depth of sapwood decreases, however, the ratio of the percents of

sapwood in timbers of different diameters approaches as a limit the

ratio of the surface areas.

Similar concentrations of preservatives cannot be expected to give as

good protection to the pieces of round timber with thin sapwood as to

pieces with thick sapwood. Poles or pihng with thin sapwood will

normally lose preservative more rapidly from leaching and evaporation

than timbers with deeper sapwood. For this reason it would be logical

to insure heavier concentrations of the preservative in the thinner sapwood than in the thicker sapwood to help compensate for the more

rapid loss of preservative, since both depth of penetration and concentration of preservative are important factors affecting the service life.

Furthermore, poles or piling of one species might have an average

sapwood depth considerably less than that of another species and,

depending on the diameter, might require a much smaller retention to

give a similar concentration of preservative. If the sapwood of the two

species was about equally permeable, the depletion of preservative would

probably be considerably faster in the thinner sapwood pieces and higher

retentions might be needed to help compensate for this more rapid loss.

On the other hand, if the species with the thinner sapwood was more

resistant to penetration, the higher sapwood resistance would probably

help materially in retarding preservative losses from leaching and evaporation. These factors should be given careful study in specifying retentions

in round timbers. In the case of poles, also, it might be necessary to

consider the matter of bleeding depending on the preservative used and

other variables.

Disadvantages of Treating Both Large- and Small-diameter

Timbers in the Same Charge

Although it is often necessary to treat both large- and small-diameter

timbers in the same charge, this practice should be avoided as much^ as

possible, especially when some timbers have a much larger cross section

than others.
104 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

When mixed sizes are treated in the same charge, the smaller-diameter

pieces will usually take more than their share of the preservative because

the percentage of sapwood in the smaller timbers usually comprises a

greater percentage of the total volume; however, the depth of sapwood

m the smaller pieces may be considerably less than in the larger pieces.

The smaller timbers will be heated to a higher temperature when

treatment is applied. Care must be exercised to make sure that in

treating large- and small-diameter timbers, the more favorable conditions

for treatment of the smaller-diameter timbers do not result in poor

treatment in the larger pieces. The trouble is considerably aggravated

if the sapwood is fairly resistant to treatment and the larger timbers

have deeper sapwood than the smaller ones.

When the specified retention is relatively low, the problem is particularly difficult, for the large pieces with fairly deep sapwood (that,

in addition, may be more or less resistant to treatment) will retain a large

proportion of the gross absorption, regardless of the initial air pressure

used. In fact, too high initial air pressures may retard rather than

improve penetration, and they become less effective as the resistance of

the wood increases.

A point too often overlooked is that the required retention cannot be

based alone on the proportion of sapwood or on the proportion of surface

area. Other considerations such as the depth of sapwood, resistance of

the wood to treatment, and proportion of the gross absorption retained

after treatment, are very important factors affecting the final retention.

Maximum Absorptions in Round Timbers

Figure 7 is also useful in estimating the maximum amount of preservative that poles or piling having different average depths of sapwood can absorb, since in general only the sapwood is treated. The

approximate percentage of air space P can be found from figure 11 for

the species treated. If Ps is the percent of sapwood found from figure

7 and W is the weight in pounds per cubic foot of the preservative at

the treating temperature, the maximum amount of preservative that

could be absorbed, assuming all the available air space is filled, is

I -TKrT I ( ip.! 11 ^ ) pounds per cubic foot.

The maximum absorption will necessarily be somewhat less than this

because all the air space in the treated sapwood cannot be completely

filled regardless of the method of treatment used.

Table 17 shows the surface area and volume per foot of length of

different sizes of sawed and round timbers. The areas given in this table

are actual surface areas for timbers of the dimensions shown and do not

take into account relations of end to side penetration. In the table the

nominal sizes are used; that is, in computations of volume and surface

area for the sawed timbers account is not taken of the fact that the

actual dimensions of lumber and timber are usually somewhat less than

the nominal dimensions. For simplicity, the round timbers were assumed to be cylindrical in shape, which, although not correct, is sufficiently

close for practical purposes, since the average diameter can be used in

computing volumes or percentages of sapwood.
PKESERVATIVE TREATMENT OF WOOD 105

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PRESERVATIVE TREATMENT OF WOOD 107

MEASUREMENT OF ABSORPTION

In making preservative treatments the absorptions are usually determined by one of the following methods: (1) By taking gage readings

and determining the difference in the volume of preservative contained

in the working tank before and after treatment; (2) by using a working

tank mounted on scales and weighing the total amount of preservative

absorbed, or (3) by taking weights of the charge of timber before and

after it is treated. The oldest and most commonly used method of

determining absorption is to reduce the volumetric measurement of the

amount of preservative absorbed to pounds per cubic foot.

The advantages of the weighing-tank method as compared with tankgage readings in measuring absorption are: More accurate determinations of the absorption can be made because the method reduces the

possibility of errors that may be introduced by such factors as the expansion of preservative, cylinder, and measuring tanks due to change

in temperature, friction affecting the movement of the gage, and the

difficulty of making accurate readings in the measuring or working tank

of small changes in the height of the liquid. Weighing the wood on

track scales before and after treatment is a very dependable method of

measuring absorption in well-seasoned material, but it has the disadvantage when the wood has a high moisture content or when the charge

is given a preliminary steaming treatment, that the moisture content

and hence the weight of the wood is changed by the steaming. The

weight of the timber cannot be conveniently determined after completing

the vacuum following steaming and before the preservative is admitted

to the cylinder. When the charge contains wood at a high moisture

content, some of the moisture is lost when pressure is released and a final

vacuum is applied. This will introduce an error when the final weight

is taken on track scales.

The same objection applies when the material is conditioned by the

Boulton method, unless the amount of water withdrawn from the wood

during the vacuum period is measured accurately. The amount of

water condensed during the Boulton treatment is not a true measure of

the amount of moisture removed from the wood unless the oil is free from

water when it is admitted to the cylinder and all of the vapors withdrawn

by the vacuum pump are condensed.

Unless proper care is exercised, absorption measurements made by

taking gage readings or by weighing the working tank may be subject

to considerable error. Small leaks in valves may allow a large amount

of preservative to pass out of the cylinder during the pressure period;

and unless this preservative is returned to the working tank before

treatment is completed, a false reading of absorption will be obtained.

Specifications of the American Wood-Preservers' Association i^ require

that in all tank-volume readings the temperature be recorded and the

reading be reduced to that of the preservative at 100° F.

Figure 32 will be found convenient to use in determining the volumetric

absorption, in percent of the total volume of the charge, for a given

specified retention in pounds or in gallons per cubic foot. The range of

specific gravities shown is from 0.92 to 1.12 for differences of 0.04 in each

13 American Wood-Preservers' Association Manual of Recommended Practice,

Instructions for the Inspection of Preservative Treatment of Wood.
108 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

interval. Intermediate values can be easily determined by interpolating.

The readings are net absorptions (retentions). Since there is always a

kick-back after treatment, the gross absorption should be proportionately

increased.

Example of the use of figure S3.—Suppose than an absorption of 15

pounds per cubic foot is specified, and the specific gravity of the preservative at the temperature used is 1.04. What is the required net

retention in percentage of total volume of charge and in gallons per

cubic foot?

Starting in figure 32 on the horizontal scale with the specified absorption of 15 pounds per cubic foot, follow vertically until an intersection is made with the diagonal line labeled specific gravity 1.04. The

reading on the left vertical scale corresponding to the intersection point

is found to be about 23.2 percent, which indicates that for each cubic

foot of wood in the charge 0.232 cubic foot of preservative should be

injected; projecting horizontally across to the diagonal dotted hne

labeled gallons per cubic foot and then directly above to the horizontal

scale, it is found that the required retention is about 1.73 gallons per

cubic foot. The lines with arrows (fig. 32) shows the procedure for this

example. If in this case it is found from experience that a gross absorption of 18 pounds is necessary to obtain the net retention of 15 pounds,

the chart shows that this absorption (18 pounds) requires about 28

percent, or 0.28 cubic foot, of preservative per cubic foot of wood. Expressed in gallons, the gross absorption is found to be about 2.1 gallons

per cubic foot of wood.

It may be noted that when absorptions are specified in pounds per

cubic foot of wood, a greater volumetric absorption of the preservative

will be obtained with a preservative having a low specific gravity than

with one having a high specific gravity, since the absorptions by volume

are inversely proportional to the specific gravities.

. Assume for illustration that a creosote-petroleum mixture has a

specific gravity of 0.94 at the treating temperature and that a creosotetar solution has a specific gravity of 1.08 at the same temperature. For

a given absorption in pounds per cubic foot the ratio of the volumetric

a)Dsorption of the creosote-tar solution to that of the creosote-petroleum

mixture will be 0.94-^1.08, or about 87 percent. In other words, the

unit volumetric absorption of the preservative oil with the higher specific

gravity will be about 13 percent less than that of the oil with the lower

specific gravity.

Differences in the specific gravity of oils used in preservative mixtures

made by volume will affect the proportion of each absorbed by weight.

For example, assume that a petroleum oil having a specific gravity of

0.9 at the treating temperature is mixed with an equal volume of creosote

having a specific gravity of 1.02 at the same temperature. If W is the

specified absorption in pounds per cubic foot, then the weight of petroleum

injected per cubic foot will be

^to^

^0.9+1.02^

ill be

yo.9+i.02y

The weight of creosote will be

1 no \

= .0531F'
PRESERVATIVE TREATMENT OF WOOD 109

ABSORPTION REQUIRED (GALLONS PER CUBIC EOOf)

0 0.4 0.Ô 1.2 1.6 2,0 2.4 2.8 3.2 3.6

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ABSORPTION REQUIRED (POUNDS PER CUBIC FOOT)

FIGURE 32.—Volumetric absorption, in percent of total volume of the charge, required

for preservatives at different specific gravities to obtain a given absorption in pounds

or gallons per cubic foot. (Dotted line is for absorptions in gallons per cubic foot

Scale for absorptions in pounds per cubic foot is at bottom of figure, and scale for

absorptions in gallons per cubic foot is at top.)

207777°—53 8
no AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

An S-pound treatment of the 50-50 mixture would then contain 3.75

pounds of petroleum and 4.25 pounds of creosote per cubic foot.

If the mixture contained 45 percent of petroleum and 55 percent of

creosote, the weight of petroleum per pound of absorption of the mixture

would be

(0-9) (0-45) ^ 0 42 pound

(0.9) (0.45)+ (1.02) (.0551-Â^-^^ ^^'"''^

and the weight of creosote (1-0.42), or 0.58 pound. With a mixture

of this proportion the 8-pound treatment would therefore give an absorption of 3.36 pounds of petroleum and 4.64 pounds of creosote per

cubic foot. In the first case the absorption of creosote is more than

13 percent greater by weight than the absorption of petroleum, while in

the second case, when the difference in volumetric proportions is 10

percent, the absorption of creosote is more than 38 percent greater by

weight than that of the petroleum.

Initial Absorption

During the filling of the cylinder with the preservative, when airseasoned or steamed material is treated, and also during the boilingunder-vacuum period when the Boulton process is employed, there is

more or less absorption of preservative, known as initial absorption. The

amount of this initial absorption will vary, depending on the treating

conditions and similar factors.

Experimental air-seasoned hemlock ties, when treated by the full-cell

process with preservative oils and water solutions, had an initial absorption ranging from about 1.5 to 3 pounds and averaging about 2

pounds per cubic foot. The initial absorption in several charges of

seasoned red oak ties, impregnated by the full-cell process with water

solutions and preservative oils, ranged from about 3.5 to 6 pounds per

cubic foot and averaged about 4 pounds. The allowance that should be

made for the initial absorption and kick-back can usually be estimated

fairly closely after a few charges of timber have been treated.

EFFECT OF TREATMENT ON THE STRENGTH AND

PHYSICAL PROPERTIES OF THE WOOD

EFFECT OF PRESERVATIVES

Experiments indicate that the preservatives commonly used for the

treatment of wood have no significant effect on the wood properties.

Preservative oils such as creosote, creosote-coal tar, or creosote-petroleum

mixtures do not contain chemical substances that would have a harmful

effect on the wood, and this is substantiated by results obtained in

strength tests.

Very strong solutions of some water-borne salt preservatives may

affect the strength, depending on the concentration, but the concentrations used for protection against decay and insect attack are usually

so low that the salt retention in the wood should not have an important

effect on the strength or other wood properties.
PRESERVATIVE TREATMENT OF WOOD HI

VARIABLES AFFECTING THE STRENGTH PROPERTIES

There are a large number of variables that determine the effect of

treatment on the strength properties. Some of the more important

are (1) species, (2) heating medium used, (3) temperature of the heating

medium, (4) length of the heating period, (5) dimensions of the timber,

(6) preservative pressure used, (7) moisture content of the wood, and (8)

length of the pressure period.

Some species are more susceptible to injury under any given heating

conditions than are others. The heating medium is also an important

factor, for the surface coefficient of heat transfer varies for different

mediums and this affects the rate at which the wood is heated.

Both the temperature and the duration of heating naturally have an

important bearing on the results, since the two variables are interrelated. Increasing the temperature will decrease rapidly the time

required to produce a given effect on the strength properties of wood.

Laboratory studies have shown that the effect of temperature on both

the strength properties and on the rate of deterioration, as indicated by

loss in oven-dry weight, is accelerated as the temperature is increased.

For example, the effect of raising the temperature from 250° to 300° F.

is considerably greater than the corresponding effect on strength that

occurs when the temperature is raised from 200° to 250° F. In general,

it is better practice to use moderate heating temperatures and a somewhat longer heating period than to use high temperatures in order to

hasten the rate of heating. Points near the surface of a timber quickly

approach the temperature of the heating medium, while much of the

wood at the interior is still at a relatively low temperature. It is therefore necessary to bear in mind that the wood at and near the surface is

exposed to a temperature close to that of the heating medium for most

of the heating period, unless water is being evaporated while the timber

is in contact with the heating medium.

Experiments have shown that the size of the timber has a bearing on

the results obtained. In strength tests it was found that the strength

properties of timbers of large dimension were more easily injured by high

temperatures and long-continued heating than those of timbers of smaller

sizes. On the other hand, both air-seasoned and steamed timbers with

small cross-sectional dimensions were apparently more easily collapsed

during the preservative treatment.

Similarly different results are obtained with water solutions than with

oils. Timbers treated with water solutions showed a greater tendency to

collapse than the same kind of material impregnated with preservative

oils under the same treating conditions. This is probably because the

water solutions soften the wood more than do preservative oils. Figure

33 shows cross sections from the centers of 4- by 4- by 48-inch heartwood

Douglas-ñr specimens treated with a water solution under pressures of

150 and 175 pounds. The shaded portion indicates the original size of

each specimen. The two specimens shown in the figure were the most

severely collapsed of those treated, but the results demonstrate that the

pressures used were too high for the species and size of specimens. Nearly

all of the pieces treated under these pressures were more or less severely

injured, but little or no collapse occurred in specimens treated under the

same conditions but with pressures of 100 and 125 pounds.
112 AGRICULTURE HANDBOOK 40, U. S. DEFT. OF AGRICULTURE

A B

FiGURK 33.—Collapse in 4- by 4- by 4S-iiich Jíounlas-ñr .spécimens treated with a

«ater solution: A, At 150 pounds' pressure; H, at 175 pounds' pressure.

Although these specimen.s were coUiqj.sed by the treating conditions

used, less easily injured woods might show no indication of collapse

under the same treatment. Some woods can be treated at much higher

l^rcssures than others, particularly when dry. As previously mentioned,

it is important to keep the preservative pres.sure low enough to avoid

collapse and checking under the temperature conditions necessary for

satisfactory treatment and for the species being treated. In general,

the denser AVOOCIS, such as many of the hardwoods, show less tendency to

check and collapse under high ¡¡ressures than the lower-density woods.

The moisture content of the wocxl is a factor, since hydrolysis is increased when the moisture content is high.

EFFECT OF STEA]MI\G ON STRENGTH AND

CONDITION OF THE WOOD

PHYSICAL

Steaming at temperatures that are too high for the species treated or

steaming for long periods may materially reduce the strength of the

wood (IS, lli). The effect of steaming on the strength of wood is influenced by so many variables that no way has yet been fovmd to determine the loss in strength to be expected in timbers of different size,

shape, or character for any given set of steaming conditions. Furthermore, some strength properties are more affected than others. In using

the steaming process, the aim should always be to employ steam teml^eratures and steaming periods that are as mild as consistent with good

absorption and penetration. Some woods are more adversely affected

than southern yellow pine, but in present practice steaming is not used

much with other species. When it is used for other woods, it is usually

for relatively short periods to help remove excess preservative from the

surface of timbers after treatment or to assist in heating the wood prior

to treatment. In such cases lower temperatures or considerably shorter

heating periods are generally used than are specified for conditioning

southern pine.
PRESERVATIVE TREATMENT OF WOOD 113

Since the eJEfect of steaming on the strength of the wood may not be

evident to the eye, there may be considerable loss in strength in timbers

that appear to be entirely unaffected. Both the visible and the invisible damage should be avoided so far as possible.

From the standpoint of checking and collapse the sapwood of most

species is less likely to be damaged by steaming than the heartwood. For

this reason round timbers, such as poles and piling, particularly those

that are largely sapwood, are able to withstand steaming treatments

that would cause serious checking and collapse in sawed material having

exposed heartwood faces. The end surfaces of round timbers, however,

may check badly in the heartwood, and objectionable ring shakes may

develop if the steaming conditions are too severe. The same is true of

sawed timbers that are largely sapwood but have boxed heartwood.

The ends are exposed to more severe conditions than any other part of

the timber because they are heated in a longitudinal direction as well as

from the sides. Heating also takes place most rapidly from the ends.

In some timbers the growth conditions apparently affect the resistance of the heartwood to checking and collapse. In one instance

sawed timbers of rapid-growth wood were found badly end-checked and

collapsed after steaming and treatment in a commercial charge, whereas

slower-growth material of the same species treated in the same charge

was not visibly damaged. It is possible that other characteristics than

the rate of growth also had an important bearing on the results observed.

Very often wood that has been steamed will appear uninjured if examined immediately after the steaming-and-vacuum treatment, but

serious checking and collapse may occur during the subsequent pressure

treatment. This indicates that the wood has been at least temporarily

weakened by the steaming treatment. The remedy is to lower the pressure of the preservative to a point where appreciable checking and collapse

will be avoided and, if necessary, to lengthen the pressure period to

some extent. Contrary to common belief, reducing the preservative

temperature below 200° F. is not necessary.

In experiments on several species of conifers it has been found that

wood having a high moisture content at the time it is steamed usually

shows less tendency to check and collapse during the preservative treatment than air-seasoned material that has been similarly steamed before

treatment. On the other hand, some woods, like the oaks, usually check

severely even when the timber is green at the time of steaming. Experiments indicate that steaming may increase checking and shakes, depending on the species, on the treating pressures used, and on the steaming

conditions

EFFECT OF BOULTON PROCESS ON STRENGTH

One of the principal reasons for using the Boulton process on Douglasfir and for some hardwood timbers is to overcome the damaging effects

that would result from the higher temperatures normally used in the

steaming process or from boiling in creosote at hii^h temperatures, as

was formerly the practice. With the milder heating conditions used m

the Boulton process, these effects are much less (03). In the use of the

Boulton process, as in the steaming-and-vacuum method, the preservative

temperature and the length of the heating period should be restricted as

much as practicable, consistent with good treatment.
114 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

BLEEDING OF TREATED WOOD

In wood treated with creosote or creosote mixtures the oil may sometimes ''bleed'' or ooze to the surface after the wood is in use and thus

cause inconvenience under certain conditions of service. The phenomenon

is not encountered with water solutions, except perhaps for a very brief

time immediately after removal from the treating cylinder; for the

water evaporates in a short time, leaving no free Hquid that can bleed

from the wood. Bleeding may occur, however, with any oil sufficiently

hquid to flow at atmospheric temperatures. This trouble is sometimes

of concern in poles, cross arms, or timbers used in places where the

dripping preservative or the oily surface may cause damage or public

complaint, but it is seldom of importance in cross ties or piling.

The causes of bleeding are not well understood, but it is apparently

influenced chiefly by the intensity of exposure to sunshine or to other

sources of heat. The dark color of the treated wood causes it to absorb

considerable heat from the sun. Temperatures as high as 140° F. have

been measured at the surface of a creosoted timber in direct sunshine.

It is quite possible that higher temperatures may be reached under

favorable conditions. The bleeding may occur either in summer or

winter, but it is usually greatest in the summer. Although this oozing

of the preservative is usually at its worst during the first year after

treatment, it may occur to a certain extent on some poles over a period

of several years. In a pole or other timber the bleeding will usually be

confined to the surface facing the sun. Apparently, it is a direct result

of the heating and expansion of air and oil in the affected portion and of

the inability of the expanded oil to flow in any direction except toward

the surface. Bleeding, of course, would not occur if the preservative

could not flow at the temperatures reached by the wood in the sunshine.

The character of the preservative used has an important influence

upon bleeding. Straight coal-tar creosote is less likely to cause trouble

than mixtures of creosote with tar or petroleum. Perhaps the mixtures

do not bleed more, but they remain on the surface longer and are more

troublesome that straight creosote. Similarly, low-boiling creosotes are,

in general, less troublesome than high-boihng creosotes or those that

show a high residue above 355° C.

The quantity of preservative injected affects bleeding. Very high

absorptions are more likely to bleed than lower absorptions injected by

the same method. Differences in method of injection, however, must

also be considered. In experiments with paving blocks (58) it was found

that the amount of air in the wood had a very marked effect on bleeding.

Blocks treated by the Rueping process bled much more than those treated

by the full-cell process. Those treated by the full-cell process, after

preliminary steaming and vacuum, bled least. The air used in emptycell treatments of heartwood timbers does not all escape with the completion of the treating process. The expansion of the air that remains

in the wood apparently adds to the effect of the expansion of the oil on

exposure of the treated wood to strong sunshine. This is not such an
PRESERVATIVE TREATMENT OF WOOD 115

important factor in the more easily treated sapwood, since the resistance

to the movement of air through the sapwood is much less than in the

heartwood.

Heartwood and resistant woods in general appear more subject to

bleeding than easily penetrated sapwood, but it is true that sapwood may

also bleed copiously. On sawed surfaces, however, on which both heartwood and sapwood may be exposed, the heartwood seems to have the

greater tendency to bleed. Resistant sapwood is also much more incHned to bleed than that which is less resistant to treatment.

It must not be assumed from the above statements that all creosoted

wood gives trouble from bleeding. Many pole lines and structures of

various kinds have been built of creosoted wood that have given no

trouble whatever from bleeding. The difficulty is not that the wood

always bleeds, but that the treating-plant operator is unable to guarantee

that his treated material will be entirely free from bleeding.

Methods generally used in an effort to reduce bleeding are the application of an expansion bath, as discussed on page 94, or the application

of a final steaming-and-vacuum treatment. When final steaming is

employed, the cylinder is emptied of preservative when the pressure

treatment is completed. Steam is then admitted until a pressure of

10 to 20 pounds is reached, and this is held for 30 minutes or longer,

depending on the steam temperature used and the material treated. A

final vacuum is applied after the steam treatment. This final steaming

treatment, like the expansion bath, is intended to reduce the amount of

air in the wood cells and to remove some of the free oil near the surface.

It also helps to clean off the surface of the wood.

A third method employed to a very limited extent by a few plants in

an effort to reduce bleeding has been to use a final air pressure after

the preservative has been withdrawn from the cylinder. Air at pressures as high as 150 pounds or more has been applied for a period of 3 to 4

hours for the purpose of forcing the free oil farther into the wood. There

is considerable question whether the use of a final air pressure would be

effective as a means of reducing bleeding. As far as known, adequate

experiments have not been made to settle the question.

A considerable amount of observation and study will be necessary to

catalog and determine the relative importance of the several factors that

encourage bleeding and to find the most effective means of preventing it.

TREATING CONDITIONS USED IN COMMERCIAL

PRACTICE

Tables 18 to 20, inclusive, summarize the commercial treating methods

employed for various species and different kinds of material. These

tables include data obtained from a large number of the wood-preserving

plants operating in the United States, as well as from some of those in

Canada, and show representative treating practices for different regions.

These data were obtained in 1949 and 1950. The diversity m conditions

used in treating the same or similar material reflects some of the lack of

information and difference of opinion as to conditions that are considered most satisfactory.
116 AGRICULTURE HANDBOOK 40; U. S. DEPT. OF AGRICULTURE

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PRESERVATIVE TREATMENT OF WOOD 121

TABLE 20,—Coinmercial treating conditions and retentions in use for

species m the Eastern and Southern States

AIR-SEASONED TIES ^

Species treated

and number of

plants studied

Pine, southern

yellow:

2

1

i__. _

1

1

1

1

2

3

Gum:

2

1

1

1

1

Oak:

1

1

1

1

1

1

1

1

1

1

Preservative

Coal-tar

creosote

Percent

100

70

2 25

3 50

70

100

100

GO

100

100

SO

100

70

80

70

60

80

100

100

70

100

100

100

80

60

80

80

Coal tar

Percent

30

30

'4Ó

20'

30

20

30

40

20

Preliminary

air pressure

Maximum

treating

pressure

Preservative

temperature

30

20

40

20

20

Pounds per

square inch

70- 90

90-100

30

45 90

65- 90

90-100

75- 80

75- 80

60- 85

60

70- SO

80

65- 80

100

65- 80

75- 80

60

25

70

90

50

80

80

70

20

Pounds per

square inch

175

200

125

200 I

200

200

200 '

200

175

200

200

175

200

200

200

195-200

200

175

200

200

175

200

175

225

225

200

200

200

212

195-200

200

200

200

200

200

200-210

200

200-210

200

200-210

200-210

200

200

200

200

200

200

190-200

200

200

205

205

200

200

Net

retention

Pounds per

cubic fool

6,8

6

10

Wo

7 "

8

6

8

6,8

8

8

6

6,8

10

7

6

6,7

7

6

6

8

8

8

STEAMED ( TREEX ( 'ROSS ARMS

Pine, southern

yellow:

1

1

1

100

100

100 '"''"'

60 :

80 :

60 1 1

200

200

175

200

200

200

10

6

6,8

STEAMED GREEN LUMBER AND TIMBERS ^

Pine, southern

yellow:

100

100

100

100

100

100

100

100

50

70-75

60-70

50-70

40-75

25-50

40-50

50

140

150

175

175

150

175

200

200

210

185-200

200

200

200

200-205

205

200

8

8

8-12

1 X 8,10, 12

8,10

8-lC,

12

12

STEAMED G^EEN I *( >ST>

Pine, southern

yellow :

100

100

100

100

100

50

70-80

75-80

70-75

85

130

150

150

150

200

200

185-200

200

200

200

6

8

6

6

8

A9 yy?6*iii.
122 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

TABLE 20.—Commercial treating conditions and retentions in use for

species in the Eastern and Southern States.—Continued

AIR-SEASONED POSTS "^

Species treated

and number of

plants studied

Preservative Preliminary

air pressure

Maximum

treating

pressure

Preservative

temperature

Net

Coal-tar retention

creosote Coal tar

Pine, southern

yellow:

1

Percent

100

100

Percent

Pounds per

square inch

60

80

Pounds per

square inch

175

180

200

200

Pounds per

cubic foot

8

1 5

STEAMED GREEN PILING AND POLES 8

Pine, southern

yellow:

4 100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

70-80

50-60

40

50-60

40-80

50-60

40-50

20

75-80

70-80

70-75

40-50

60-80

60-70

35

55

50-60

{')

80

60-75

50

175

175

175

200

200

160

160

200

250

150

175

200

200

175

200

125

175

175

175

175

175

175-200

190-200

200

200

190-200

210

190-200

190-200

200

200-210

185-200

200

200

200

200-210

200-205

200

200-210

185-200

185-200

200-210

200-205

200

8

3 8,10

1 12

2 8,10,12

1 8,12

1 8

1 10,12

1 18

1 8

2 8

2 8,10,12

1 10,12

1 8

3 16,18, 20

4 8,10

1 12

3

2

1

8,10

8,10

16

1 __ .__ 8

2

1

8,10

8,10,12

AIR-SEASONED POLES ^^

Pine, southern

yellow:

1 100

100

100

100

100

80-90

60-80

75-80

70

70-80

175

175

200

200

200

200

205

200

205

200-210

8

1 8

1 8,10

1 8,10

2 8,10

1 Pressure periods for southern yellow pine ties, 2 to 4 hours; gum ties, 2 to 5 hours;

oak ties, 4 to 6 hours. Final vacuum periods range from 3^ to maximum of 2 hours,

with average about 1 hour.

2 Remaining 75 percent is petroleum oil.

3 Remaining 50 percent is petroleum oil.

^ Lowry process.

^ Pressure periods, 1J^ to 2 hours; final vacuum period, J^ hour.

6 Pressure periods, 2 to 6 hours; final vacuum periods, }^ to 23^ hours.

^ Pressure periods, 1 to 2}^ hours for green and seasoned posts; final vacuum periods,

J/^ to 1 hour.

8 Pressure periods for poles, 2 to 4 hours; piles, 4 to 6 hours. Final vacuum periods,

3^ to 1 hour.

9 Full-cell treatment.

10 Pressure periods, 2 to 4 hours; final vacuum periods, % to 1 hour.
PRESERVATIVE TREATMENT OF WOOD 123

SPECIFICATIONS FOR TREATMENT

GENERAL CONSIDERATIONS

A large proportion of the wood that is treated commercially is treated

under purchasers^ specifications. Sometimes the specifications employed are the general specifications developed by various associations

that are mterested. In other cases the purchaser develops his own

specifications and puts into them the provisions that he considers important. Frequently, whatever the source, the specification fails to

protect the purchaser or contains provisions that are actually harmful

to the wood or unnecessarily increase the cost of the treatment. Competent inspectors placed at the treating plants by the purchaser can

enforce compliance with most of the requirements of a specification,

¡Dut even then the treatment will be unsatisfactory unless the specification

is properly prepared or the treating-plant operator gives good treatment despite the specification.

As far as is practicable, the author of a treating specification should

take into consideration the following items: Species, form, and dimensions of the timber to be treated, proportion of heartwood and sapwood, degree of seasoning at the time of treatment, method of seasoning

or conditioning for treatment, purpose for which the treated timber is

to be used, kind of preservative, retention, penetration, and the details

of the treating process. The specification, however, should avoid unnecessarily fixing the details of the treating operation. Provision should

be made against the use of temperatures, pressures, and treating periods

that are Hkely to damage the wood, but within these limits the plant

operator should be given as much freedom as practicable.

As far as possible, the specification should cover the finished product

rather than minor details of the treating process. It should also clearly

define the methods that will be used in judging the quality of the finished

product. ^ In developing new specifications or improvements to old

specifications, there should be cooperation and thorough understanding

between the purchaser and the treating-plant operator to be sure that

the requirements of the specification are reasonable and can be fulfilled.

The general specifications that have been adopted by the Federal Government, the American Wood-Preservers' Association, and other associations afford a good starting point, but they are necessarily quite general

in character and must often be modified or limited in some respects in

order to meet the needs of the individual purchaser.

AVOIDING INJURIOUS TREATING CONDITIONS

While the knowledge of wood-impregnating technic is still too incomplete to show the dividing line between safe and unsafe conditions,

enough is know^n to permit some definite limitations. In the steamingand-vacuum process of conditioning green timber, various steaming

pressures and lengths of steaming periods have been used. With our

present state of knowledge, however, there appears to be no good reason

why steaming pressures higher than 20 pounds' gage pressure (about

259° F.) should be employed in treating green pine timber. No advantage that might be gained by higher pressures is known that would

offset the greater danger that the higher temperatures might damage the
124 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

timber. Further study and experience may show that the use of pressures lower than 20 pounds will be practical and preferable for the pines.

With other species, specifications customarily greatly limit the use of

steaming and usually do not permit higher steam temperatures than

240° F.

The data given in figures 13 to 22 can be useful as a guide in selecting

a steaming period. From these temperature curves it may be noted

that material of small dimension, such as that 3 inches or less in thickness, can be heated in a very short time, so that the temperature at the

center is nearly equal to that of the outside steam temperature. With a

steam pressure of 20 pounds, about 4 to 5 hours of steaming should be

ample for timbers about 6 to 7 inches in diameter, about IJ^ to 2 hours

for lumber 2 inches in thickness, about 2 to 3 hours for lumber 3 inches

in thickness, and 3 to 4 hours for material 4 inches in thickness. The

foregoing time intervals assume that full steam temperature is applied

at once. In actual practice, from 3^ to 1 hour additional may be required

to reach the maximum steam temperature. It is important that sawed

material be properly spaced or ''stickered'' in loading the trams so that

the steam can circulate freely around every piece; otherwise longer steaming periods will be required and uniform distribution of heat throughout

the charge may not be obtained.

In the larger sizes it becomes impractical and also unnecessary to

heat the center to a temperature approaching that of the heating medium.

It is impractical because of the very long heating periods required and of

the injury that might result to the wood. It is unnecessary because the

heat at the interior of the large timber cannot be very effective in evaporating moisture during the subsequent vacuum period. Although it is

not known just what the wood temperature should be at a given distance

from the surface to obtain the most effective results in steam conditioning,

it is probable that very little would be gained by heating the wood much

above 200° F. at a distance of about 3 inches from the surface. In some

of the larger sizes of timber it would be necessary to be satisfied with even

lower temperatures at this depth because of the undesirably long steaming periods otherwise required. If particularly long steaming periods

are employed, they should be used only with the full knowledge of both

operator and purchaser that the risk of damage increases as the steaming period is lengthened. In steaming timbers to sterilize the wood, it is

important to bear in mind that the temperature at the central portion

will rise to a considerable extent after cooling starts, depending on the

surface temperature conditions. This is discussed on page 67.

When conditioning wood by the Boulton process, time and temperature are again the important factors to control in avoiding damage. The

specifications of the American Wood-Preservers' Association permit a

maximum oil temperature of 220° F. during the boiling-under-vacuum

period when treating green Douglas-fir poles and piling, and of 210° F.

when treating Douglas-fir lumber and sawed timbers. In the light of

present information these temperatures should not be exceeded when

conditioning green wood by the Boulton method.

Treating pressures specified should be maximum allowable pressures;

not required pressures. The plant operator should be allowed to use any

pressure within the maximum limit that will give the required retention

and penetration and avoid damage to the timber.
PRESERVATIVE TREATMENT OF WOOD 125

The maximum pressures employed in practice for various species are

given in tables 18 to 20 inclusive. Some of these pressures, however,

have been found to be too high if applied for a considerable period or

when used with preservative temperatures that are more favorable for

treatment. With preservative temperatures of 195° to 200° F., gage

pressures of 100 to 125 pounds are generally as high as should be used

in the full-cell treatment of low-density species such as the spruces and

true firs. This also applies to other species that show susceptibility

to checking and collapse during treatment. In the empty-cell treatment

of such woods the maximum preservative pressure will depend somewhat

on the initial air pressure, but it should always be less than an amount

equal to the pressures recommended for a full-cell treatment plus the

initial air pressure. For example, if an initial air pressure of 50 pounds

per square inch is used, the preservative pressure in most cases should

not be more than 150 pounds as a maximum with low-density woods or

with those that are susceptible to checking and collapse. The more

dense woods can, of course, withstand somewhat higher pressures, but

if it is found that collapse or checking occurs during treatment, the preservative pressure should be lowered.

SELECTION OF TREATING PROCESS

There is often considerable misunderstanding regarding the relative

merits of the full-cell and the empty-cell methods of treatment. The

effectiveness of treatment depends upon the preservative, and the retention and depth of penetration, and not upon the treating process,

except as the process used may affect the penetration and the retention

specified.

The terms ''full-celF' and '^empty-cell,'' as applied to treatment, are

very misleading, since the so-called full-cell process does not leave the

wood cells completely filled with preservative even when an effort is

made to obtain this objective, nor does empty-cell treatment leave the

cells empty. As previously mentioned in the discussion of the effect of

initial air on penetration, the empty-cell process loses much of its value

when applied in the treatment of wood that is fairly resistant under

normal pressure-treating conditions. The empty-cell treatment is most

effective when employed in the treatment of reasonably permeable sapwood, such as that of the pines or the penetrable heartwood of species

like the red oaks, black túpelo, or ponderosa pine. In selecting the

treating process, the object in all cases should be to obtain the maximum

penetration practicable with the absorption specified.

This important consideration has been frequently overlooked in many

specifications. Some specifications formerly used required full-cell treatment of timbers that were largely sapwood, and with this stipulated

that all sapwood be penetrated. These specifications at the same time

named net retentions that would not permit complete sapwood penetration by the full-cell method. An empty-cell treatment should be employed whenever the penetration can be improved thereby, and the

purchaser harms himself in restricting the plant operator to the use of the

full-cell process when a limited retention is specified. Full-cell treatment

should be specified only when, because of a limited amount of sapwood,

resistance of the material, or for other reasons, the specified net retention

cannot be obtained by an empty-cell treatment.

207777°—53 9
126 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

In preparing specifications, it is also important to specify retentions

that are not too low to give the required penetration. For the same

depth of penetration, resistant material such as heartwood timber or

round timbers with resistant sapwood generally require a higher concentration of preservative than material that is easily penetrated. Even

when an initial air pressure is used, only a limited kick-back can be obtained from resistant wood.

UNNECESSARY REQUIREMENTS

A serious fault with some specifications is that they attempt to fix all

the details of treatment regardless of the fact that, even when the same

species and the same class of timber are treated, each charge may require

some modification of the treating conditions to obtain satisfactory results. For example, in the treatment of green material some timbers

will require different steaming or boiling-under-vacuum periods, depending on the size of the timber, moisture content, amount of sapwood and

heartwood, and other variables. Similarly, a certain degree of latitude

should be permitted in the treating pressures and pressure periods employed because pressure conditions that give good results for some

timbers may prove unsatisfactory for others. Specifications, however,

should set maximum limits on the steaming pressure, steaming periods,

preservative pressures, and similar treating operations that may affect

the condition of the timber.

An attempt has sometimes been made to insure deep penetrations by

specifying the gross absorption as well as the net retention. Since the

net retention may vary widely for any given gross absorption, such

specifications are impractical and often impossible to meet. Again, if

the sapwood is not very deep or if the wood has a high moisture content

when treated, it may be impossible to get the required gross absorption.

In any case, it is suflScient to specify the net retention and whatever

penetration can reasonably be required.

Specifications sometimes severely limit the amount of sapwood permitted in the timber. As a rule, this merely adds to the cost of the

timber, makes it more difficult to get, and may result in poorer treatment

than if no limit is placed on the amount of sapwood. This provision is

evidently carried over from specifications covering timber to be used

without preservative treatment, when the proportion of heartwood had

a marked inñuence on the decay resistance of the wood. Preservatives,

however, make sapwood as durable as heartwood; and, since sapwood

takes better absorptions and penetrations than heartwood, there is

seldom good cause to require a high percentage of heartwood in timber

that is to be treated. Since the mechanical properties of sapwood average

as high as those of heartwood, strength requirements afford no basis for

discriminating against sapwood.

With good vacuum pumps, long vacuum periods are apparently unnecessary at any stage in the treating operation with the exception of

the boiling-under-vacuum process. Preliminary vacuum for full-cell

treatment of air-seasoned material needs to be extended little, if any,

beyond the point at which approximately the maximum vacuum is

reached. In order to allow sufficient time for the excess preservative to

drip from the wood, final vacuums after full-cell or empty-cell treatments

sometimes need to be held longer than a preliminary vacuum.
PRESERVATIVE TREATMENT OF WOOD 127

The vacuum applied after steaming should not be held longer than

necessary to obtain a practicable moisture reduction, since after a time

the effect is merely to cool the wood to a less favorable treating temperature without appreciably lowering the moisture content. Sufficient experimental work has not yet been done to determine the most effective

vacuum periods for various sizes of timber, but the data at hand indicate

that some plants use vacuum periods that are appreciably longer than

needed to accomplish the best results.

RETENTIONS

The net retentions that should be specified for various types of material

such as ties, poles, posts, piling, lumber, and structural timbers of different

types will depend on a number of variables. These include kind of

material, dimensions, species, service conditions, preservative used,

amount of sapwood and heartwood in the timbers, the degree to which

the wood is subject to mechanical wear, kind of wood-destroying agencies

to which the material is exposed, cost of renewal, possibility of obsolescence, and the like.

The tendency of some purchasers is to specify relatively low retentions

in the hope of reducing the cost to the lowest safe minimum. This proves

to be false economy in many cases, for the minimum safe retention

cannot be named with exactness for any form or use of treated timber.

The cost of a little additional preservative is relatively small in comparison

with the total cost of the treated timber in place in the finished structure,

and the saving in cost that may result from the use of low retentions is

very small in comparison to the risk of poor service from inadequate

treatment. When long service is required, it is better to specify retentions somewhat above the minimum in common practice.

Most of the general specifications, such as those of the American WoodPreservers' Association, state the mininumi retention required. Engineers preparing specifications for the treatment of timbers, however,

may overlook the fact that the figures stipulated are minimum values and

may specify retentions even lower than the minimum named. Although

it is desirable to avoid the use of retentions that are higher than necessary

for satisfactory service and economical treatment, it is important to

consider all the variables involved, for, under certain conditions, retentions considerably higher than the minimum may be needed from the

standpoint of economy in use.

It is quite natural for one to assume that most timbers in a charge

will have about the average volumetric retention specified. This assumption, however, is not justified because of the large number of variables

that affect the results. There is often a \\ide variation in both the

retention and the penetration obtained in individual timbers in the same

charge; some will have considerably less and others considerably more

than the average retention or penetration for the charge as a whole.

Some of the important factors that affect the treatment of individual

timbers are differences in dimensions, presence of incipient decay in

some pieces, differences in the temperature of the preservative in contact

with the various pieces while pressure is api:)lied, variations in moisture

content, variable growth conditions that affect the relative resistance to

penetration of the preservative, proportion of sapwood and heartwood,

depth of sapwood, type of preservative used, method of treatment employed, and similar variables.
128 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Because of these variables it is not practicable to conclude from sample

timbers or from sections cut from sample pieces that the charge has

received the specified average retention. Even a careful determination

of the amount of preservative in the treated portion of the sample would

be of little value in estimating the average retention per unit volume of

the entire charge with which the sample timbers were treated. In

other words, it is necessary to determine the average retentions at the

time of treatment, for there is no satisfactory way to obtain this information at a later date. Penetrations, however, can usually be measured at any time, especially when preservative oils are used, and this is

one of the most important means of determining whether timbers have

been properly treated.

Following are some of the advantages of specifying retentions that are

higher than the minimum it is believed will be ''just enough.''

(1) The higher retentions provide a better reserve against depletion

by leaching and evaporation.

(2) They usually result in better distribution and deeper penetration

of the preservative.

(3) When the preservative has an uncertain toxicity, the heavier

absorptions help protect against a deficiency in this property.

(4) There is less danger of inadequate retention because of careless

treatment.

(5) Good retentions help protect against the possibility of not providing sufficient treatment for the more resistant timbers in a charge.

Likewise, they help protect against failure to give sufficient treatment to

timbers of the larger sizes when the charge contains timbers of different

dimensions.

(6) Ample retentions reduce the danger of wide variations between the

maximum and minimum penetrations obtained in the same or in different

timbers.

The principal disadvantages of using heavier retentions are: The

initial cost of treatment is somewhat higher and there is a possibility that

objectionable bleeding will take place when oily preservatives are used,

as in the case of poles. A higher initial cost of treatment, however, is

not necessarily a criterion of the ultimate cost. Under certain circumstances, somewhat higher retentions than commonly specified may

result in marked economy by substantially increasing the life in service

and thereby reducing the annual charge.

Ties

Coal-tar creosote, solutions of coal-tar creosote and coal tar, and

solutions of coal-tar creosote and petroleum oil are the preservatives

most widely used for ties. The preservative solutions containing petroleum or tar have been extensively used for many years to help reduce the

cost of the preservative and have proved very satisfactory. Table 21

shows the preservatives used and the average retentions specified by

various railroads in the United States and Canada as found in a survey

made in 1948.

The creosote-petroleum mixtures are extensively used in the Far

Western and Rocky Mountain States, and the solutions of creosote and

coal tar in the Eastern and Southern States. Most of the ties impregnated with preservative oils are treated by the empty-cell method
PRESERVATIVE TREATMENT OF WOOD 129

TABLE 21—Preservatives and absorptions used for ties hy various railroads

in the United States and Canada in 194-8 ^

Number

of Coaltar

creosote

Mixtures of

coal-tar creosote

and petroleum

Mixtures of

coal tar creosote

and coal tar

Specified retention

railroad

companies Coaltar

creosote

Petroleum

Coal

tar

creosote

Coal

tar Per cubic foot Per tie

1

Percent

100

100

100

100

Percent Percent Percent Percent Pounds

5.0

6.5

7.0

8.0

10.0

7.0, 8.0

7.0

8.0

8.5

9.0

9.5

8.0

8.0-10.0

11.0

9.5

6.0

7.0

8.0

Gallons Gallons

2

1

2

1 30

50

50

50

50

50

50

40

40

70

25

70

50

50

50

50

50

50

60

60

30

75

3

5

6

2

80

80

80

60

60

60

60

60

60

70

70

70

70

70

50

50

50

50

50

40

20

20

20

40

40

40

40

40

40

30

30

30

30

30

50

50

50

50

50

10

0.86

2.5-3.Ö

7.5

7.0-8.0

8.0

6.0

8.0 2.5-3.Ô

2.5

0.75

7

6.0-8.0

2.5

8

6.0-7.0

9.5

40 10 3.Ó

1 Preservative Survey, 1948. Amer. Ry. Engin. Assoc. Bui. 474, p. 402. 194S.

(Lowry or Rueping process), and the specified net retentions usually

vary from about 5 to 8 pounds per cubic foot. Retentions of 5 to 6

pounds appear to be too low for ties that can be deeply penetrated, such

as red oak or sapwood material.

Although the empty-cell treatment may give good penetrations when

the wood has been properly seasoned, the possibility of erratic penetrations is great when low net retentions are used. The chance of erratic

penetration decreases with higher retentions. The question also arises

whether, even with complete penetration, the concentration ot pre207777°—53 10
130 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

servative with these low retentions is sufficient to protect the ties over

the period of service that should be expected of properly treated wood.

Ties with a considerable proportion of heartwood, treated with the

lower retentions, would naturally show a greater concentration of preservative in the treated part than those that take practically complete

penetration. The life obtained from the heartwood ties having a heavy

concentration of preservative might be much longer than would be

obtained from sapwood material or woods that take practically complete

penetration, unless the absorption in the latter was increased to compensate for the greater amount of wood treated. In view of the unsatisfactory results that have been noted in some installations of ties

treated with the low net retentions of 5 to 6 pounds of preservative oil

per cubic foot, it is recommended that, for general use, specifications

should require a net retention of at least 8 pounds per cubic foot in ties

having 50 percent or more sapwood or when the species is one that can

be well penetrated in the heartwood.

If the ties are used under tropical or semitropical conditions, minimum

retentions of 10 to 12 pounds of preservative per cubic foot, or treatment with the maximum absorption obtainable, are recommended regardless of whether the creosote is used alone or in mixtures. Some specifications call for 5 to 6 pounds of creosote per cubic foot on the assumption

that the greater toxicity of creosote, as compared with that of creosote

mixtures containing coal tar or petroleum, will permit these lower retentions when creosote alone is used. Successful results are not assured

by a consideration of toxicity only, since permanence plays a very important part. Data on leaching and evaporation indicate that doubling

the retention should increase permanence fourfold.

Marine Timbers

The protection of wood against marine borers is a much more difficult

problem than protection against decay and insects and requires much

higher concentration of the preservative. The Forest Products Laboratory has tested a large number of preservatives to study their effectiveness

in protecting wood against marine borers {41^ 4^)- Results obtained in

these experiments, as well as experience in general, have shown that

heavy retentions of coal-tar creosote are essential if the best protection

is to be obtained. The heavy retentions insure better penetrations and

also furnish a reserve supply of creosote to provide against early depletion

by leaching. Over much of the coastal region of the United States,

marine timbers are exposed to severe borer attack, and it is poor economy

to specify retentions that will not give the maximum protection under

such conditions. Specifications for such timbers should require treatment to refusal by the full-cell process, and the specified retention should

be the minimum that wiU be accepted. No maximum should be specified.

The species most commonly used for marine timbers in the United

States are southern yellow pine and coast Douglas-fir. Southern yellow

pine piling treated with the maximum retention practicable by the fuUcell process usually has average net retentions of 20 to 25 pounds of

creosote per cubic foot, provided the moisture content of the wood is

not too high at the time of treatment. Since the depth of sapwood on

Douglas-fir piling is usually less than that on southern yellow pine,

the net retentions are somewhat less in the Douglas-fir for the same
PRESERVATIVE TREATMENT OF WOOD 131

treatment. The maximum retention that can be conveniently obtained

m tha Douglas-fir piling may vary from about 12 to 16 pounds per cubic

foot, depending on the size, moisture content, and depth of sapwood.

Posts

Although posts are exposed to service conditions similar to those

under which poles are used, the cost of post replacements is small compared with the cost of pole renewals. Furthermore, unlike poles, posts

are relatively cheap, and they can usually be replaced at any convenient

time and without use of the special equipment and skilled labor commonly required for poles. Preservative retentions specified for posts

are therefore usually somewhat less than those for poles and similar

timbers that have a high initial cost and that are difficult and costly to

replace.

Since posts are exposed to conditions in which leaching is important,

preservative oils are generally specified for post treatment. Preservative

salts or some of the chemicals carried in suitable nonaqueous solutions

may be used when the posts are to be painted. Minimum retentions of

about 6 pounds of coal-tar creosote or of creosote and coal-tar solution

per cubic foot, or about 7 pounds of creosote-petroleum solution, are

usually specified. Some plants, however, treat southern pine posts

with retentions of 8 pounds of creosote per cubic foot. Specified retentions of preservative salts or chemicals carried in a nonaqueous solution vary, depending on the preservative used.

Poles

Prior to World War II, most of the pressure-treated poles used in

the United States were treated with American Wood-Preservers' Association specification grade 1 coal-tar creosote with a specified distillation

residue above 355° C. of not more than 20 to 25 percent.

In recent years solutions of pentachlorophenol have attracted attention

as substitutes for creosote or for use in mixtures with creosote, and

large quantities have been used. Thousands of poles have been treated

with pentachlorophenol dissolved in the lighter petroleum oils or with

solutions containing various proportions of coal-tar creosote and pentachlorophenol dissolved in a petroleum-oil solvent. These poles have

not been in service for sufficient time to determine how the results will

compare ultimately with those obtained from creosoted poles. Experimental installations under observation, however, are giving excellent

results, so that this preservative may find a wide field of use in the future.

Most of the poles that have been pressure-treated and on which the

best service records are available, are southern yellow pine and coast

Douglas-fir. Of these two woods, southern pine has been much more

extensively used. Other pine species have also been used to a variable

extent during recent years, particularly lodgepole, red, jack, and some

ponderosa. Several pole users have also started using pressure-treated

western larch poles in varying quantities.

Retentions of 8 to 10 pounds of preservative oil per cubic foot for

southern yellow pine and about 8 pounds per cubic foot for coast Douglasfir have been commonly specified. Poles with these absorptions are
132 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

treated by the empty-cell processes. Some specifications require retentions of 12 to 14 pounds per cubic foot in southern yellow pine and

10 to 12 pounds in coast Douglas-fir poles.

In some cases lodgepole pine, jack pine, and western larch poles have

been treated with retentions of 4.5 to 6 pounds per cubic foot, although

8-pound retentions are more commonly specified. The low retentions

have been used partly because of the thinner sapwood in poles of these

species and partly because many of the poles are used in regions where

decay conditions are not so severe, as in the South where southern pine

poles are most extensively employed. Another reason for using the

lower retentions is to reduce the possibility of objectionable bleeding as

much as possible. Some poles treated with these lower retentions have

given good results, but most of them have not been in use a sufficient

time to show what service life will be obtained.

Piles

The principal woods used for piling are southern pine and coast Douglasfir, although a few other woods, such as red pine, lodgepole pine, western

larch, and oak, are used in some localities.

Specifications for most species used for land piling generally require

retentions of about 10 to 16 pounds of creosote solution, creosote and

coal-tar solution, or creosote-petroleum solution per cubic foot of wood.

When particularly heavy concentrations of the preservative are needed,

retentions should be specified that will insure good sapwood penetration

with the full-cell treatment.

Pile renewals are very expensive compared with renewal costs of

many other types of timber, such as ties or medium-size poles. In fact,

the cost of renewal may be considerably more than the cost of the treated

timber. Furthermore, as in the case of bridge structures, traffic obstruction that commonly occurs while repairs are being made, may be an

important problem. For this reason it is not good economy to specify

retentions that may later prove to be too low to insure a satisfactory

service life.

Lumber and Timbers

Coal-tar creosote and its mixtures are commonly employed for sawed

material, such as bridge timbers, used under relatively severe conditions. Retentions specified for such timbers vary from about 6 to

20 pounds per cubic foot, about 10 to 12 pounds being most common.

Both empty-cell and full-cell methods are employed, depending on the

amount of sapwood, retention required, size of timbers, and similar

factors. The full-cell process is commonly employed in the treatment

of resistant heartwood timbers and timber for use in salt water.

Water-borne salts are widely applied in the treatment of sawed lumber

under conditions that make it impractical to employ preservative oils.

Specifications for retentions of both preservative oils and water-borne

salts often fail to take into consideration the relation of the timber

dimensions to penetration and retention (p. 96). Frequently the treating

plant has been blamed for unsatisfactory treatment when the fault lies in

the specification, which does not call for a sufficient retention to insure

a good penetration. Again, the specifications may require a net retention

in large heartwood timbers that cannot be obtained because of the small
PRESERVATIVE TREATMENT OF WOOD 133

ratio^ of surface area to volume, although the same retention might be

obtained without difficulty in heartwood timbers of tie size or smaller

or in large-size timbers containing a large proportion of sapwood. Table

16 will be found useful in estimating the proportional retention for various

sizes of heartwood timbers. In timbers containing 50 percent or more

sapwood, it is recommended that at least 10 pounds of preservative oil

be specified per cubic foot of wood, i^

Small and large sizes of heartwood timber should not be treated in

the same charge if it can be avoided, but sometimes the total volume

of timber in an order is less than enough for one charge and it would be

uneconomical to make more than one charge. In such cases the average

absorption should be calculated on the proportions of small and large

sizes, as shown on p. 101.

PENETRATION

Whenever practicable, the minimum acceptable penetration should

be specified, but this should be done with considerable care or the requirement may be impracticable. Complete sapwood penetration is

obtained many times in various sizes and shapes of timber, but a strict

enforcement of this requirement on every piece of timber treatedwould generally prove impractical. There should be a definite understanding between

purchaser and plant operator as to what will be acceptable and what

will not. A complication in this connection is that it is not always easy

to locate in treated timber the dividing line between heartwood and sapwood. To say that ^^as much of the heartwood as practicable shall be

penetrated'^ is too indefinite and adds nothing of value in the specification.

It is possible to require minimum penetrations in heartwood faces that

are practical to obtain by good treating methods and that are suitable

for the service desired of the timber. This is not often done, however,

except in incised timber. It is, of course, impractical to demand deep

penetrations in boards or timber of small dimensions when insufficient

retentions are specified. This is especially true for heartwood material

because of the smaller amount of kick-back from heartwood in comparison with that obtained from sapwood.

In the treatment of some kinds of timber, such as posts, ties, and

lumber, it is not practical to make penetration measurements on all

pieces. For such material it is common practice to specify that a certain

number of pieces from each charge shall be bored for penetration measurements and that some high percentage of them shall meet the minimum

penetration specified. In such material it is not always practical to

require that every piece meet the minimum penetration requirement.

In larger, more costly material, however, it is often required that every

piece be bored for penetration and that only those meeting the minimum

requirement shall be acceptable.

Penetrations of creosote and other dark-colored oils can be measured

on increment borings or bit holes made at a sufficient distance from the

ends of the piece to escape the effect of end penetration. The oil has a

tendency to creep over the surface of the wood in a short time, so that

the observation should be made promptly after boring. With sufficient

14 Specifications of the American Wood-Preservers' Association and Federal specification TT-W-571C include various preservative salts and give retentions recommended.
134 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

skill and a sharp knife or blade, the core taken out by the increment

borer may be split open lengthwise and a more accurate determination

made of the penetration. This avoids the effect of oil creeping along the

surface during the boring operation.

The benzidine stain is very satisfactory for showing the demarcation

between the heartwood and sapwood of the pines. This is prepared

in two separate solutions as follows: Solution 1 is made by dissolving

1 gram of benzidine in 5 grams of 25-percent hydrochloric acid and 194

grams of water. Three parts of commercial hydrochloric acid mixed with

1 part water will give the 25 percent strength required. Solution 2 is

made by dissolving 20 grams of sodium nitrite in 80 grams of water.

When ready to use, mix 50 percent of solution 1 with 50 percent of

solution 2 by volume. Immerse the increment-borer core in the mixture

for a few seconds or spray the mixture on the surface of the timber to

show the sapwood region. The heartwood turns a dark red, while the

sapwood is yellow.

The penetration on borings from timber treated with water-borne

salts can usually be shown by spraying the boring with a solution that

gives a distinct color reaction with the preservative. If a timber treated

with water-borne salts is sawed for penetration measurements before it is

seasoned, the saw may carry some of the solution over the cut surface.

This could very easily give an indicated penetration much deeper than

is actually obtained. Even well-seasoned wood may have particles of

sawdust carried over the surface from the treated portion, which would

give misleading results. It is therefore important to guard against conditions that will spread the preservative over unpenetrated wood when

the timber is either bored or sawed for penetration measurements.

All holes made in treated timber for observing penetration should be

tightly plugged with thoroughly treated plugs.

FRAMING AND BORING

Insofar as is practical, the specifications should provide for complete

framing and boring before treatment. This is much more practical than

is ordinarily supposed (P, 62). Cutting into timber after treatment is

very likely to expose untreated wood and thus to permit the entrance of

decay or insects beneath the treatment. Cutting after treatment is

especially dangerous in timber that will be exposed to marine borers.
APPENDIX

SYMBOLS USED IN FORMULAS RELATING TO PHYSICAL

PROPERTIES OF WOOD

C = Percentage shrinkage per unit volume.

D = Average diameter of timber in inches.

M = Percentage moisture based on oven-dry weight,

m = Percentage moisture based on original weight.

Ml and Mo = Percentage moisture-content values below the fiber saturation point where moisture-content range is Mi to M2, and Mi is

less than M2.

Mf = Maximum percentage moisture wood will hold when all void

space is filled with water.

Mh = Percentage moisture of heartwood (when considered separately

from that of the sapwood) based on oven-dry weight.

Ms = Percentage moisture of sapwood (when considered separately from

that of the heartwood) based on oven-dry weight.

P = Percentage air space in wood.

Py; = Percentage volume occupied by water in wood.

Ps = Percentage sapwood in timber based on total volume.

py, = Density of water in wood.

S = Specific gravity based on the weight when oven-dry and the volume

at current moisture content.

Sa and Sb = Specific gravity values when wood has moisture contents of

Ml and Mo, respectively. Sa greater than AS^ and Mi less than Mo.

S¿ = Specific gravity based on the weight of the oven-dry wood and the

volume when oven-dry.

Sg = Specific gravity based on the weight of the oven-dry wood and the

volume when green.

Sh = Specific gravity of heartwood (when considered separately from

that of the sapwood).

Ss = Specific gravity of sapwood (when considered separately from that

of the heartwood).

T = Average thickness of sapwood in inches.

W = Original weight of moisture specimen.

Wa = Weight of moisture specimen when oven-dried.

^"Pf^ = Weight of oven-dry per unit volume at moisture content M.

"if^ = Weight of water in wood at moisture content M (weight per unit

volume).

Wo = Weight per unit volume of wood at moisture content M = weight per

unit volume of wood before oven-drying.

f7^ = Weight per unit volume of water at maximum density. {Ww= 1 in

C.G.S. system and equals 0.0361 pound per cubic inch or 62.4

pounds per cubic foot in the English system.)

lAQ = Spedñc gravity of wood substance determined in helium gas.

1.53 = Specific gravity of wood substance determined in water.
136 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

FORMULAS RELATING TO PHYSICAL PROPERTIES

OF WOOD

Moisture Content in Percent

Moisture content based on weight when oven-dry = M

Moisture content based on original original weight (weight (weight before oven-drying) oven-drying) =m

m=100|

M expressed in terms of m

m expressed in terms of M

,, 100 m ._.

100 —m ' ^ ^

100 M

Maximum Moisture Content in Percent

(All air space filled)

M,= 10o[ (w.-'^^^W^ =IOo(l-jÍ3) (5)

If the weights are expressed in pounds Dund per cubic foot,

Ms =M^^-^^ (6)

Specific Gravity

Specific gravity based on weight when oven-dry and volume at current

moisture content=ä

'S = ^= -7 iTFY- (7)

W

If the weights are expressed in pounds per cubic foot,

^ = 621 = 62.4/ ° -^ ^^^

V 100/

Specific gravity at any moisture content M, below the fiber saturation

point, determined from the specific-gravity values based on weight when

oven-dry and volume when green and on weight and volume when ovendry.

8.=S,-{Sl-S,)^ (9)
PRESERVATIVE TREATMENT OF WOOD 137

Weight of Water per Unit Volume of Wood

w..(w.-w,).w.(ßyw.(^^'^ (10)

If the weights are expressed in pounds per cubic foot,

Percentage of Volume Occupied by Water

(11)

-»-»<ff7?)=^' Pw

If the weights are expressed in pounds per cubic foot,

\{Q2A)p^J

Values of Pu» can be found from figure 8 for moisture-content values up

to the fiber saturation point. Above the fiber saturation point

M .

Total Weight of Wood and Water at Any Moisture Content

When Wo and Wd are in pounds per cubic foot :

H'.=S(62.4)(l+-^) (15)

-'^{'+^)-«('^")('+ÎSi) (")

cubic foot:

When the moisture content and specific gravity of the heartwood and

sapwood are to be considered separately:

TF.=Tr„[s.(^)(i+^)+>s.(i-^)(i+Mi)] = (Tr,+Tr.) (i6)

where Ws and Wh are the proportional weight of sapwood and heartwood,

respectively, per unit volume.

If the specific gravity of the sapwood Ss is assumed to be the same as

the specific gravity of the heartwood, which is usually sufficiently accurate for green timbers,

^,^,snw,\(^)(^-^) + (^+^)\ (17,

Percentage of Sapwood in Total Volume

P.^roob-U^l (IS,
138 AGRICULTURE HANDBOOK 40, U. S- DEFT. OF AGRICULTURE

Percentage of shrinkage in volume in seasoning from moisture content

M2 to a moisture content Mi (where M2 is greater than Mi and not more

than 30):

C=100(i^V^M (19) i^)

Percentage of Air Space in Wood

If Mk is the percentage of moisture content at or below the fiber saturation point (taken as 30 percent) and Ma is the percentage of moisture

content above the fiber saturation point,

When the moisture content is at or above the fiber saturation point and

M=Mk+Ma, or the total amount of moisture in the wood, equation 20

may be written:

METHOD OF USING FIGURES WHEN FINDING

TEMPERATURES TO BE EXPECTED IN TIMBERS HEATED

UNDER ANY GIVEN CONDITIONS

A list of symbols employed in the following discussion relating to

temperature changes in wood is given on pp. 149-150. Temperatures

are designated by using the letter t with various subscripts, while heating periods, expressed in hours, are designated by using Greek letter dj

also with corresponding subscripts.

All computed temperatures plotted in figures 13 to 22, inclusive, are

designated as tc, and the corresponding heating periods found from these

figures are designated as dc.

In computing the time-temperature curves, the initial wood temperature was taken as 60° F., the heating-medium temperature as 200° F.,

and the diffusivity a as 0.00025.

If dc represents the time required to obtain a temperature tc at some

particular distance from the surface of a timber when the diffusivity is

a = 0.00025, then the time 6x required to obtain the same temperature

under the same heating conditions in the same size timber and at the

same distance from the surface when the diffusivity is a 1 is shown by the

relation:

6x ai=dca or ßx = (o^^)0^,(,),_g_(,)

Since the diffusivity was assumed as 0.00025 in computing temperature

data plotted in the figures, the required heating period dx for wood with

any other diffusivity designated as a 1 is easily determined by multiplying

the ratio —'- = / by the time 6 c found from the proper chart for

ai

the temperature desired. Likewise, when 6x is given,

Y0.00025J y^J
PRESERVATIVE TREATMENT OP WOOD 139

S 0000 00 s o

m Ö * ' * 1-Î r--i

Q ^

P bC *i-»

o tí^ ^ '>^. 02 »—( tn

TJ -4-3 TV _S ^ S loio -^ -^ o Oi ¿^ (^jc^_ i-| q Oi 00

a» ^ 1—1 1-H 1—1 1—1 -^ *>-.

^"^'

g

>> OO (M '^ :0 00

i < íM Oí CSI <M (M (M

iz; ^ -^ ^ > r O o o o o

Q-l ^ q q q q q q

o3 :3 o * * * CD m

C^ CD rt^ QO CO 00

Oi Oi O O T-H (M,

o * 1-^ 1-Î ^* 1-H

bO <^ fi^

^ ">^M

o

0)

ac

H C5Tt< CO CO CO 00

O

<:^^<Z)^ <^^ Oi^ 00 l>;

s •^ rH 1—t

^

0) ^

c3

Oí w ce Tt^ CO t^ 05 »M ^ C^ <M <M <N C^ CO 1 -4-3

-c 5l« oo So o o

o ^ d ' '

<4-l

í*

o 00 Cl X oí CO CO

?

q ^^ ci CO ^^ lO

o

g

vT^

T—( í—l 1—1 rH 1-H

5^ c3

CO 0> 00 CO o 'f

02 Oi 00 t^ t^ 1>

Ö * * '

q

•r^

'^

0^

' I>00 (M CO CO os ^ (M (M CO CO CO CO

, -^^

^ ^ r O o o o o o

Q-S^ oo o o o q

^

o *

11 i

fí T 1

-<"" 1 1 i-gj iáiii: is

ü

OÍ

m c3a;>^o i-t^o;;:;^-??;^

^^ ¡

o¿:-a¿^g-Joa^£^

02 ^m O ^ ^

111

Í.ÍO

o

o

o

03 fe

»O

<M

O

O

O

d

fe
140 AGRICULTURE HANDBOOK 40, U. S. BEPT. OF AGRICULTURE

For convenience table 22 has been prepared showing the multiplying

factors/and/i for both green and seasoned wood of different species when

heated in steam or in creosote. As an example, this table shows the multiplying factor/ for green longleaf pine heated in steam as 0.89= ' '

while for green shortleaf or loblolly pine it is about 0.78. For practical

purposes these factors may be taken to the nearest decimal, which gives

0.9 and 0.8, respectively. The simple adjustment in the treating period

for differences in the diffusivity of different woods can thus easily be

made by using the multiplying factor/ in finding dx when Oc is known or

the multiplying factor/i when dx is known and 6c is to be determined.

For commercial treating conditions the desired information relating

to temperature changes in timbers may be obtained either (1) by finding

the time 6x required to obtain a desired temperature tx at any particular point in a timber when the initial wood temperature ta and the

heating temperature tb are known, or (2) by finding the temperature tx

obtained at a given point in a timber when the heating period ^^ is

TABLE 23.—Corresponding wood temperatures tx obtained with different

heating-medium temperatures

Temperature tc^

(°F.)

Corresponding temperature tx when heating-medium

temperature is—

210° F. 220° F. 230° F. 240° F. 250° F. 260° F.

60_

65_

TOTO.

SOSO90.

95-

100-

105.

110-

115-

120-

125-

130.

135-

140.

145-

150.

155.

160-

165-

170.

175.

180.

185.

190_

195-

•200-.

°F.

60

65

71

76

81

87

92

97

103

108

114

119

124

130

135

140

146

151

156

162

167

172

178

183

189

194

199

205

210

°F.

60

66

71

77

83

89

94

100

106

111

117

123

129

134

140

146

151

157

163

169

174

180

186

191

197

203

209

214

220

op

*60

66

72

78

84

90

96

102

109

115

121

127

133

139

145

151

157

163

169

175

181

187

194

200

206

212

218

224

230

'60

66

73

79

• 86

92

99

105

111

118

124

131

137

144

150

156

163

169

176

-182

189

195

201

208

214

221

227

234

240

op

'60

67

74

80

87

94

101

108

114

121

128

135

141

148

155

162

168

175

182

189

196

, 202

209

216

223

229

236

243

250

°F.

60

67

74

81

89

96

103

110

117

124

131

139

146

153

160

167

174

181

189

196

203

210

217

224

231

239

246

253

260

1 From figures based on heating-medium temperature of 200° F.
PRESERVATIVE TREATMENT OF WOOD 141

60 80

TEMPERATURE

100 120

fc C'F)

60 80 100 120

TEMPERATURE

140 160 180 200

fc f'T-J

FIGURE 34.—Chart for finding the corresponding temperature tx within the timber

when the initial wood temperature ta, the heating-medium temperature /&, or both,

are different from those assumed in computing the plotted temperature, (iemperature ic taken from figs. 13 to 22, inclusive.)

known. In the first case, 0.=/ öc, and in the second case, when 6. is

assumed, Qc = jex=i\ ^xEither table 23 or figure 34 can be used in finding the corresponding

wood temperature t, when the heating conditions are different from those

assumed in making the temperature computations. In most cases it
142 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

will be sufficient to assume that the average initial wood temperature

ta is about 60° F. (as has been done in preparing table 23 and figure 34),

but the heating-medium temperature h may differ for any charge treated.

A simple algebraic expression that can be used in finding the corresponding wood temperatures tx when the initial wood temperature is any

value ¿a, which in winter may be well below 60° F. (the lowest temperature

shown in figure 34), is given on page 150, with examples illustrating how

to use it.

ADJUSTING FOR DIFFERENT HEATING-MEDIUM

TEMPERATURES

In table 23 the first column shows temperatures tc (temperatures found

from the figures) in 5° intervals up to the assumed heating-medium

temperature of 200° F. The other columns, 2 to 7, show the corresponding wood temperatures tx for heating-medium temperatures of 210°,

220°, 230°, 240°, 250°, and 260° F. In this tabulated form, which covers

the range of heating temperatures most commonly used, it is easy to

compare the increase in wood temperature tx for each increase in heatingmedium temperature listed. The data in table 23 show that the rate of

temperature rise at any point will be proportional to the temperature

of the heating medium. For example, with a heating-medium temperature of 260° F. the rise in temperature will be ^^7;;^—-^f or 1.43° for each 1°

JiKjy) — oU

temperature rise when the heating-medium temperature is 200° F. and

the initial wood temperature is assumed as 60° F. Similarly, when the

heating-medium temperature is 250° F., the temperature will rise^rrrr^j^'

or 1.36° for each degree rise when the heating-medium temperature is

200° F., and so on.

A general expression for any value of ta and ¿& is . ° = the temperature

change for each 1° temperature rise for the temperature conditions

assumed in computing time-temperature data plotted in the figures.

Although the rate of temperature rise will be faster or slower than shown

for tc (column 1, table 23), depending on values of ta and ¿&, it will be

more convenient and sufficiently accurate to neglect these differences in

the 5° intervals of tc when using this table.

For example, assume ¿^ = 170° and i& = 260° F. In using table 23 to

find the corresponding value of ¿c, column 7 (fo = 260°) reveals that 170°

lies between the tabulated temperature of 167°, corresponding to tc= 135°,

and 174°, corresponding to ¿c= 140°. The required temperature ¿x = 170°

is 3° higher than 167. Adding 3° to 135° gives 138° as the value for tc.

On the other hand, by assuming tc is given as 143°, the corresponding

temperature tx from column 7 would be taken as 174° plus 3° = 177°.

Figure 34 can be used if desired instead of table 23. This figure

provides a convenient means of finding the corresponding temperature

tx for any assumed initial wood temperature ta between 60° and 260° F.

or for any heating-medium temperature ¿& within the same temperature

range.

In using this figure, connect the initial wood temperature ta on the

left-hand scale with the heating-medium temperature tb on the right-
PRESERVATIVE TREATMENT OF WOOD 143

hand scale. This can be done conveniently by means of a straightedge.

Values of tcj which are temperatures found from the time-temperature

charts, are to be read on either the bottom or top scale, which are the

same.

The lines connecting the initial wood temperatures of 60° F. and

various heating-medium temperatures are merely drawn in for convenience in using the chart. The dotted line drawn from the initial

wood temperature ¿« = 75° to the heating-medium temperature ¿¿, = 245°

shows how the chart can be used for different assumed heating conditions.

In this case the dotted line shows how the chart would be used when the

initial wood temperature ta is 75° and the heating-medium temperature

tb is 245°. If initial wood temperatures lower than 60° are to be considered, the corresponding temperatures tx or tc can be computed from

the simple equation (A) given on page 150.

In figure 34 the line a drawn from 190° = ¿x (found on left-hand scale)

to the line connecting the initial wood temperature ¿„ = 60° F. and the

heating-medium temperature ¿6 = 260°, shows how to find the corresponding temperature ¿c, the plotted temperature to be found from the proper

figure. In this case where ¿^ is 190, the desired wood temperature tc

is found to be 151°. The heating period dc can then be found when tc

is determined.

TEMPERATURES IN ROUND TIMBERS

Since timbers such as posts, poles, and piling can be assumed to have

approximately a circular cross section, it is possible to compute the

temperature in timbers of various diameters after any heating period

when the temperature distribution is computed for any given diameter D

and for any assumed heating conditions. (See p. 149 for list of symbols

used.) This is done by expressing the radial distance from the circumferential surface as a proportion of the radius. In this case the

proportional radial distance from the surface is simply the distance F

from the surface to the point at which the temperature is to be determined, divided by the radius R. Figure 35 will be convenient to use for

this purpose, as it shows either the distance F from the surface (in inches)

F

for any assumed proportional radial distance -5 of timbers rangmg m

diameter from 4 to 22 inches, or the corresponding proportional radial

F

distance -^ for any assumed distance F from the surface. The two

R

formulas (B) and (C), p. 150, are simple algebraic relations that are the

only formulas needed in using figure 13. Since figure 13 is based on a

heating-medium temperature of 200° F., table 23 or figure 34 can be

used, as explained on pages 140-141, to find ¿x when other heating temperatures are used.

The time required to obtain the same temperature at the same proportional radial distance in any two timbers of diameters d and D inches,

respectively, will be directly proportional to the squares of the diameters

or squares of the radii. For example, assuming the same kind of wood

and same heating conditions, if Be represents the time required to obtam

a given temperature tc at a proportional radial distance -=—, where
144 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

II.O

10.5

fO.O

9.5

9.0

DIAMETER (INCHES)

.4567 8 9 10 II 12 13 14 15 , ,'^f, ,. ff. .. ^f,, , .^f., . f¡^ , , ff,, , ^f//.¿?

ÏÏÏÏ iïïï ÏÏÏÏ T^ Tïï

F= DISTANCE FROM SURFACE

R= RADIUS OF TIMBER

^=PROPORTIONAL RADIAL DISTANCE

m

10.5

10.0

9.5

9.0

8.5

8,0

7.5

7.0 Q

6.5

6.0 5

5.5 I'^

5.0 I

3.5 «^

3.0

2.5

2.0

1.5

1.0

0.5

4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22

DIAMETER (INCHES)

FIGURE 35.—Chart showing relation among distance from surface, proportional radial

distance, and diameter of round timbers.

r=^, in a timber d inches in diameter, and if 0^ represents the time required to reach the same temperature at the same proportional radial

1 OF T\ P /72

distance—=^:r-, where i2 = —, then—^ = -=r^. One may then write,

R

D e. D^'

(¿2 \ /DA

jr^ j 6xy and ^x={-j2 I ^c When the proportional radial distances are equal,

2F 2Fi

d D

timber having a diameter D =

j or the distance Fi from the surface of the

-K^)-
PRESERVATIVE TREATMENT OF WOOD 145

Figure 13 has been computed for a timber diameter of 10 inches,

and because of the foregoing relation, can be used for timbers of any

diameter D if the time So computed for the 10-inch diameter is multiphed

by ( YQö ) • It is because of this fact that it is an advantage to express

the distances from the circumference as a proportion of the radius.

The relation between diffusivity and heating period as given on page 138

. ^ /0.00025\ , ^ / ai \ ^, ,

^^ \ ~~o^i / ' "^ \ 0 00025 ) ^"- ^^ ^^^ temperature t^^ is to

F

be calculated at any given proportional radial distance -p. for a timber of

K

diameter D that is greater or less than 10 inches, the required heating

period 6^ can be calculated easily by using the simple algebraic relation

(B),page 150, which is 6^:= ( T7^ I (/) ( ^c ). Values of / are found in {m)<J^{')

table 22.

On the other hand, if the heating period 0^ is given and it is desired to

compute the temperature tx obtained at any proportional radial distance

F—, this can be computed from the relation (C) given on page 150, which is

(-)

Values of / and /i are given in table 22 for various species. In using

the foregoing equation (B), the value tc corresponding to the required

temperature tx for the given heating-medium temperature h is first found

from table 23 or figure 34. If the initial wood temperature ta is greater

that 60° (the initial wood temperature assumed in preparing table 23),

either figure 34 or the algebraic expression (A), page 150, can be used to

find tc. Equation (A) can be used for any assumed values of ta and fo.

Next, find from figure 13 the heating period dc corresponding to the

temperature tc for the proportional radial distance -^. Then substitute

jß

in (B) the factors y^^, /, and öc, and from the expression determine the

required heating period Q^- (See example 1, p. 146.)

In using equation (C), when dx is given and ^c is to be determined,

substitute /i, from table 22, and the assumed heating period Q^ and

compute öc. Next find from figure 13, for the proportional radial distance

under consideration, the temperature tc corresponding to the computed

heating period dc- (See example 2, p. 146.)

Third, find the value of tx from figure 34 or table 23 that corresponds

to tc for the heating-medium temperature used.

The following two examples illustrate the use of figure 13 in com-

])uting temperatures obtained in round timbers when different heating

temix'ratures and diffcnmt heating mediums are used. (See relation of

(9., dc, and diameter D given in (B) and ((^), for round timbers, p. 150).
146 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Example 1

Data given

Material—Green shortleaf pine timbers with average diameter assumed

as 12.5 inches ( = D).

Heating medium —Steam.

Steam temperature = 260"^ F. i = tb).

Initial wood temperature = 60° F. (ta).

Required data

Find the heating period ox needed to obtain a temperature tx, of

approximately 212° F. in the timber at the proportional radial distance

F /l2 5\ of 0.3 (='w) .This is equal to a distance from the surface of 0.3 I —^ j

or l!^8 inches.

In this example the required heating period ^x is easily found by

using equation (B), page 150.

From table 23 or figure 34 it is found that if the heating-medium

temperature is 260° F. when ¿^ = 212°, the corresponding value of tc

(given in the first column of table 23) is about 167°.

F

The heating period dc corresponding to 167° when — = 0.3, is found

K

from figure 13 to be about 4.9 hours.

The factor/ is found from table 22 to be 0.78 (or approximately 0.80)

for green steamed shortleaf pine, and the required value of ^x is then

/(12.5)A

V 100 J

computed as (0.80) ( ^ ^^/ j (4.9) = 6.1 hours.

Example 2

Assume that air-seasoned Douglas-fir piles, with an average diameter

of about 14 inches are heated in creosote at 200° F. for 8 hours and that

the initial wood temperature ta is taken as 60°. Compute the temperature at 0.5, 1.5, 2.5, and 3.5 inches from the surface and at the center.

In this case ¿6 = 200°, öa; = 8.hours, and —=—^or 0.07 for 0.5 inch from

K 7

the surface, -;^or 0.21 for 1.5 inches, -^ or about 0.36 for 2.5 inches,

3.5

and^z-or 0.50 for 3.5 inches from the surface, and 1.00 at the center.

These ratios could also be found by using figure 35.

First find the heating period dc from equation (C), p. 150, which gives

^c = -^ (/i) Ox.

From table 22 the factor/i is found to be 0.96 for seasoned Douglas-fir

heated in creosote. Bysubstituting the numerical values, Oc = j7r^ (0.96) (8),

or approximately 3.9 hours. Next find from figure 13 the temperature tc obtained in 3.9 hours for the proportional radial distances. These
PRESERVATIVE TREATMENT OF WOOD 147

temperatures t^ are found to be approximately 190° F. for ^ =0.07,

171° for ;^ = 0.21, 150° for f= 0.36, 133° for ^= 0.50, and 103° at the

center.

Since the initial wood temperature and heating-medium temperature

have been assumed as 60° and 200° F., respectively (the same as were

used m computing the data for fig. 13), tc = t.. It is therefore unnecessary m this case to use table 23 or figure 34 to adjust for a difference in

heating-medium temperature.

It is easy to compare the temperatures that would be obtained if

different heating-medium temperatures were employed. For example,

after tc had been determined, table 23 or figure 34 would show that if

the creosote temperature h had been 220° F. instead of 200°, the computed

temperatures at 0.5, 1.5, 2.5, and 3.5 inches from the surface would be

about 209°; 187°, 163°, and 143° F. instead of 190°, 171°, 150°, and 133° F.,

while the temperature at the center would be about 109° instead of 103° F.

Examples (1) and (2), pp. 150-151, show how to find the temperatures

tr or tc when the initial wood temperature ta is any temperature below

60° F., the value used in preparing table 23 and as the minimum initial

wood temperature for figure 34.

TEMPERATURES IN SAWED TIMBERS

The relative symmetry of material in the round form such as in

posts, poles, and piling makes it possible to compute the approximate

temperatures for timbers of different diameters when the temperature

distribution has been computed for any particular diameter (for example,

10 inches). This possibility has been shown in the discussion of temperature data for round timbers.

A similar relation holds for square timbers, but it does not apply,

however, for sawed material when the width and thickness are not equal.

It is therefore necessary to compute temperatures at various assumed

distances from the surface for sawed timbers of different dimensions.

Figures 14 to 20, inclusive, show computed time-temperature curves

for lumber and timbers, 1, 2, 3, 4, 6, 8, 10, and 12 inches in thickness and

having widths varying from the square dimensions to 16 inches. Figures

21 and 22 show the computed temperatures midway between the surface

and the center, and at the center of timbers 4 by 4, 4 by 6, 7 by 9, 8 by 8,

8 by 10, 10 by 10, 10 by 12, 12 by 12, and 12 by 14 inches in cross section.

These time-temperature curves for sawed timbers, like those shown for

round timbers in figure 13, were computed by assuming an initial wood

temperature of 60° F., a heating-medium temperature of 200° F., and a

diffusivity of 0.00025. The data given in tables 22 and 23 will therefore

apply for sawed as well as for round timbers.

The temperatures given in figures 14 to 22, inclusive, for sawed timbers

and lumber are temperatures obtained along the transverse axis on

which the temperature changes take place most slowly. In square

timbers the temperature would normally be the same along either transverse axis perpendicular to two parallel faces, since the rate of temperature change may be assumed to be the same in both the radial and

tangential directions. In all timbers, however, in which the width and
148 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

thickness are different, temperature changes will take place more slowly

along the axis perpendicular to the wider faces, which is the shorter axis.

For example, in a timber 2 by 4 inches in cross section, temperature

changes will be slower along the 2-inch axis.

The following examples will illustrate the use of the temperature

curves shown in figures 14 to 22, inclusive. (See relation of 6^^ and dc

given in (D) and (E) for sawed timbers, p. 150.)

Example 1

Data given

Material—Green loblolly or shortleaf pine.

Cross-sectional dimension of timber—8 by 10 inches.

Initial wood temperature = 60° F. ( = ¿a).

Heating medium—Steam.

Steam temperatures 250° F. ( = iè).

Data required

Find steaming period needed to obtain a temperature tx of 200° F.

at a distance of 2 inches from the surface. Table 23 or figure 34 shows

that this would correspond to a temperature tc of approximately 163°.

The corresponding heating period dc is found from figure 18 to be about

63^ hours. Table 22 shows that the correction factor / for the difference

in diffusivity is about 0.8; and since the required time 6x=fSc, this

equals (0.8) 6.5 or about 5.2 hours.

Example 2

Data given

Material—Air-seasoned coast Douglas-fir.

Cross-sectional dimensions—10 by 12 inches.

Heating medium—Coal-tar creosote.

Creosote temperature—210° F. ( = fc).

Initial wood temperature—60° F. ( =¿a).

Data required

Find (1) the heating period 6^ required to reach a temperature tx of

190° F. at a distance of 1 inch from the surface, and (2) find the temperature tx obtained at the center when the timber has been heated for the

time determined.

For part (1) table 23 or figure 34 shows that a temperature tx of 190° F.,

when ¿6 = 210°, corresponds to a temperature of about 182° ( = ¿c) when

the heating-medium temperature is 200°. Figure 19 shows that the

heating period 6 c required to reach a temperature of 182° at 1 inch from

the surface is about 9 hours.

In table 22 the correction factor/to correct for differences in diffusivity

is shown as 1.04. The required heating period Ox is then/öc= (1.04) (9)

= 9.36, or about 9.4 hours.

For part (2) figure 19 shows that a temperature of about 142° F.

{ = tc) is obtained at the center when öc = 9 hours, corresponding to 0^ = 9.4

hours. This temperature corresponds to a temperature of about 147°

when ¿6 = 210°, as shown by table 23 or figure 34.
PRESERVATIVE TREATMENT OF WOOD 149

Example 3

Data given

Material—Air-seasoned red oak ties 7 by 9 inches in cross section.

Heating medium—Coal-tar creosote.

Creosote temperature during pressure period = 200° F. { = tb).

Initial wood temperature = 60° F. ( = ¿a).

Date required

Assuming that the wood is heated only during the pressure period,

which is 4 hours long (= öj, what temperature ¿^ can be expected at the

center of the ties when the pressure period is completed, negk^cting the

effect of any heat that might be carried in by penetration of the preservative?

Since the heating-medium temperature is assumed to be 200° F.

(which was the temperature used in computing the time-temperature

curves), tc = tx.

Table 22 shows that the factor/i for air-seasoned red oak is 0.8; then

öc=(0.8) (4) = 3.2 hours. In this case the temperature that can be

expected at the center would be the temperature shown in figure 22 after

heating for 3.2 hours. This is found to be about 112° F.

If the ties were coast Douglas-fir, black túpelo, or sweetgum, table 22

would show that /i = 0.96 for these woods. The heating period 6c

corresponding to öx = 4 hours is then (0.96) (4), or about 3.8 hours. The

temperature tx that can be expected at the center of ties of these species

will then be the temperature shown in figure 14 after heating for 3.8

hours. This is found to be about 126° F.

Computations such as those in example 3 are of interest to determine

whether air-seasoned timbers, such as ties, have been heated long enough

during the pressure period to sterilize the wood at the center if there is

danger of incipient decay being present at the time of treatment.

SYMBOLS AND FORMULAS RELATING TO TEMPERATURE

CHANGE IN WOOD

a =Diffusivity factor (0.00025) used in computing temperature curves

shown in figures 13 to 22, inclusive. (Diffusivity=y7-where K

is conductivity through unit area of a layer of the substance having

unit thickness, C is the specific heat, and p is the density.)

ai = Apparent diffusivity for the wood and heating medium under

consideration found from figure 12.

D = Diameter of timber = 2R.

F = Distance from surface of timber to point at which wood temperature

is to be determined. F is measured on radius for round timbers

and along axis of sawed timbers. This is measured along the

shorter axis if width is different from thickness.

— = Proportional radial distances where R is radius of timber.

R

t =Any wood temperature.

ta = Any initial wood temperature.

tb =Any heating-medium temperature.
150 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

tc = Temperature found from figures 13 to 22, inclusive, which were

computed by assuming ¿„=60"* F., ¿& = 200°, and diffusivity

a = 0.00025.

t^ =Wood temperature to be determined at any given point when the

heating period Ox, diffusivity ai, initial wood temperature ia,

and heating-medium temperature ¿6 are assumed. If ía: is assumed, 6x is to be determined.

The relation (A) of t^, tc, ta, and h, when 60° and 200° F. are the assumed

initial wood temperature and heating-medium temperature, respectively,

used in computing the plotted temperature curves in figures 13 to 22, is

the following:

h-t:. 200-tc^.j^ ^ +"

h-ta 140

Since tby ta, and either ¿x or ¿^ are assumed to be known, the unknown

value of ¿X or ¿c can be readily determined from the foregoing expression.

For most normal ranges of initial wood temperature ta, or of heatingmedium temperature tb, the temperatures tx and tc can be found conveniently by using figure 34 or table 23. :^

Examples (1) and (2), pp. 149-150. illustrate the use of formula (A).

This formula applies for both round and sawed timbers.

In figures 13 to 22 the computed temperature tc is plotted against the

corresponding heating period 6c for round timbers of different diameters

F

and for different proportional radial distances ^. Similarly, for sawed K

timbers the temperature tc is plotted for different distances from the

surface measured along the axis.

For Round Timbers

e.-f (^) e. ' (B)

öc=/i (^) 0. (C)

For Sawed Timbers

e.=fe. (D)

Ö.=/iÖ. (E)

Values of / and /i are given in table 22 for various species. Figure 13

can be used for timbers of any diameter and for computing the temperaF ¿Yf^-h ture at any proportional radial distance p. (See list of symbols, p. téft.

Example 1

(1) Assume the heating-medium temperature Í6 = 250° F., the initial

wood temperature ¿a = 0°, and tc found from one of the figures = 85°.

Find the corresponding temperature t^ by using equation (A). (2) With

the same assumed temperatures for ta and tbj 0° and 250° F., respectively,

find the temperature when tx = tc and the time dx required to reach this
PRESERVATIVE TREATMENT OF WOOD 151

jp

temperature at a distance of-^ = 0.3, or 1.8 inches from the surface of a

It

shortleaf pine timber 12 inches in diameter when heated in steam at 250*".

For part (1), substituting in equation (A),

250-^,^200-85

250-0" 140

For part (2), substituting in equation (A),

250-Í, 200-Í,

Since tc = tx, then 250-0 140

,. t.=2,0-Y''-\^^''-''^\ ,which gives ..= 136° F.

In this case the temperature tc is the temperature obtained at some

particular point in a timber when ¿« = 60° and /6 = 200° F., while t^ is

the temperature obtained at the same point in the same time when

¿a = 0° and fe = 250°. Although t,, at the start is lower than /„ because

the initial wood temperature is assumed to be zero instead of 60°, the

higher heating-medium temperature of 250° and the greater temperature

difference make tx increase more rapidly than ¿c. After reaching 136°,

F

tx = tc at -^ = 0.3, or 1.8 inches from the surface, and will then continue

to increase faster than tc and reach the heating-medium temperature of

250° when tc reaches the corresponding heating-medium temperature of

200°. From table 22, /is found to be about 0.8, and from figure 13, for

F— = 0.3, it is found that 6c is approximately 2.2 hours when ¿^=136°.

K

Then öx=i T^)(0-8) (2.2) = 2.53, or about 2.5 hours, which is the

steaming period required to reach a temperature of 136° in the 12-inchF

diameter timber when ¿a = 0°, ¿6 = 250° F., and— =0.3, or 1.8 inches from

the surface.

Example 2

Assume the heating-medium temperature ¿6 = 220° F., the initial wood

temperature ta = 75°, and the temperature ¿. = 190°. Find temperature tc.

. ,,, . 220-190 200-tc

Substituting in the equation (A) gives 220-75 ^—Ï40~

i'-^^^ [ (220-75) ]
152 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

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1938. WOOD PRESERVATION. 457 pp., illus., New York.

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179, illus.

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1933. EXPERIMENTS WITH THE BOULTON PROCESS IN THE TREATMENT OF GREEN

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1935. TEMPERATURE AND I^IOISTURE CHANGES IN COAST DOUGLAS-FIR. Amcr.

Wood-Preservers' Assoc. Proc, pp. 77-103, illus.
154 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

AVERAGE TEMPERATURE AND MOISTURE REDUCTION CALCULATIONS FOR

STEAMED ROUND SOUTHERN PINE TIMBERS. Amer. Wood-Preservers'

Assoc. Proc, pp. 256-279, illus.

WEIGHTS AND ABSORPTION LIMITATIONS OF ROUND TIMBERS. Amcr.

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RELATION OF WOOD DENSITY TO RATE OF TEMPERATURE CHANGE IN WOOD

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INFORMATION OBTAINED FROM MARINE PILING EXPERIMENTS ON THE GULF

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Lab. Rpt. R1406, 26 pp., illus.

1936.

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1938.

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1940

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1941.

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1941.

(A^i) _

1942.

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1943. RATE OF TEMPERATURE CHANGE IN LAMINATED TIMBERS HEATED IN AIR

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1944. EFFECT OF MOISTURE CHANGES ON THE SHRINKING, SWELLING, SPECIFIC

GRAVITY, AIR OR VOID SPACE, WEIGHT, AND SIMILAR PROPERTIES OF

WOOD. U. S. Forest Serv. Forest Prod. Lab. Rpt. R1448, 44 pp.,

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1946. RATE OF TEMPERATURE CHANGE IN SHORT-LENGTH ROUND TIMBERS.

Amer. Soc. Mech. Engin. Trans. 68 (1) : 1-16.

1946. TEMPERATURES OBTAINED IN TIMBERS WHEN THE SURFACE TEMPERATURE

IS CHANGED AFTER VARIOUS PERIODS OF HEATING. Amei. WoodPreservers' Assoc. Proc. 42: 87-139, illus.

1949. RESULTS OF EXPERIMENTS ON THE EFFECTIVENESS OF VARIOUS PRESERVATIVES IN PROTECTING WOOD AGAINST MARINE BORER ATTACK. U. S.

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1951. RATE OF DISINTEGRATION OF WOOD UNDER DIFFERENT HEATING CONDITIONS. Amer. Wood-Preservers' Assoc. Proc. 47: 155-168, illus.

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1949. AIR-SEASONING OF RED OAK CROSS TIES. Amer. Wood-Preservers'

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(52) MCMILLEN, JOHN M.

1950. METHODS OF DETERMINING THE MOISTURE CONTENT OF WOOD. U. S.

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1933. MOVEMENT OF THE PIT MEMBRANE IN CONIFEROUS WOODS, WITH SPECIAL

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1929. THE FIBER SATURATION POINT OF WOOD AS OBTAINED FROM ELECTRICALCONDUCTIVITY MEASUREMENTS. Indus, and Engin. Chem. Analyt.

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1937. THE BONDING FORCE OF CELLULOSIC MATERIALS FOR WATER. (From

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1935. ABSORPTION COMPRESSION OF CELLULOSE AND WOOD. 1. DENSITY

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1943. COATINGS THAT PREVENT END CHECKS. U. S. Forest Serv. Forest Prod.

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1918. RELATIVE RESISTANCE OF VARIOUS HARDWOODS TO INJECTION WITH

CREOSOTE. U. S. Dept. Agr. Bui. 606, 36 pp., illus.

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1918. TESTS OF THE ABSORPTION AND PENETRATION OF COAL-TAR CREOSOTE INTO

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1940. PREFRAMING OF TREATED TIMBER FOR USE IN BRIDGES. Amer. WoodPreservers' Assoc. Proc. 36: 200-205.

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1930. THE EFFECT OF CREOSOTE ON STRENGTH OF FIR TIMBERS. Tlmbcrman

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1924. TEMPERATURE CHANGES IN WOOD UNDER TREATMENT. Amer. WoodPreservers' Assoc. Proc. 20: 285-303, illus.

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1925. TEMPERATURE AND MOISTURE CHANGES IN WOOD UNDER STEAMING

TREATMENT. Amer. Wood-Preservers' Assoc. Proc. 21: 271-289, illus.
156 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

INDEX

Page

Absorptions 127

Full-cell and empt3'-cell 95

Gross, definition of 90

Initial, definition of 90

Mixed charges 98

Net, definition of 90

Net, not measure of effectiveness of

treatment --- 87, 94

Not practical to specify gross absorbtion 126

Of oils and water solutions in cell walls 81

Relation of measurements by weight and

by volume 107

Relation of volumetric absorption and

surface area 96-107

Should be determined at time of treatment 128

Specifications for 127

Adzing ties before treatment 39

Air, cannot be entirely removed from wood. 2

Diffusion in wood 86

Entrapped in cylinder during steaming. 43

Preliminary pressure 86

Removed from wood by vacuum __ 2

Seasoned wood, temperature changes in 84

Space in wood, amount of 28

Space in wood, formula for 138

Air-seasoning 32

Climatic conditions 34

Drainage conditions 35

Locality 34

Peeling 33

Piling 35

Sanitation of seasoning yard 35

Seasoning of heartwood and sapwood. _ 32

Seasoning periods 36

Shrinkage during 25

Species and size of timber 31

Time of cutting 34

Alpine fir, penetrability classification 17

Alternate steaming and vacuum, effect on

moisture content 42

Altitude, relation to vacuum required __ 85

Amount of sap in trees in summer and in

winter 34

Annual ring, illustration of 7, 8

Antichecking irons 33

Arsenic 5

Basswood, penetrability classification 17

Beech, commercial air-seasoning periods 36

Commercial treating conditions for ties,

Central and Rocky Mountain States. 119

Commercial treating conditions for ties,

Eastern and Southern States 121,122

Penetrability classification 17

Beech (red heartwood), penetrability classification 17

Bethell process —_— 2

Bigtooth aspen, penetrability classification. 17

Birch, yellow, commercial treating conditions for ties, Central and Rocky Mountain States 119 t!¡Íommerf'ial treating conditions for

ties. Eastern and Southern States 121,122

Black locust, penetrability classification 17

Black túpelo, penetrability classification 17

Black willow, penetrability classification 17

Bleeding of treated wood 114

Boiling points of water at different altitudes 84

BoiUng points of water under different vacuums ' 85

Bordered pits, discussion of 10

Illustration of 11-12

Boring ties before treatment 39,134

Boulton process, advantages and disadvantages of: 51

Amount of water removed by 46

Commercial conditions employed 45

Definition of 45

Discussion of 44-52

Effect on strength II3

Temperatures obtained in wood 50

Page

Use with full-cell and empty-cell processes__ 45

Bristlecone pine, penetrability classification 17

Broadleaf trees, definition of 6

Burnett process, definition of 2

Casehardening, definition of 37

Checking, coatings used to reduce 33

Effect of pressure period 89

Effect of rate of seasoning 35

Effect of steaming 113

Effect of treating pressure 77, 88

Checking and collapse, effect of improper

steaming or boiUng conditions on 113

Chemicals dissolved in solvents other than

water 5

Chestnut, penetrability classification 17

Chestnut oak, penetrability classification 17

Classification of species with respect to ease

of penetrability 16

Coal-tar creosote 4

Coal tars 4

Coatings used to reduce end checking 33

Collapse, definition of 28

Effect of length of pressure period 89

Effect of steaming 113

Effect of treating pressure 89

Effect on specific gravity of wood 28

In sapwood and in heartwood 113

Compensating for time needed to reach

maximum temperature of heating medium 56

Compression of wood under pressure 92

Effect of timber dimensions 111

Effect of water solutions and preservative oils 111

Conditioning processes 39

Boulton 44

Steaming 39

Vapor-drying process 52

Conifers, species included 6

Copper sulfate 5

Corkbark fir, penetrability classification 17

Relative penetration of coal-tar creosote

and zinc chloride in 81

Cottonwood, penetrability classification 17

Creosote coal-tar solutions 4

Proportions of tar used 5

Creosote-petroleum solutions 5

Penetration 69

Proportion of petroleum used 5

Cross arms, commercial steam-and-vacuumtreating periods 43, 44

Decay, resistance of heartwood 9

Resistance of sapwood 9

Temperatures required to kill decayproducing fungi 67

Density, effect on preservative treatment.. 13, 14

Differences in structure of hardwoods and

softwoods 6

Diffuse-porous woods, definition of '^ 7

Effect on penetration 10

Dimensions of timber related to absorption

and penetration 96

Douglas-fir (coast type), absorptions for

marine timbers 130,131

Absorptions for poles and land piling.. 131

Air-seasoning periods 36

Air-seasoned piling, commercial treating

conditions for 117

Air-seasoned ties, commercial treating

conditions for 116

Conditioning by Boulton process 44-52

Effect of Boulton process on strength. _ 113

Effect of different pressures and temperatures on absorption of zinc-chloride solution 75

Green lumber and bridge timbers, commercial treating conditions for 116

Green piling, commercial treating conditions for 117
PKESERVATIVE TREATMENT OF WOOD 157

Page

Green ties, commercial treating conditions for J 116

Penetrability classification 16

Temperature changes in green Douglasfir when heated in creosote 50, 51

Water removed by Boulton process 46^9

Douglas-fir (Rocky Mountain tj^e), commercial air-seasoning periods 36

Commercial treating conditions for ties,

Central and Rocky Mountain States 119,120

Effect of different pressure periods on

treatment 89

Effect of different pressures and temperatures on treatment 78

Penetrability classification 16

Preliminary air pressure used with

Rueping process 86

Relative penetration of coal-tar creosote

and zinc-chloride solution 81

Dripping, after removing the charge, method

of reducing 81

Eastern hemlock, effect of viscosity and temperature on absorption and penetration 69

Penetrability classification 17

Effect of Boulton process on strength 113

Effect of density on absorption 13, 14

Effect of incising on penetration 38

Effect of keeping steam coils heated during

vacuum period 42

Effect of moisture content on rate of heating 54

Effect of vacuum on rate of heating_ 55

Effect of snow or ice on rate of heating 55

Effect of specific gravity on rate of heating, 55

Effect of preliminary vacuum on net absorption 86

Effect of treatment on the strength and physical properties of wood 110

Effect of tyloses on penetration 9

Effect of wood structure on treatment 6

Effectiveness of alternate steaming-andvacuum treatment 42

.Empty-cell process, absorptions..2, 95, 101-104,

115, 127-132

Advantages of 2, 3

Definition of 2, 3

Discussion of 2, 3

Lowry (without initial air), definition of 2

Preliminary air pressure 3

Rueping (with initial air), definition of • 3

Selection of treating process 125

Use after Boulton treatment 45

End coatings to reduce checking 33

End surface, illustration of 7, 8

Engelmann spruce, penetrabiHty classification 17

Equivalent surface area 96

Equivalent treatment 102

Expansion bath 94

Extractives, definition of 9

Factors affecting temperature changes in

wood 53

Factors that influence penetration and absorption 67

Fibers 6

Illustration of 7, 8

Fiber saturation point, definition of 19

Final vacuum after steaming 40

After treatment 126

Formulas for computing moisture content,

specific gravity, shrinkage, and air space

in wood 136-138

Formulas for computing temperatures in

timbers 149,150

Framing.

Free water in wood, definition of

Full-cell process, absorptions

After Boulton treatment

Definition of

Fungi, killing temperatures

Fusiform wood ray, illustration of

Green ash, penetrability classification.

Green wood, temperature changes in__

Gross absorption, definition of_

^ . . Page

Gmn, air-seasomng periods 36

Commercial treating conditions for ties,

Central and Rocky Mountain States 119

Commercial treating conditions for ties,

Eastern and Southern States 121,122

Gums in wood, effect on penetration 10

Hackberry, penetrability classification 17

Hardwoods, conditioning by Boulton process 47

Definition of ; 6

Heartwood, absorptions in 96

Difference between heartwood and sapwood 6

Ease of preservative treatment of 9

Effect on bleeding 115

Moisture content 19

Penetration in 96-101

Seasoning of 32

Heating periods for a given temperature in

green timbers boiled under vacuum 50

Hernlock eastern, commercial treating conditions for ties, Central and Rocky Mountain States 119

Hemlock, western, commercial treating conditions for ties, Central and Rocky Mountain States 119

Horizontal resin duct, illustration of 8

Ice, effect on rate of heating wood 55

Incising, definition of 38

Timber for treatment 38

Infiltrating substances 9

Influence of different initial wood temperatures or of different heating-medium temperatures on temperature obtained in

wood 140-143

Influence of structure on direction of penetration 14, 15

Influence of viscosity and temperature of

preservative on treatment 67

Initial absorption, definition of 110

Injecting preservatives 67

Injurious treating conditions, avoiding 123

Inner bark, effect on penetration. 33

39

20

95

45

2

67

8

17

54

90

Specification not practical 126

Jack pine, penetrability classification. 17

Kick-back, after empty-cell treatment 90

After full-cell treatment 93

Definition of . 90

Discussion of 90-93

Effect of compression of wood 92

Effect of expansion of air in wood cells. 90

Effect of preservative temperature 92

Effect of pressure 89

Effect of time pressure is applied 92

Relative kick-back from heartwood and

sapwood 93

Land piling, absorption in 132

Commercial steam-and-vacuum-treating periods _- 115

Larch, western, commercial treating conditions for ties, Central and Rocky Mountain States 119

Literature cited 152

Loblolly pine, penetrability classification— 17

Lodgepole pine, air-seasoning periods 36

Penetrability classification 17

Longitudinal penetration ^ 14

Longleaf pine, penetrability classification. __ 17

Lowland white fir, penetrability classification -,---.i- ^^

Lowland white pine, penetrability classification 17

Lowry process 2

Maple, penetrability classification 17

Maple, air-sea.soning periods 36

Commercial treating conditions for ties.

Central and Rocky Mountain States 119

Commercial treating conditions for ties.

Eastern and Southern States 121

Marine piling, retentions in 130

Commercial steam-and-vacuum-treating periods 115
158 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Page

Maximum absorption in round timbers 104

Maximum moisture content, formula for 136

Maximum vacuum commonly obtained 86

Maximum weight of wood 28

Measurement of absorption 107

Mechanical preparation 38

Adzing 39

Boring 39

Framing 39

Incising --- 38

Medullary rays, influence on preservative

treatment 14

Methods of determining moisture content in

timbers 21

Mixed charges, effect on absorptions 98

How handled 101

Mockernut hickory, penetrability classification 17

Moisture content, definition of 18, 30

Discussion of 18

Effect on weight of wood 20

Formula for 137

Increase during steaming ^ 40

Of green sapwood and heartwood 19

Relation to amount of water in wood 18

Net absorption, definition of 90

Not criterion of treatment 90

Variables affecting 127

Noble fir, penetrability classification 17

Norway pine, penetrability classification 17

Oak, red and wnite, commercial treating conditions for ties. Eastern and Southern

States 121

Oak, red, commercial treating conditions for

ties. Central and Rocky Mountain States 119

Commercial treating conditions for ties.

Eastern and Southern States 121

Oak, white, commercial treating conditions

for ties. Central and Rocky Mountain

States 119

Oaks, structural difference of red oaks and

white oaks 10

Oils, absorption in cell walls 81

Peeling before seasoning 33

Penetration, determined from increment

borings 133

Penetration of preservative, clsissification of

species with respect to 16

Effect of bordered pits 10

Effect of density 13

Effect of dimensions 96

Effect of gums and resins 10, 13

Effect of resin passages 13

Effect of simple pits 12

Effect of springwood 11

Effect of summerwood 11

Effect of tori 11

Effect of tyloses 9

Effect of wood structure on 14

In heartwood and sapwood 8

Longitudinal 14

Method of determining 133

Not proportional to net absorptions 94

Oils compared with water solutions 81

Radial 14

Specification of 133

Tangential 14

Pentachlorophenol 5

Permanence, important quality of preservatives _ 130

Petroleum oils 4

Piling, absorption in 130, 132

Piling timber for seasoning 35

Pin, cherry, penetrability classification 17

Pine stain for showing depth of sapwood in

pine species 134

Pine, lodgepole, commercial treating conditions for poles. Central and Rocky Mountain States 120

Pine, ponderosa, commercial treating conditions for poles, Central and Rocky Mountain States 120

Commercial treating conditions for ties,

Central and Rocky Mountain States 119

Page

Pine, southern yellow, commercial treating

conditions for green lumber and bridge

timbers 121

Commercial treating conditions for

steamed green cross arms 121

Commercial treating conditions for

steamed green piling and poles 122

Commercial treatmg conditions for ties,

Central and Rocky Mountain States 119

Commercial treating conditions for ties,

Eastern arid Southern States 121

Pinon, penetrability classification 17

Pits, bordered and simple 10-12

Poles, retention in 131

Poles and piling, commercial steam-andvacuum-treating periods 43, 44

Ponderosa pine, air-seasoning periods 36

Penetrability classification 17

Pores, definition of J. 6

Effect of penetrability of hardwoods 7

Illustration of_i 7

Posts, retentions in 131

Preframing 134

Preheating wood before applying pressure. _ 89

Preliminary air pressure 86

Relation to preservative pressure 88

Preliminary vacuum, function of 81

Preliminary vacuum periods, effect on net

absorption 86

Length of __ 86

Preparation of timbers for treatment 30

Adzing, framing, and boring 39

Air-seasoning 32

Boulton process 44

Incising 38

Steaming-and-vacuum process 39

Preservatives 3-6

Preservative pressure 88

Relation to pressure period 88

Preservative temperatures recommended 69-81

Pressure, maximum 124,125

Pressure period, effect on checking and collapse 89

Effect on treatment 88, 89

Effect on wood temperature 89

Pressure processes 2-3

Bethell full-ceU 2

Lowry empty-cell (without initial air) 2

Rueping empty-cell (with initial air) 3

Selection of 125

Proportional absorptions, sapwood timbers

and easily treated woods 101-102

Sawed heartwood timbers 96-101

Proportional absorptions in round timbers 102-107

Proprietary preservatives 6

Purpose of final vacuum 81

Purpose of preliminary vacuum 81

Radial penetration 14, 15

Radial surface, illiistration of 7, 8

Rate of applying preservative pressure 88

Rate of temperature change d\iring heating

in creosote 50

Rate of temperature change during steaming 53

Rate of temperature change in transverse

and longitudinal directions 54

Red oak, air-seasoning periods 36

Effect of tyloses on penetration 9, 10

Treatment when green 45

Red oaks, penetrability classification 17

Relation of preservative pressure and preliminary air pressure 88

Relation, of specific gravity, moisture content and air space in wood 28

Relation of wood density and penetration 13

Resin passages, definition of 13

Effect on penetration 13

Species containing : 13

Retentions 127-133

Commercial treatments 115

Effect of air space 92,125

Effect of density 13

For marine timbers 130

For mixed sizes in a charge 98

For poles and land piling 131,132

For posts 131
PRESERVATIVE TREATMENT OP WOOD 159

For sawed timbers_i 132

For ties 128

River birch, penetrability classification 17

Rock elm, penetrability classification 17

Round material, sapwood treatment essential 16

Round timbers, absorptions and depths of

sapwood 102-104

Rueping process 3

Sap, amount in trees in summer and in

winter 34

Sapwood, absorptions in 102

Amount in young trees 8

Decay resistance of 8,9

Difference between sapwood and heartwood 8,9

Durability when treated 9

Ease of preservative treatment of 9, 101

Effect on bleeding 115

Moisture content 19

Penetrations in 16,126

Resistance to penetration 16

Seasoning of . ^ 32

Strength in comparison with that of

heartwood 9

Sawed timber, commercial ateam-and-vacuum-treating periods 43

Sawed timbers, absorptions in 96-101

Seasoned wood, temperature changes in 54

Seasoning, effect of climatic conditions 34

Effect of size of timber on 36

Effect of species 36-38

Commercial seasoning periods 36-37

Optimum degree of 37

Yard sanitation 35

Selection of treating process _ 125

Shortieaf pine, penetrability classification. _ 17

Shrinkage and swelling _ 25

Shrinkage in volume in seasoning, formula

for -----. 137

Silver maple, penetrability classification 17

Simple pits 10-12

Sitka spruce, penetrability classification 17

SJippery elm, penetrability classification 17

Sodium fluorioe-- 5

Softwoods, definition of 6

Southern pine, effect of steaming on strength

and physical condition of wood 112,113

Steaming of 39-44

Steam pressure and steaming period 42

Tangential penetration 14

Temperature changes in round timbers. 143

In sawed timbers 147

Ingreen and seasoned timbers heated

in creosote 138-142

Temperatures obtained at top and butt

ends of poles or piling when heated. _ 56

Water removed by Boulton process.... 46

During steaming period and during

vacuum --- 40, 41

Southern yellow pine, absorption for marine

timbers 130

Absorption for poles and land piling. _ 131, 132

Air-seasoning periods 37

Empty-cell retentions in poles 131, 132

Species, classification with respect to ease of

penetration IJ

Hardwoods, description of 6t 7

Softwoods, description of 6,7

Specific gravity ^ 26

Change with shrinkage 28

Definition of " 26

For any moisture content below fiber

saturation point, formula for 136

Formula for 136

Method of computing 26

Of various species 2o, 24

Of wood substance 28

Specifications, discussion of 123

Absorptions for marine timbers 130

Absorptions for poles and land piling 131, 132

Absorptions for sawed timbers 132

Absorptions forties _-- 128

Avoiding injurious treating conditions. 123

General considerations 123

Page

Not practical to specify gross absorption 146

Penetration 133

Preservative pressures 88

Preservative temperatures 67-81

Selection of treating process 125

Spiral dowels 33

Steaming periods 123

Steam pressures 123

Unnecessary requirements 126

Springwood, definition of 11

Illustration of 8

Penetrability of * 11

Steaming, effect on checking 113

Effect on collapse 113

Steaming and vacuum periods used by commercial plants 43, 44

Steaming-and-vacuum process 39

Advantages and disadvantages of 44

Amount of water removed 40, 41

Commercial steaming practice 43, 44

Effect on strength and physical condition of wood 112

Steam coils heated during vacuum

period 42

Steam pressure and steaming periods.. 42

Sterilizing temperatures 67

Strength properties of heartwood and sapwood 9

Effect of treatment on 110

Structure of wood, effect on penetration 6

Sugar maple, penetrability classification 17

Summerwood, definition of 11

Illustration of 8

Penetrability of 11

Surface area and volume per foot length of

different sizes of timber 105,106

Surface hardening 37

Sweet birch, penetrability classification 17

Rweetgum, penetrability classification 17

Sycamore, penetrability classification 17

Symbols and formulas relating to temperature changes in wood 149,150

Tamarack, penetrability classification 17

Tangential surface, illustration of _._ 7, 8

Tank gage, method of measuring absorption 107

Tars 4

Temperature computations 56

Effect of ice or snow on temperature 55

In air-seasoned wood heated in creosote

at atmospheric pressure 54

In green wood when boiled under vacuum or when heated in creosote without vacuum 50, 51

In tops and butts of round timbers 56

Temperature of preservative, effect on treatment 69-81

Temperature of preservative oil, maximum. 81

Temperature required to kill fungi 67

Temperatures obtained in timbers during

pressure treatment...1 79

Ties, retention in 128

Commercial steam-and-vacuum-treating periods 43, 44

Time of cutting timber, effect on condition. 34

Time required to reach maximum preservative pressure 88

Total weight of wood and water at any moisture content, formula for 137

Toxicity alone is not measure of effectiveness

of preservative 130

Tracheids, definition of 6

Discussion of 6

Illustration of 8

Treating conditions used in commercial practice 115

Treating pressure, effect on checking 89

Effect on collapse 89

Specifications for 124, 125

Treating process, selection of 125

Treating temperatures 123-125

Tupelo, black, penetrability classification._. 17

Tyloses, definition of 9

Effect on penetration _ 9
160 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE

Page

Vacuum, amount of water removed by vacuum after steaming 40, 41

Boiling point of water for different vacuum conditions 85

Boiling under vacuum (Boulton process) 44-52

Circulation of heat in . 42

Corrections at different altitudes to correspond with sea-level conditions...- 84

Effect of keeping steam coila heated with

vacuum cylinder 42

Effect of vacuum on rate of temperature

change in wood during Boulton treatment 50

Effect on air in cylinder 43

Function of in boiling under vacuum

process 45

Function of in steaming-and-vacuum

process 40

Maximum commonly obtained 86

Minimum specified ^ 84

Preliminary and final vacuum periods. _ 86

Preliminary, function of 81

Use of_ 81

Use with full-cell process 81

Vacuum periods after steaming 86

Vapor-drying process 39, 52

Variables affecting treatment 67

Vertical resin duct, illustration of 8

Vessel, illustration of 7

^'^essels, definition of 6

Viscosity, effect on treatment 67-79

Viscosity of water solutions 75-79

Viscosity varied, temperature constant 73

Visible injury, comparison of sapwood and

heartwoodl 112-113

Volume occupied by moisture, formula for__ 137

Water, per unit volume of wood, formula for 137

Removed by Boulton process 46

Removed by steaming-and-vacuum

process 40-41

Water-gas tar 4

Water-gas-tar creosote 4

Water-soluble salts, use of 5

Water solutions, absorption in cell walls 79

Definition of 5

Effect of pressure on penetration of

zinc-chloride solution 75-79

Effect of temperature of zinc chloride on

penetration 75-79

Relative penetration of water solutions

and preservative oüs 81

Weighing-tank method for measuring absorption 107

Western hemlock, penetrability classification 17

Western larch, air-seasoning periods 37

Penetrability classification 17

Page

Western redcedar, penetrability classification 17

White ash, penetrability classification 17

White fir, penetrability classification 17

White oak, air-seasoning periods 37

Effect of tyloses on penetration 9

White oaks, penetrability classification 17

White spruce, penetrability classification 17

Wood preservatives 3-6

Arsenic 5

Chemicals dissolved in solvents other

than water 5

Coal-tar creosote 4

Coal tars 4

Copper sulfate 5

Creosote, coal-tar solutions 4

Creosote-petroleum mixtures 5

Naphthenates 5

Pentachlorophenol 5

Petroleum oils 4

Proprietary products 6

Sodium fluoride 5

Water-gas tar 4

Water-gas-tar creosote 4

Water-borne preservatives 5

Wood-tar creosote 4

Zinc chloride 5

Wood rays, illustration of 7,8

Wood structure 6

Air space 28

Bordered pits 10

Chemical infiltrating substances 9

Density 13

Diffuse-porous structure 7,10

Fibers 6

Hardwoods and softwoods, difference

between 6

Heartwood and sapwood, difference between 8

Pores 6

Resin passages 13

Ring-porous structure 7

Simple pits 10

Springwood, definition of 11

Summerwood, definition of 11

Tracheids 6,9

Tyloses 9,10

Wood-tar creosote 4

Wood temperatures during treatment 79

Yellow birch, air-seasoning periods 37

Penetrability classification 17

Zinc chloride

Effect of viscosity and temperature on

penetration of solution 79, 80

Relative penetration of zinc-chloride

solution and coal-tar creosote 81

TÎrU. s. GOVERNMENT PRINTING OFFICE: 1953-207777