DRYING EASTERN

HARDWOOD LUMBER

By

JOHN M. McMILLEN

and

EUGENE M. WENGERT

Forest Products Laboratory

Forest Service

U. S. Department of Agriculture

(Maintained at Madison, Wisconsin, in

cooperation with the University of Wisconsin)

AGRICULTURE HANDBOOK NO. 528

September 1978

Library of Congress Catalog No. 77-600073
MCMILLEN, JOHN M.

1977. Drying eastern

and WENGERT, EUGENE M.

hardwood lumber. U.S. Dep. Agric., Agric. Handb. 528, 104 p.

Presents recommendations based on recent research and

industry practice for drying eastern hardwood lumber, including dimension items. Accent is on comparing methods

for energy-saving management decisions, but practical guidance is also given to wood drying personnel. Air drying,

accelerated air drying, and kiln drying are covered.

KEYWORDS: Hardwoods, drying, air drying, kiln drying,

forced-air drying, high-temperature drying, steaming, dehumidifier drying, cost accounting, energy saving, degrade

reduction.

ABOUT THE AUTHORS . . .

JOHN M. MCMILLEN is a forest products technologist who has

specialized in the drying of wood for more than three decades

at the Forest Products Laboratory.

EUGENE M. WENGERT, formerly a member of the wood drying group at the Laboratory, is now extension specialist in

wood technology at Virginia Polytechnic Institute and State

University, Blacksburg, Va.

For sale by the Superintendent of Documents

U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 001-000-03761-7

ii
PREFACE

Interest in broader utilization of hardwoods has been spurred by shortages of softwoods

and an apparent surplus of hardwoods. However, to greatly increase use of hardwoods will

require simplified drying procedures. At the same time, the better grades of hardwoods must be

seasoned by methods less wasteful of both material and the energy required to dry it. These

facts make this publication desirable.

The handbook is confined to eastern hardwoods because most of the hardwood timber grows

in the Eastern United States. Much of the general methodology will apply to western

hardwoods, but some species have special problems and some localities have severe air drying

weather; western hardwood users should consult local authorities on specific questions.

Without supplanting the two existing handbooks on kiln drying and air drying lumber, this

handbook combines improvements in practice with new research findings to provide savings in

costs and energy. It assembles scattered technology on accelerated air drying and presents new

facts and interpretations on air drying. Kiln drying information is concentrated here, and other

methods of drying are discussed. An overall guide is also provided to combine various drying

procedures most economically for various product requirements.

This publication should be of the greatest value to operating and managerial personnel

responsible for hardwood lumber processing in hardwood mills, custom drying operations, and

furniture plants. It should also help individuals dry small quantities of lumber inexpensively

without sacrificing quality. Teachers and students should find this handbook helpful in career

development programs.

The authors acknowledge the assistance of Val Mitchell, State Climatologist, University of

Wisconsin, Madison in supplying and interpreting some of the climatological information

needed for this publication. Other vital contributions were made by many Forest Service

colleagues and coworkers in industry. Particular recognition goes to Walton Smith and Paul

Bois for photographs, to Kenneth Compton for specific information on steaming of walnut, and

to members of the U.S. Forest Products Laboratory research unit on improvements in drying

technology. For all these forms of assistance, we are most grateful.

JOHN M. MCMILLEN

E UGENE M. WENGER T

Use of trade, firm, or corporation names in this publication is for

the information and convenience of the reader. Such use does

not constitute an official endorsement or approval of any product or service by the U.S. Department of Agriculture to the

exclusion of others that may be suitable.

iii
CONTENTS

Drying Hardwoods . . . . . . . . . . . . . . . . .

Chapter I.—Comparison of Drying

Methods for Hardwoods . . . . . . . . . . . .

Basic Drying Concepts . . . . . . . . . . .

Methods . . . . . . . . . . . . . . . . . . . . . . . . . .

How Dry is Dry Enough . . . . . . . . . .

Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 2.—Stock Preparation and

Stacking . . . . . . . . . . . . . . . . . . . . . . . . . . .

From Logs to Lumber . . . . . . . . . . . .

Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . .

Stacking . . . . . . . . . . . . . . . . . . . . . . . . .

Further Warp Control . . . . . . . . . . . .

Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 3.—Air Drying . . . . . . . . . . . . .

Advantages and Limitations of

Air Drying . . . . . . . . . . . . . . . . . . . . .

Utilizing Air Movement . . . . . . . . . .

Other Factors Relating to Drying

Rate and Defects . . . . . . . . . . . . . . .

Drying Time and Final Moisture

Content . . . . . . . . . . . . . . . . . . . . . . . .

Deterioration of Lumber While

Air Drying . . . . . . . . . . . . . . . . . . . . .

Air Drying Small Quantities

of Lumber . . . . . . . . . . . . . . . . . . . . . .

A Quick Guide for Improving Air

Drying Efficiency of Hardwoods

Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 4.—Accelerated Air Drying

Research Basis . . . . . . . . . . . . . . . . . . .

Types of Dryers and Procedures . .

Advantages and Disadvantages of

Accelerated Air Drying . . . . . . . . .

Drying Times . . . . . . . . . . . . . . . . . . . .

Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 5.—Conventional Kiln

Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Drying Procedures . . . . . . . . . . . . . . .

Operational Considerations . . . . . . .

Drying Times . . . . . . . . . . . . . . . . . . . .

Literature Cited . . . . . . . . . . . . . . . . . .

Page

1

2

2

2

4

4

5

5

7

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8

10

11

12

13

13

19

21

21

22

22

23

23

24

31

31

33

34

36

52

54

55

Chapter 6.—High-Temperature

Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Basic Concepts . . . . . . . . . . . . . . . . . . .

Research and Practice to Date . . .

Drying Times and Species

Potentialities . . . . . . . . . . . . . . . . . . .

Apparent Process Requirements .

Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 7.—Special Predrying

Treatments . . . . . . . . . . . . . . . . . . . . . . . .

Steaming . . . . . . . . . . . . . . . . . . . . . . . . .

Chemical Seasoning . . . . . . . . . . . . . .

Polyethylene Glycol Process . . . . . .

Surface Treatments to Prevent

Checking . . . . . . . . . . . . . . . . . . . . . . .

Other Pretreatments . . . . . . . . . . . . .

Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 8.—Other Methods of

Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Heated Room Drying . . . . . . . . . . . . .

Dehumidifier Drying . . . . . . . . . . . . .

Solar Drying . . . . . . . . . . . . . . . . . . . . .

Press Drying . . . . . . . . . . . . . . . . . . . . .

High-Frequency and Microwave

Heating . . . . . . . . . . . . . . . . . . . . . . . .

Solvent Seasoning . . . . . . . . . . . . . . . .

Minor Special Methods . . . . . . . . . . .

Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 9.—Storage of Dried

Lumber . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Air-Dried Lumber . . . . . . . . . . . . . . . .

Kiln-Dried Lumber . . . . . . . . . . . . . . .

Chapter 10.—Economics and Energy

Typical Costs for Various Drying

Procedures . . . . . . . . . . . . . . . . . . . . .

Degrade Costs and Causes . . . . . . .

A Cost Accounting Approach . . . . .

Energy Considerations . . . . . . . . . . .

Literature Cited . . . . . . . . . . . . . . . . . .

Appendix A.—Species, Thicknesses,

Drying Schedules, and

Comments . . . . . . . . . . . . . . . . . . . . . . . .

Page

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56

57

58

59

60

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81

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84

iv
CONTENTS (Con.)

Page Page

Appendix B.—Mixed Drying of Some Appendix D.—Lumber Names, Tree

Species by Simplified Species, Seasoning Types, and

Schedules . . . . . . . . . . . . . . . . . . . . . . . . 87 Botanical Names Mentioned in

This Handbook . . . . . . . . . . . . . . . . . . . 92

Appendix C.—Special Kiln Schedules

for Special Purposes . . . . . . . . . . . . . . 89 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Requests for copies of illustrations contained in this publication

should be directed to the Forest Products Laboratory, Forest

Service, U.S. Department of Agriculture, P. O. Box 5130, Madison, Wis. 53705.

LIST OF TEXT TABLES

Table

1

2

3

4

5

6

Title

Expected average interior

relative humidities and

recommended moisture

content values for most

wood items for interior

uses . . . . . . . . . . . . . . . . . . . . . . .

Estimated time to air dry

green 1- and 2-inch hardwood lumber to approximately 20 percent average

moisture content . . . . . . . . . .

Forced-air dryer schedules for selected eastern

hardwoods . . . . . . . . . . . . . . . . .

Index of forced-air drying

schedules . . . . . . . . . . . . . . . . . .

Low-temperature kiln

schedules for selected

eastern hardwoods . . . . . . . .

Index of low-temperature

kiln schedules . . . . . . . . . . . . .

Page

4

18

27

28

29

30

Table

7

8

9

10

11

12

Title

Approximate Canadian

low-temperature kiln

schedules for 4/4 maple

and birch . . . . . . . . . . . . . . . . . .

Suggested low-temperature

kiln schedule for roughsawed 4/4 red and

white oak . . . . . . . . . . . . . . . . . .

Accelerated air drying times

for common hardwood

items in slightly heated

forced-air dryers (lowtemperature kilns) . . . . . . . . .

Grouping species for basic

kiln schedules . . . . . . . . . . . . .

Alphabetical index of

species and basic kiln

schedule table numbers . . .

Basic kiln schedules for

red and white oak, southern lowland . . . . . . . . . . . . . . . .

Page

30

30

32

36

37

38

v
LIST OF TEXT TABLES (Con.)

Table

13

14

15

16

17

18

19

20

21

Title

Basic kiln schedules for red

oak, northern or upland . . .

Basic kiln schedules for

white oak, northern

or upland . . . . . . . . . . . . . . . . . .

Basic kiln schedules for rock

elm, dogwood, persimmon,

and similar woods . . . . . . . . .

Basic kiln schedules for

American and slippery elm,

holly, and black walnut . . . .

Basic kiln schedules for

beech, sugar maple,

and pecan . . . . . . . . . . . . . . . . .

Basic kiln schedules for

white ash, yellow birch

cherry, sweetgum

and similar woods . . . . . . . . .

Basic kiln schedules for

magnolia, paper birch,

butternut, normal cottonwood, and similar woods . . .

Basic kiln schedules for yellowpoplar and cucumber-tree .

Basic kiln schedules for

black tupelo (black gum)

and sweetgum sapwood

(sap gum) . . . . . . . . . . . . . . . . . .

Page

38

38

39

39

40

40

41

41

42

Table

22

23

24

25

26

27

Title

Basic kiln schedules for

basswood, aspen (sapwood or box lumber). . . . . . . .

Accelerated kiln schedule

for northern or upland

red oak 4/4, 5/4 with

a high initial moisture

content (some risk of

slight surface checking) . . .

Approximate kiln-drying

times for 4/4 hardwood

lumber in conventional

internal fan kilns . . . . . . . . . .

Eastern hardwood species

for which high-temperature

kiln-drying potential has been

indicated, and estimated

drying time for 1-inch

lumber . . . . . . . . . . . . . . . . . . . .

Amount temperature must

be raised above average

outdoor temperature for

various equilibrium moisture content values in

heated-room drying . . . . . . . .

Energy consumption and

45

54

cost estimates in kiln

drying hardwood lumber . .

Mention of a chemical in this handbook does not constitute a

recommendation; only those chemicals registered by the U.S.

Environmental Protection Agency may be recommended, and

then only for uses as prescribed in the registration—and in the

manner and at the concentration prescribed. The list of registered chemicals varies from time to time; prospective users,

therefore, should get current information on registration status

from Pesticides Regulation Division, Environmental Protection

Agency, Washington, D.C. 20460.

Page

42

59

68

81

vi
DRYING HARDWOODS

Drying is a vital but sometimes troublesome procedure in the manufacturing of most

hardwood products. Only after much of the

moisture has been removed from wood is the

material ready to be made into useful products. And drying raises problems-specially

with hardwoods. During drying, degrade can

occur, resulting in both loss of value and loss

of valuable wood resource; considerable

amounts of energy can be wasted; and mistakes can be made that will cause problems in

subsequent manufacturing. Fortunately, a

variety of procedures are available that satisfactorily dry hardwood lumber for shipment

and ultimate use.

This report combines the information

published by many public laboratories, universities, and associations, with other information developed at the Forest Products Laboratory and other units of the Forest Service,

U.S. Department of Agriculture. It concentrates on the common ways of drying 1-inch

through 2-inch thick lumber, free from drying

defects, for the finest uses. It also suggests

ways to economize on drying time, cost, and

energy demands whenever hardwoods are to

be used for construction or less demanding

uses. It also provides a standardized method of

computing drying costs.

Much general information needed for air

drying and kiln drying hardwoods properly is

contained in two Agriculture Handbooks: The

“Dry Kiln Operator’s Manual” 1 and “Air Drying of Lumber.” 2 This report does not

attempt to repeat all details from those volumes. Rather, it presents new information on

these methods and on accelerated air drying,

as well as other methods available for drying

hardwoods.

1 Rasmussen, Edmund F. 1961. Dry Kiln Operator’s

Manual. U.S. Dep. Agric., Agric. Handb. 188,197 p., illus.

2 Rietz, Raymond C., and Rufus H. Page. 1971. Air Drying of Lumber: A Guide to Industry Practices. U.S. Dep.

Agric., Agric. Handb. 402, 110 p., illus.

-1-
CHAPTER 1

COMPARISON OF DRYING METHODS FOR HARDWOODS

Basic Drying Concepts

A great deal of scientific knowledge is not

necessary to dry wood. Some appreciation of

the basic concepts, however, will help in the

selection of drying methods and in the application of the general information on drying to

specific situations.

To dry wood, three basic requirements

must be met:

(1) Energy must be supplied to evaporate

moisture throughout the drying process.

As the wood becomes drier, additional

energy is needed to release moisture from

the hydroscopic forces that bind some of

the water to the wood. Very green wood

with large amounts of free water requires

580 calories per gram of water evaporated. Below 30 percent moisture content,

more energy is required (fig. 1). Energy

also is required to raise the temperature

of the wood to increase the rate of moisture movement from the interior to the

surface and to increase the rate of evaporation.

(2) The environment adjacent to the

wood must be capable of receiving moisture from the wood’s surface. In air and

kiln drying, the relative humidity must be

below 100 percent. The lower the relative

humidity, the faster the drying. (But relative humidity must not be so low as to

cause degrade. )

(3) Air movement through a stack of

lumber must be adequate to bring heat

energy in, to remove evaporated moisture, and to maintain desired relative

humidities within the stack. The relative

humidity within a stack of drying lumber

tends to rise above the general relative

humidity in the drying yard or kiln. Thus

the inside of the stack tends to dry slower

than the outside.

These three factors can be manipulated to

control the drying process, minimizing defects while drying the wood as rapidly as possible. See Dry Kiln Operator’s Manual and Air

Drying of Lumber for more information on

wood properties, the drying process, causes of

drying defects, and the methods of avoiding

them. More highly technical information is

given in other references (Brown et al 1952,

MCMillen 1969, Siau 1971, and Stamm 1964).

Methods

A number of methods are available for drying hardwood lumber, ranging from air and

kiln drying to special seasoning processes.

The customary method in the United States,

except in the northernmost States, has been

to air dry the lumber to 20 percent moisture

content, then kiln dry it as much as needed. If

lumber is in short supply, more lumber is kiln

dried from the partly air dry or green condition. Kiln drying from the green condition has

been common where air drying conditions are

poor most of the year.

During years of adequate energy and wood

supplies, the shortest air drying time and the

best target moisture contents for air dried

lumber were not well defined. Today these are

much more important; the combination of air

drying and kiln drying probably is the best

energy-conserving method for drying better

quality hardwood lumber in most of the

United States. A later chapter on air-drying

contains a map of the average number of

months of good air-drying weather available

in various regions, and a table of air-drying

times by species and thicknesses in these regions.

In regions where good air-drying weather

prevails only 4 to 6 months of the year, consideration should be given to accelerated air

drying procedures. These have good promise

for reducing lumber inventories and bringing

lumber production closer to the market. They

also permit quality drying of hardwoods with

character marks, or minor natural defects.

The chapter on this subject includes a discussion of the several types of equipment avail-

-2-
M 144691

Figure 1.—Energy required in drying of wood as wood

moisture content changes (after Skaar and Simpson

1968).

able, the schedules recently perfected for a

variety of Appalachian hardwoods at the

Forest Products Marketing Laboratory,3 and

a table of drying times for common hardwood

items in slightly heated forced-air dryers.

Very small quantities of hardwood lumber

can be air dried and then stored in a slightly

heated or dehumidified room to bring moisture content low enough for furniture and

other indoor uses. If fully air dried, such small

quantities can be kiln dried with almost any

other species of hardwood in a commercial

kiln, if a person can find a custom kiln drying

firm that will accept them.

Drying hardwood lumber in a conventional

kiln has several advantages. Modern conventional kilns have been designed to accelerate

drying greatly by using temperatures from

100° to 180° F with air velocities from 200 to

600 feet per minute through the load. Under

these conditions, a schedule of relative

humidities high enough to prevent drying defects can be maintained in reasonably tight

site-constructed or prefabricated buildings.

One advantage of kiln drying is the ability

to achieve a low, uniform final moisture con3 Northeastern Forest Experiment Station, Forest

Service, U.S. Department of Agriculture, located at

Princeton, W. Va.

tent. Another is to relieve drying stresses

(case-hardening). Relief of drying stresses is

necessary to prevent warp if more wood is

planed off of one side of a board than the

other, or if the wood is to be resawed or

machined deeply from one side. Uniformity of

moisture content and relief of drying stresses

are especially important to large-scale operations that produce hardwood dimension cuttings from small trees. If the stock is not

properly equalized and conditioned, warpage

and poor joints occur when the cuttings are

glued into panels.

While high-temperature drying (above 212°

F) is commonly used for softwoods, it has generally been considered inapplicable to most

hardwoods. However, recent research and industrial applications indicate more optimistic

prospects for this time- and energy-saving

method. Drying time may be only one-fourth

the time required for conventional kiln drying. There are two reasons why energy is

saved: (1) The vents are kept closed and (2)

the shortened drying time greatly reduces

heat loss through the structure and floor.

Slight discoloration may bar hightemperature-dried wood from some uses. The

method, however, may be fully satisfactory

for drying yellow-poplar and other easily

dried woods for concealed uses and for drying

similar hardwoods for construction purposes.

Besides major information on the above

drying processes, this handbook covers special predrying treatments that reduce defects

when drying especially refractory or defectprone woods, a number of other drying

methods, the proper storage of dried lumber,

and economic and energy considerations.

Predrying treatments include steaming for

color promotion or drying rate acceleration,

and treatment with the hydroscopic and bulking chemical (polyethylene glycol) for defect

reduction. Also included are wax, sodium alginate, or salt-paste surface treatment for the

prevention of surface checks.

A cardinal rule in drying hardwoods is to

reduce the material to the smallest practical

size before starting to dry it. Drying time is

more than doubled if thickness is doubled.

Therefore, it would be better, from the drying

standpoint, to make 2-, 3-, and 4-inch material

by gluing together the required number of

pieces of dry 1-inch material. This is one of the

-3-
considerations that makes press drying so of these appear likely to replace air and kiln

promising among the special drying methods. drying for the majority of hardwood drying

Other methods discussed are dehumidifier operations but they do have possibilities for

drying, solar drying, high-frequency and mi- some purposes.

crowave drying, and solvent seasoning. None

Table 1. —Expected interior relative humidities and recommended moisture content values for

most wood items for interior uses in the United States 1

Relative humidity Wood moisture content

Are as

Average Range Average Range

Percent Percent Percent Percent

Much of United States 40 30-55 8 6-10

Dry Southwest 2

30 15-50 6 4-9

Damp, warm coastal 60 40-70 11 8-13

1 From Wood Handbook, 1974.

2 Also applies to areas where interior environment is dry year around.

How Dry is

In the many decades of dealing with the use

of wood, the U.S. Forest Products Laboratory

has noted that problems often arise when the

wood is not dry enough. Too high a moisture

content can bring about difficulties in drying,

manufacturing, and use. Today, with tighter

construction in homes, with heated manufacturing plants, and with year-around climate

control in offices, the in-use moisture content

of wood products is quite low, lower even than

in the 1950’s and before. To avoid shrinkage,

warping, checking, and splitting in the finished product, lumber must be dried to a final

moisture content (after conditioning) close to

the middle of the range of expected in-use

moisture contents (table 1).

Furthermore, dried wood must be stored,

manufactured, and warehoused at or near the

expected moisture content values. Failure to

Dry Enough?

dry wood low enough and then keep it this low

as it gets into use may result later in open

glue joints, checking and splitting, finishing

imperfections, and warping. And these all decrease profits and waste this natural resource.

The moisture content for special uses may

be higher; if so, stringent storage precautions

generally are not required. Wood to be bent or

used in boats should be somewhere between

15 and 20 percent moisture content. After

bending, wood for interior uses should be

brought down to the moisture levels indicated

above. Wood to be used as framing or supports

in construction should be in the 10 to 19 percent moisture content range. Wood to be

treated with preservatives before use should

be between 20 and 35 percent moisture content.

Literature Cited

-4-
CHAPTER 2

STOCK PREPARATION AND STACKING

Before actual drying operations begin, a

number of steps are important in reducing

warp and avoiding other seasoning defects.

Unless great care is exercised in early phases

of the seasoning process, subsequent drying

From Logs

Log Protection

Logs should be taken from the woods and

sawed into lumber as soon as possible, particularly during warm weather. At a small

sawmill where only a few logs are accumulated, no other precautions are needed. At

larger sawmills, a considerable supply of logs

is often kept on hand, and some precautions

should be taken to protect against deterioration.

Logs that have been piled without protection may end check and be attacked by fungi

and insects, especially during warm weather

when higher temperatures speed up drying

and fungus and insect activity. Spraying with

cold water to keep the logs wet, especially

during warm dry weather, will reduce end

checking, splitting, staining, and decay. Continuous spraying is the most beneficial, so

spray equipment should be as clog-free as

possible. One type is illustrated in figure 2.

Chemical sprays effective against fungi and

insects also are available.

Protection of Green Lumber

It is important that lumber be protected

from the time it leaves the sawmill until it can

be transported to the seasoning site. Unless

the weather is cold, freshly sawed lumber

may be attacked by fungi and insects, and is

susceptible to checking and splitting if exposed to direct sunshine. Dipping the green

lumber in a solution of toxic chemicals can

protect it against attack by fungi or insects.

Prompt piling of the lumber for air drying

also makes attack by fungi and some types of

steps may be ineffective in reducing degrade

and costs. The precautions start with the logs.

Major points are discussed here, but others

are included in later sections where they are

specifically appropriate.

to Lumber

insects less probable. Protection against

sticker stain, interior graying, and other

oxidative discoloration also begins with the

green lumber (MCMillen 1976).

Protection against insects is accomplished

along with dipping or spraying to prevent

blue stain, the most common fungal hazard.

Principal chemicals used in the past included

mercurial and chlorinated phenols but environmental considerations have been changing the picture. Check with your County

Agricultural Agent or State Agricultural Experiment Station for approved chemicals.

Manufacturers will supply directions for use.

Treatment is imperative during most months

in the South and has been found beneficial

during the warmer months in many Northern

States. A general recommendation is to consider dipping during any period of a week or

more when a daily mean temperature of 60° F

or over is expected.

Dipping lumber to prevent fungus attack

does not protect it against oxidative discoloration; in fact, the excessive moisture left on

the wood may promote discoloration under

some circumstances. The best preventative is

to rapidly dry the surface moisture. Oxidation

stain can be minimized in hardwood lumber

by keeping the wood in the strictly green

state until rapid drying can be started, but

bulk storage should not be prolonged. If green

or excessively wet sapwood lumber has been

held more than 2 weeks before stacking, fast

dry the lumber surfaces before regular air

drying is started. Such fast drying can be accomplished by 1 day of accelerated air drying

or 3 days of very openly spaced yard drying. If

dried with mixed loads of sapwood and

- 5 -
M 144 876

Figure 2.—Water spraying equipment used effectively on decked logs.

heartwood, or if the sapwood boards have

heartwood on one face, the heartwood should

be observed frequently so that slower drying

can be started as soon as minor checks appear.

One mistake in handling green lumber is to

sticker it and then hold it in closely spaced

stacks on a temporary storage yard under the

assumption that it will be properly piled or

kiln dried “soon.” Damage from discoloration,

stain, or even decay can be very costly, even

with dipped lumber, if this storage is inadvertently prolonged. Spacing equal to that recommended for air drying is advisable during

such storage.

Preventing Surface Checks

and End Checks

Surface checks can start during exposure of

2-inch heartwood of any hardwood or l-inch

heartwood of oak, hickory, and beech 4 to di4

 Wood species are generally listed in this handbook by

their common names. But, because different names are

so often used for the same species, both common and

botanical names are included in Appendix D.

rect sunlight or strong winds during hot or

dry weather. Basically, the surface of the material is drying much faster than the interior.

The material can be protected from direct

sunshine by a roof. This might be over the

area used to sort and accumulate the freshly

sawed lumber, or even a temporary roof on

any pile that will be exposed an hour or more.

Research has shown that presurfacing

lumber to remove all fine saw marks reduces

surface checking in oak (McMillen 1969; Rietz

and Jenson 1966; Simpson and Baltes 1972;

Wengert and Baltes 1971). Although some

field trials have been successful (Cuppett and

Craft 1972, Rice 1971) industrial adoption will

depend upon economic considerations. Presently, surface checking in oak and other refractory woods can be economically controlled

by other precautions described later. Surfacing to a uniform thickness, when the lumber

has not been precisely sawed, is beneficial in

control of warp, a major cause of degrade.

Certainly another of the most common

forms of degrade is end checks. In uncoated

thick stock, end checks that are not even hair-

-6-
line cracks on air-dried material can extend

12 inches or more from the end of the piece.

This emphasizes the importance of end coatings.

Many hardwood manufacturing operations

routinely trim 3 to 10 inches off the ends of

every board. This is a 6 to 20 percent loss of

lumber for 100-inch lengths!

Almost 6/4 and thicker hardwood stock

would benefit from end coating. Two-inch and

larger squares usually are end coated, and

gunstock blanks must be coated.

End coatings may be divided into two classes: Cold coatings are liquid at ordinary temperatures and can be applied without being

heated; and hot coatings are solid at ordinary

temperatures and must be applied hot. Cold

coatings can be applied readily to logs and

lumber. Special end coatings of the coldcoating class are available from manufacturers and from dry kiln companies. Cold

coatings are usauly applied by brush, although they can be sprayed with proper

equipment. For small-scale use, heavy pastes

such as roofing cement can be used. Sprayable

wax emulsions, which have been very effective

on redwood lumber, gennerally do not provide

sufficient protection on thick hardwoods.

Hot coatings (pitch, asphalt, and paraffin)

are well suited for small stock that can be

easily handled. They are low in cost and high

in water resistance when applied in a single

coat. Hot coatings are generally applied by

dipping, but they also can be applied by holding the lumber end against a roller that is

rotated while partly submerged in the coating. Paraffin is a material with a low softening point; it is suitable only for air drying- or

for temporary protection of the ends of

squares that will be kiln dried by a kiln

schedule that uses a high relative humidity

during the first stages.

Some specialty items, such as walnut

gunstock blanks, require a highly waterresistant coating with a high softening point.

Without a good coating, end checks can extend themselves through the length of the

piece as honeycomb.

End coatings should be applied as soon as

possible to freshly cut end surfaces; end coatings applied after checking has begun usually

do not prevent deepening of the checks. Cold

coatings should be allowed to dry a few hours

before being subjected to kiln temperatures.

Sorting

From the standpoint of stacking and control of warping, hardwood lumber must be

sorted by individual nominal thicknesses.

Sorting by length and width also simplifies

piling. Sorting on the basis of species also

tends to increase the efficiency of the drying

operation, because species differ in their drying characteristics. A further sorting on the

basis of heartwood and sapwood would contribute to drying efficiency within some individual species. Further discussion of the most

common sorting practices is contained in Air

Drying of Lumber.

In the North, where the number of species

is small and the value of each species may be

high, sorting by individual species is the rule.

In the South, especially when removing

hardwoods from sites where pine is desired, it

may be expedient to combine species that can

be kiln dried together. However, hickory–

pecan, red oak, and white oak should be handled individually and southern lowland oaks

should be separated from upland oaks.

Sorting for quality has long been practiced

with some species, but there are new scientific findings explaining the quality differences and why low quality material presents

drying problems. So-called mineral streak or

mineral portions of sugar maple are now

known to be wound-induced discolorations

(Shigo 1965). Such material is very low in

permeability, greatly hindering water removal. Some wood of the oaks, aspen, cottonwood, and several other hardwood species

have streaks of wood infected with bacteria

(Ward et al 1972; Sachs, Ward, and Kinney

1974). Efforts should be made to recognize

these abnormal types of wood. When they are

present in large quantity, they should be

sorted out so that special kiln schedule and

moisture monitoring measures can be

applied.

-7-
Stacking

The importance of proper stacking cannot

be overemphasized. Sloppiness at this point

cannot be compensated for later on. The

major purposes of stacking lumber in a

specified manner are twofold: (1) To promote

uniform air circulation, which in turn results

in good drying; and (2) to reduce or eliminate

warp.

Hardwood lumber is most typically piled for

drying in packages with horizontal layers,

using stickers as spacers between each layer

(fig. 3). If the lumber is first end racked, end

piled, or cribbed for rapid surface drying, it

should. be repiled in flat piles within 3 or 4

days. Degrade from warp can be very costly if

the stock is end racked or cribbed very long.

The length of the lumber package is usually

the same as the length of the lumber being

cut. When several sizes are being sawn, several different package lengths may be used.

Sometimes 6-, 10-, and 12-foot lengths are put

in one sorted-length pile and 8-, 14-, and 16-

foot lengths in another. The shortest pieces

are piled end to end and the intermediatelength pieces all pulled to one end. Hardwoods

sawed in both odd and even lengths should be

box piled (fig. 4). In either method, it is important to have full-length pieces on the outer

edges of each layer and full support under

each end of each board.

Width of the pile is frequently governed by

the requirements of the dry kiln or forced-air

dryer. For most modern kilns with air velocity

through the load of 200 feet per minute or

more, the most common package width is 48

M 135 021

Figure 3.—Stacking stickered package of lumber.

inches. However, in air drying, and in older

kilns with low air velocity and insufficient

venting, less air is moving. Thus, the narrower the pile, the faster the wood will dry.

Hand-stacked piles should not exceed 6 feet

wide unless they are provided with vertical

flues. See chapter 3 for more comments on pile

construction.

Hardwood lumber stacking should aim for

square, level, and straight-sided piles with all

stickers and bolsters in perfect alinement.

Acceptable piles are the rule with machine

stacking, which is widely used in medium- and

large-sized hardwood mills and concentration

yards. It is also possible to meet quality

specifications with hand stacking if sticker

guides are used with proper care.

Further Warp Control

In addition to perfect, vertical alinement of of a unit package and reduces the amount of

the stickers, good bolster placement, and good warp. Precision sawing to a uniform thickness

pile roofs, several other considerations help in also will reduce warp.

reducing degrade, particularly warp:

1. Basic warp control can be obtained 3. The foundation, kiln floor, or kiln trucks

through proper sawing procedures, especially must provide a firm, flat bearing surface for

for small logs. Saw especially warp-prone ma- the lumber pile. A crooked or uneven surface

terial, such as city-grown elm, into wide will cause twist, bow, or kink in the lumber

boards. during drying.

2. Presurfacing lumber (planing both faces) 4. Properly sized and placed stickers are

or blanking (one face) to a uniform thickness highly important, with uniform thickness a

before stacking increases the usable volume prime requirement. Broken or distorted

-8-
stickers can increase

paired or discarded.

M 144695

Figure 4.—Method of box piling for random-length lumber.

warp and should be re5. Sticker spacings of 16 to 24 inches are

satisfactory for many hardwoods, provided all

tiers of stickers are fully supported and their

alinement is perfectly vertical. Very valuable

lumber or especially warp-prone material

may benefit from spacing as close as 12

inches.

6. Several drying firms restrain the tops of

the loads by spring clamps anchored lower in

the pile, or by added weight, Research showed

a load of 50 pounds per square foot was

adequate for aspen 2 by 4’s dried by high temperature. More dense woods probably would

require more pressure. Research on 1-inch

blackgum, for instance, showed 150 pounds

per square foot more effective than 90 pounds

per square foot in reducing cupping. Two

types of hold-down clamps for restraint of

warp are shown in Air Drying of Lumber. If

difficulty is experienced in locating sources of

hold-downs, the utilization specialists in state

forestry or forestry extension offices may be

able to help.

7. Rapid natural air drying or equivalent

accelerated air drying may be a necessary

first step in drying warp-prone woods such as

American elm. Such methods tend to develop

a large amount of tension set in the outer

shell, which helps to hold the lumber flat in

later stages of drying (MCMillen 1963). When

kiln drying lumber green from the saw, use

schedule modifications that provide initial

conditions of lower temperature and lower

relative humidity than those generally recommended.

Most of these procedures will require the

operator to spend some money to implement

them. However, as the value of high-quality

lumber rises and quantities lessen, the costbenefits may justify employing these

methods.

-9-
Literature Cited

-10-
CHAPTER 3

AIR DRYING

Where annual mean temperatures are not

too low and space for a drying yard is not too

limited, air drying is the most economical and

energy-conserving method to get most of the

water out of hardwoods. To achieve that end,

however, keep in mind a thought that is best

stated colloquially: “Wood can’t help drying,

if you help it!”

The green moisture content of U.S.

hardwoods ranges from 45 to 160 percent, on

the ovendry basis. The moisture above 30 percent is present as water or water vapor in the

wood cell lumens (cell cavities) and is called

“free water.” This water comes out of the

wood very readily in air drying. The rest of

the water, in the wood-cell walls, is called

“bound water.” It comes out more slowly, particularly as the moisture content decreases

below 25 percent.

In the past, the moisture content goal for

air-dried lumber was 20 percent. One reason

was to reduce the chance that mold, stain, or

decay would develop during transit, storage,

or use of the wood. Another was to reduce the

amount of shrinkage that would occur if the

wood were used for an exterior purpose without further drying. Time to air dry to an absolute 20 percent moisture content is unduly

long, however, in some regions at some times

in the year. This is especially true for stock

thicker than 4/4 (1 inch).

Under proper air drying procedures, the

surface zone will be at or below 20 percent

moisture content when the average is 25 percent. There are no substantial difficulties in

kiln drying air-dried stock at this moisture

level. This handbook, therefore, recommends

terminating the air drying of stock that is to

be subsequently kiln dried at the 25 percent

average moisture content level whenever it is

advantageous economically. Items to be

treated with preservatives also should stop

air drying at this level.

On the other hand, lumber to be bent to

form, used for outdoor furniture, or used in

constructing unheated barns and garages

should be air dried to 20 pereent or slightly

below. Wood to be bent in a hot press or

machined for trim and flooring in boats and

buildings that are heated only occasionally

should be dried to a lower moisture content

(12 to 18 percent). This level can be obtained

by prolonged air drying in a shelter. Material

for ultimate interior use in heated or dehumidified rooms should, of course, be dried

further, in a kiln or by heated-room drying.

The general principles of air drying are reasonably well understood. Their proper application can make this method of reducing

moisture content more efficient and profitable. During good air drying weather, slight

deviation from best practices is tolerable

without incurring serious degrade losses or

retardation of drying rate. Three or more

days of greatly abnormal weather, however,

can be devastating. High humidity, heavy

rain, and fog can cause discoloration; strong

winds in combination with low relative

humidity can cause severe checking. Therefore, it is important to adhere to good practice

year around, irrespective of normal weather

conditions. With such practices, degrade can

be limited to 1 or 2 percent of the lumber

value.

The Air Drying of Lumber gives a complete

discussion of drying practices. Several points

are expanded in this handbook, however, to

emphasize the assumption that faster drying

and conserving more energy and resources

now are paramount in air drying. Some of

these points involve promotion of practices

known to be helpful but not commonly used

because they add some cost.

-11-
Advantages and Limitations of

Air Drying

One real advantage of air-drying lumber

over drying by other processes is its low initial cost. Although there are substantial land,

installation, and operating costs for air drying, the cost of kiln-drying dense hardwoods

to the moisture content levels achieved in air

drying could be prohibitively high. However,

as the value of the wood increases, kilndrying species like beech, birch, and maple

green from the saw becomes more feasible.

This is especially so in regions having fewer

than 6 months of good air-drying weather.

A second real advantage is substantial

energy saving. Each percent moisture removed by air drying saves energy in subsequent kiln drying by roughly 50 to 85

British thermal units per board foot. For a

conventional kiln with a capacity of 50,000

board feet, this means 2.5 to 4.25 million

British thermal units can be saved for each 1

percent moisture lost in air drying. At late

1974 prices, this amounted to $3.75 to $6.38

(Wengert 1974).

The air-drying process offers the producer

or large-scale user a means of carrying an

inventory of various species, grades, and sizes

of lumber. To meet shipping schedules during

periods of the year when the sawmill cannot

be operated to capacity, the yard inventory is

built up when sawing conditions are favorable, and the lumber is air dried while being

held. Air drying also reduces weight and

shipping costs. Any degree of drying that can

be achieved, even during short periods of accumulation, is helpful. A variety of drying

processes are available at the destination to

accommodate partly air-dried stock. Some

hazard of oxidative discoloration, however,

may still exist for white sapwood species partially dried and then bulk-piled for shipment.

In combination with properly applied antistain dip treatments,5 air drying decreases

the chance that mold, stain, and decay will

5 Check with your County Agricultural Agent or State

Agricultural Experiment Station for approved recommendations.

cause degrade, as they could during bulkpiled storage and shipment. Rapid air drying

at low relative humidities produces a large

amount of set that assists in reducing warp. If

the stock is properly stacked and protected in

a well-laid-out yard, minimum degrade from

checking will be apparent, even for thick

stock of dense woods and 1-inch heartwood of

especially refractory woods.

Limitations of air drying are generally associated with the weather and the uncontrollable nature of the process. Production

schedules are at the mercies of changing

climatic conditions—temperature, relative

humidity, rainfall, sunshine, and winds. Drying generally is very slow during the cold

winter months in the northern regions of the

country. But mean annual values for the various factors are fairly consistent within any

one region from year to year, so general production schedules and plans can be worked

out. Specific technical information and good

judgment are needed, however, to complete

detailed planning.

Lack of absolute control of drying conditions poses some hazards of excessive degrade. Unusually fast drying of the surface,

caused by sun or wind and low relative

humidity, can cause surface checking of beech

within a day, even in the winter in the North.

In other times and areas, brief periods of hot,

dry winds may increase degrade and volume

loss due to severe surface checking and end

splitting. Warm, rainy, or sultry periods, with

little air movement, encourage the growth of

blue stain and aggravate chemical (oxidation)

stain and can cause excessive losses unless

proper dipping, pile spacing, and piling

methods are used.

An excessively large inventory is very

costly, especially when interest rates are very

high. Substantial deterioration of the lumber

can occur if air drying is prolonged beyond

the time needed to bring the moisture content

down to 18 percent or less. If poor market

conditions result in the holding time going

beyond 2 years, extra measures against deterioration may be advisable.

-12-
Utilizing Air Movement

Air circulation is so important that it is

considered broadly here as well as in detail

later in yard layout, foundations, and piling.

Air circulation is a principal means of bringing into the air-drying yard or shed the heat

needed to evaporate water. Air circulation

also brings heat into the lumber piles as well

as being itself a factor in drying. It is involved

in removing moisture-laden air from the pile

and from the drying area.

Because air must move into and through

the drying area, yard layout is very important. Adequate alleys and spaces must be

provided between rows and piles. By custom

in the United States, the “main” alleys for the

transport of lumber to the piles are perpendicular to the prevailing wind direction. However, recent research showed that, for some

types of yards, better circulation is obtained

by having the main alleys parallel the prevailing wind direction. Whatever such

refinements, the air space s must b e

adequately large and straight and continuous

across the yard. If these principles are observed, air flow will be adequate through the

yard whenever there is wind, regardless of

which direction the piles face.

Except for those piles exposed directly to

the wind, with their orientation perpendicular to the wind direction, no wind blows

through any pile on an air-drying yard.

Rather, all air flow across the boards is due to

small air pressure differences from eddy and

aspiration effects and to air density differences. As the water evaporates, the air is

cooled in and around all piles. Cooling makes

the air denser, and it tends to flow downward.

When the cool dense air is removed from beneath the piles, fresh air is brought into their

tops. This effect and the consequent drying go

on continuously, regardless of wind, unless

the air around all the boards is saturated

with water vapor. This is the reason why,

from the fast, economical drying viewpoint,

high and open pile foundations are very important.

One precaution should be stated about the

location of thick or especially checksusceptible material in the yard and the

manner in which the yard is filled up. Such

material should not be located on the windward side of the yard. Lower air velocities and

increased relative humidities in the central or

leeward areas of the yard modify the severity

of drying conditions and reduce the likelihood

of checking. During severe weather, however,

it would be bad practice to start refilling an

empty yard from the leeward side toward the

windward side, because each pile in turn

would be exposed directly to the wind.

Other Factors Relating to Drying Rate and Defects

The rate at which green lumber will dry between them is related to the permeability

after it is placed in the air-drying yard de- of the wood and the diffusivity of the water in

pends upon factors that involve the wood it- it. Much of this difference is due to differences

self, the yard, the pile, and other climatic con- in the proportion of sapwood and heartwood.

ditions as well as wind. Beech and maple are almost all sapwood; oak

Wood is almost all heartwood. The fibrous parts of

the oak heartwood, making up most of its

Species structure, are low in permeability and difSome lightweight hardwoods (basswood, fusivity.

willow, yellow-poplar) dry rapidly under fa- Generalized estimates of air drying time by

vorable air-drying conditions. The heavier species are given later in this chapter. It must

hardwoods require longer drying periods. be recognized, however, that some woods are

Specific gravity, then, is a physical property handled commercially in groups of species,

of wood that can guide estimations of drying and there are differences between species in

rates or overall drying time. But beech and the groups. Pecan, for instance, is regularly

sugar maple will dry faster in northern yards made up of sweet pecan (Carya illinoensis),

than will northern red oak, even though all water hickory which is often called bitter

have the same specific gravity. The difference pecan (C. aquatica), and one other minor

-13-
Carya species (see Appendix D). The sapwood

and heartwood of sweet pecan and the sapwood of bitter pecan dry very rapidly, but the

heartwood of bitter pecan dries very slowly.

A similar situation exists with the tupelos.

The heartwood and sapwood of black tupelo,

usually called blackgum (Nyssa silvatica) and

the sapwood of swamp tupelo (N. silvatica

var. biflora) and water tupelo (N. aquatica)

dry rapidly, but the heartwood of water

tupelo usually dries slowly. The heartwood of

swamp tupelo is intermediate in drying

characteristics.

Red oak is made up of several species of

Quercus and white oak of another group of

Quercus species. Southern lowland oaks, both

white and red, have drying characteristics

similar to each other but different from those

of the upland oaks. They are, therefore, separated in drying time and kiln schedule tables.

Anyone drying the woods specifically mentioned and experiencing unduly long drying

periods should consult a county or State

utilization and marketing forester to see if

some practical means of sorting the types

could be derived.

Grain Patterns

Quartersawed lumber dries more slowly

than flatsawed lumber. In quartersawed

lumber, few wood rays—which aid the movement of moisture—are exposed on the broad

surfaces of the boards. Flatsawed lumber,

which has more rays exposed, is more likely to

surface check than quartersawed lumber

under severe drying conditions.

Thickness

Thick stock naturally takes longer to dry

than thin material. One theoretical approach

suggests that drying time, under identical or

similar drying conditions, is a function of the

square of the thickness. In actual kiln drying,

the effect of thickness is slightly less. Since

thick stock takes longer than one air-drying

season to reach 25 percent moisture content

in the central and northern parts of the

United States, actual air-drying times for

2-inch stock are three to four times as long as

those for 1-inch. Also, the greater the thickness, the greater the tendency for hardwoods

to surface check and end check.

Yard

Site

An efficient yard for rapid air drying should

be on high, well-drained ground with no

obstruction to prevailing winds. One southeastern furniture firm made considerable

savings by building a new 12-acre yard on a

hill 3 miles away from three old, overcrowded,

and inefficient yards near their plants

(Minter 1961). The savings included a 45-

percent labor reduction, faster drying, and a

decrease in total inventory of 2 million board

feet.

Layout

Alley orientation and size, row and pile

spacing, and pile size for both hand-built pile

and forklift yards are discussed extensively in

Air Drying of Lumber, The general function

of alleys and pile spaces in removing

moisture-laden air from the yard has been

well known. The effect of wind-induced circulation within the piles, however, was not understood until wind tunnel-miniature model

pile studies were made using forklift layouts.

White (1963) used two orientations of a

single block of row-type model piles. Artificial

smoke was used to indicate the relative

amount and direction of air movement. When

the piles were oriented perpendicular to the

wind, much of the air was deflected over the

top of the block. No wind blew directly

through any pile except those right on the

windward side. Some of the air that flowed

between the other piles, however, entered

these piles from the rear and left from their

fronts, sides, and tops. Double eddies at the

rear of each pile moved air up into the turbulent area above the piles. When the piles were

oriented parallel with the wind, more air

moved through the spaces between the piles

and into and out of the piles.

Finighan and Liversidge (1972) used eight

different layouts. Two were single large

blocks of row-type yards; the others were

line-type yard variations. The evaporation

rate of a solid plug of moth crystal s

(paradichlorbenzene) located in each model

pile simulated its rate of drying. The rates in

all the line-type layouts were 20 to 40 percent

greater than in the row-type layouts. These

differences probably would have been less if a

greater amount of “open space” had been

-14-
used in the row-type layouts. The greatest

amount of “drying” took place in the line-type

layouts with the greatest amount of open

space-double lines of piles with 25-foot alleys. The best combination of good average

“drying” rate and good uniformity throughout the layout occurred when the model piles

and the main alleys were oriented parallel

with the wind.

For those species and sizes that may be

dried as rapidly as possible, without danger of

loss from splitting and checking, the line-type

layout parallel with the prevailing wind appears to be best. For species such as oak that

are subject to checking, the row-type layout

may be required. The length of rows and the

number of rows should be limited, however. If

the row-type layout is used and if there are

problems of getting lumber of all piles uniformly dry for shipment or kiln drying in the

shortest possible time, variable o r

nonuniform (using movable pile foundations)

pile spacing in each row should be tried

(MCMillen 1964). Such nonuniform spacing

has been widely used for redwood, one of the

softwoods. In this method, the spacing between piles in the middle of the row is about

double the spacing between the piles near the

alleys.

Whatever yard layout style is used, the

main and cross alleys and the spaces between

piles should be perfectly alined all the way

across the yard, and free of all obstructions.

Surface

The drying efficiency of a yard depends to

some extent on how well the surface is

graded, paved, and drained. If water stands in

a yard after a rain, it will decrease the drying

rate. The yard described by Minter (1961) was

surfaced to a 6-inch depth with crushed rock.

Blacktop paving also is used extensively. The

yard should also be kept clean. Vegetation

and debris, including broken stickers, boards,

or pieces of timber from pile foundations,

interfere with the movement of air over the

ground surface.

Piles

Piling Methods

The drying rate of lumber is affected by the

way the boards are stacked. For instance,

lumber dries faster when air spaces are left

between the boards of a unit package than

when the boards are placed edge to edge. The

air spaces permit greater downward flow of

air within the piles. This is important when

the outside air is calm or nearly so. Boards in

unit packages 4 feet or less in width are usually stacked edge to edge, and the downward

flow occurs in the spaces between the piles.

Minimum pile spacing is 2 feet. If the unit

packages are wider than 4 feet, the boards in

each course should be spaced. When randomlength, random-width lumber is box-piled,

enough space develops within the pile to

make additional board spacing unnecessary.

Wide unit packages of even-length lumber

and wide hand-stacked piles are often built

with flues or central chimneys.

Pile Foundations

High openly designed pile foundations are

necessary to allow the moist air to pass out

readily from beneath the pile. They also are a

significant factor in obtaining uniform drying

from the top to bottom of each pile. Finighan

and Liversidge (1972) found that the evaporative loss from top packages from wind effects

always was greater than that from bottom

packages. Foundation height for hand-built

piles should be at least 18 inches above the

yard surface. In well-laid-out and welldrained forklift yards, a 12-inch minimum is

satisfactory in many regions, but in areas of

high rainfall, the 18-inch minimum should be

used with this type of stacking. In unpaved

yards, weeds and debris should not be allowed

to block air passage.

Roofs and Sheds

Clark and Headlee (1958) showed that pile

roofs saved enough in degrade and drying

time for 4/4 No. 1 Common and Better red oak

to pay for the roofs in five uses. Also, faster

drying rates and lower final moisture contents were achieved by the use of roofs in

rainy spring weather. A report on similar

Australian air drying research, reviewed by

MCMillen (1964), showed outstanding differences between roofed and nonroofed piles

during rainy autumn and winter seasons.

Hardwoods in nonroofed piles dried to 26 percent moisture content in 70 days, then did not

dry any further for the next 150 days.

In regions of high rainfall, shed roofs give

better protection of the lumber from rewetting. They can cut customary air drying times

in half or greatly reduce the amount of water

to be evaporated in a kiln after a “standard”

length of air drying. An example of the halv-

-15-
ing of drying time is the seasoning of some

railroad cross-ties in the southeastern coastal

areas.

Climatological Conditions

The climate of the area or region in which

the hardwood air-drying yard is located

greatly influences the air-drying rate or yard

output of air-dried lumber. Perhaps the most

influential factor is temperature, but relative

humidity and rainfall also are in the picture.

In northern lumber-producing regions, the

drying rate is retarded during the winter

months by the low temperature. In the southern part of the country, where the winter

dry-bulb temperatures are higher, better drying conditions are expected. But, these higher

temperatures may be offset in some localities

by rains that wet the lumber and extend the

drying time unless the lumber is well

protected.

A map of different air drying weather regions in the Eastern United States is shown

in figure 5. There are about the same number

of “good air drying months” throughout each

region. There is no exact definition of a “good

air drying month.” Such a month, however, is

probably roughly equivalent to a month with

22 or more days equivalent to the “effective

air drying days” in the Upper Midwest (Rietz

1972). Research on beech, sugar maple, and

red oak by Peck (1954, 1957, 1959) indicated

hardwoods air dry at a moderate rate when

the daily mean temperatures are over 45° F

and at a considerably better rate when they

are over 60° F. The boundaries of the zones in

the map are largely based on the average

cumulative “growing degree days” from

March 1 to mid-October for the years 1971-5

(U.S. Department of Agriculture 1975).

(“Growing degree days” are computed to a 50° F

base with daily maximums listed to 86° F

and daily minimums to 50° F.) The northern

M 144115

Figure 5.—Air drying map for the Eastern United States.

-16-
extensions of the Central and Mid-South zones

along the Atlantic Seaboard are located to

coincide with the periods when mean temperatures are over 45° F (U.S. Department of Commerce 1973).

General confirmation of the map was obtained from a large number of persons

throughout the Eastern United States who do

commercial air drying. Within the Central,

Mid-North, and North zones, a drying yard

located on an exceptionally good site, with

local temperature and wind velocity above

the zone averages, could be expected to have

an extra month per year of “good” weather.

Obviously, a “good” air drying day is not

“the best.” To use the map in planning the

most economical mix of air drying and kiln

drying or of accelerated air drying and kiln

drying, one should consider that “good” air

drying weather is capable of bringing 4/4 oak

and other slow-drying hardwoods to 25 percent moisture content in 90 days. The “best”

weather will do it in 60 days. During unusually cold winters in the South, 90 days will not

be enough. This specific drying job should

take about 120 days during such a winter.

Another important factor is the wood

equilibrium moisture content (EMC). 6 A t

mean temperatures from 45° to 85° F, EMC is

closely related to relative humidity:

Relative humidity EMC

(Pct) (Pct)

88 20

80 16

73 14

65 12

54 10

42 8

30 6

17 4

Wood in contact with 42 percent relative

humidity obviously will dry faster than wood

in contact with 88 percent. But even air at 88

percent has a good potential for drying wood

to 25 percent moisture content, The objective

of good site selection and preparation, good

yard layout, and good pile design is to get the

outside air to move into the pile to pick up

moisture and then move out again. If the air

does not move out, saturation occurs and dry6 The EMC for a given atmospheric temperature and

relative humidity is the moisture content at which wood

in contact with that atmosphere neither gains nor loses

moisture.

ing stops. Where average relative humidities

are high for long periods, the use of spaces

between boards, or of flues and chimneys, is

necessary along with high, open pile foundations. These will promote downward flow and

remove moist air whenever any wind, however slight, occurs.

Conversely, too low a relative humidity,

especially in company with high winds, can

cause surface checking and end checking.

This can happen at any time of year, not only

during the “good” months. Danger periods

often occur in late winter or early spring

when rapid solar heating of air with a low

absolute humidity causes the relative humidity to go to very low values. A good yard manager will learn from monthly weather records

when such periods of low relative humidity

are likely to occur and will pile checksusceptible species in a manner that restricts

air circulation so as to avoid checking.

Rain is not a direct factor in governing drying rates. In low sites, with poor yard surfaces

and drainage, rain tends to raise the relative

humidity within the bottom of the piles and to

interfere with downward air flow and moist

air removal. Any lumber that is rewet, of

course, must be redried. Pile roofs keep most

of the rain out, with the water along the sides

of the pile being only superficial, except for

wind-driven rain. Where heavy rains are

common, sheds with wide overhang should be

considered.

Sunshine is another indirect factor. Solar

radiation heats nearby land areas, exposed

areas between the lumber piles, and surrounding buildings. Air moving over these

warmed areas and structures is heated by

convection, and its drying potential increases

both by raising temperature and lowering relative humidity. Black bodies absorb more

solar energy and become hotter than lightcolored materials. This characteristic is used

to advantage in air-drying yards by blacktopping the roadways and sometimes the whole

area.

In some instances it may be desirable to

arrange the main alleys of the yard north and

south to take greatest advantage of the solar

heat. This helps to quickly melt snow in the

North and to dry out the yard faster in all

regions after heavy rains. The sun also can

cause too high a temperature and too fast

drying in the tops of piles in the South and

-17-
Table 2. —Estimated time to air dry green 1- and 2-inch eastern hardwood lumber to

approximately 20 percent average moisture content

Estimated time by region 2

Species 1 Size

South Mid-South Central Mid-North

Ash

Aspen

Basswood, American

Beech, American

Birch, paper

Birch, sweet; yellow

Butternut

Cherry

Cottonwood, eastern

Elm, American; slippery

Elm, rock cedar; winged

Hackberry, sugarberry

Hickory

Magnolia

Maple, red; silver

Maple, sugar; black

Oak, lowland

Oak, red (upland)

Oak, white (upland)

Pecan

Sweetgum, heartwood (red gum)

Sweetgum, sapwood (sap gum)

Sycamore

Tupelo (and blackgum)

Walnut, black

Willow, black

Yellow-poplar

Inch

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

Days

45 - 70

180 - 210

—

—

40 - 65

170 - 200

45 - 70

180 - 210

—

—

—

—

—

—

45 - 70

180 - 210

40 - 65

170 - 200

40 - 65

170 - 200

50 - 80

190 - 230

40 - 65

170 - 200

50 - 80

190 - 230

40 - 75

170 - 220

40 - 65

170 - 200

45 - 70

180 - 210

100 -3

280

No data

60 - 120

240 - 360

60 - 120

240 - 360

60 - 120

240 - 360

50 - 80

190 - 230

40 - 65

170 - 200

40 - 65

170 - 200

60 - 110

210 - 300

45 - 70

180 - 210

30 - 65

150 - 200

40 - 65

170 - 200

Days

45 - 75

180 - 220

—

—

40 - 70

170 - 210

45 - 75

180 - 220

—

—

50 - 85

190 - 240

40 - 70

170 - 210

45 - 75

180 - 220

40 - 70

170 - 210

40 - 70

170 - 210

50 - 85

190 - 240

40 - 70

170 - 210

50 - 95

190 - 240

—

—

40 - 70

170 - 210

45 - 75

180 - 220

—

—

55 - 100

215 - 300

55 - 100

215 - 300

65 - 100

215 - 300

50 - 95

180 - 240

40 - 70

170 - 210

40 - 70

170 - 210

45 - 90

180 - 220

45 - 75

180 - 220

35 - 70

160 - 210

40 - 70

170 - 210

Days

45 - 80

180 - 230

—

40 - 75

170 - 220

45 - 80

180 - 230

—

—

50 - 90

190 - 250

40 - 75

170 - 220

45 - 80

180 - 230

40 - 75

170 - 220

40 - 75

170 - 220

50 - 90

190 - 250

40 - 75

170 - 220

50 - 90

190 - 250

—

—

40 - 75

170 - 220

45 - 80

180 - 230

—

—

50 - 90

190 - 250

50- 90

190 - 250

50 - 90

190 - 250

50 - 90

190 - 250

40 - 75

170 - 220

40 - 75

170 - 220

45 - 80

180 - 230

45 - 80

180 - 230

40 - 75

170 - 220

40 - 75

170 - 220

Days

60 –3 165

No data

50 -3 120

No data

40 -3 120

No data

60 -3 165

No data

40 -3 120

170 - 220

70 -3 165

No data

60 -3 165

No data

60 -3 165

No data

50 -3 120

No data

50 -3 120

No data

80 -3 150

No data

30 -3 120

No data

60 -3 165

No data

—

—

30 -3 120

No data

50 -3 165

No data

—

—

60 -3 165

No data

70 -3 200

No data

60 -3 165

No Data

70 -3 200

No data

60 -3 165

No data

30 -3 120

No data

70 -3 165

No data

70 -3 165

No data

30 -3 120

No data

40 -3 120

No data

1 Forest Service official tree names; corresponding botanical names are included in Appendix D.

2 Regions of approximately equal number of months of “good” air-drying weather in accordance with figure 5.

3 To an average moisture content of 25 percent.

-18-
Mid-South during the summer. This can like oak and willow. Pile roofs and closer pile

cause honeycombing and collapse in woods spacing will reduce this form of degrade.

Drying Time and Final Moisture Content

Air-drying times published in Air Drying of

Lumber have been limited to the 1-inch

thickness and generalized for the entire

range over which each wood grows. Energy

conservation and interest rate fluctuations

makes estimates for both 1- and 2-inch material and for specific regions desirable. Such

estimates are given in table 2. They are for

flat-sawed lumber in unit package piles 4 feet

or narrower in width, or having central flues.

The minimum periods given apply to

lumber piled during the best drying weather,

generally spring and early summer. One-inch

lumber piled too late in the period of best

weather to reach 20 percent that fall, or that

is piled during the fall or early winter, will not

reach 20 percent for a very long time. Thus,

maximum drying times for 1-inch lumber in

the Mid-North region are indicated to the 25

percent moisture level. In fact, none of the

times to 20 percent are to an absolute 20 percent but may be as much as 23 percent in poor

drying weather. All of the times for 2-inch

lumber may be considered as times necessary

to bring the wood to somewhere between 23

and 27 percent moisture content.

These time estimates are somewhat

speculative because they are based on

sketchy information. It is believed, however,

that they are the best base for calculating the

feasibility of revising an old yard or building a

new one with a good site, a good yard layout,

the best piling practice with high, open pile

foundations, and a roof on every pile.

An aid that should be very useful in estimating drying time so as to plan actual drying operations is the “effective air drying day

calendar” developed by Rietz (1972) for the

Upper Midwest. Each of the best months is

considered to have 30 “effective air drying

days,” and other months lesser amounts as

follows:

Month Effective airdrying days

January 5

February 5

March 10

April 20

May 25

June 30

July 30

August 30

September 25

October 20

November 10

December 5

As an example, green 1-inch northern red

oak was dried to 20 percent moisture content

in 60 days in a good forklift yard with good air

circulation when stacked in June or early

July. This of course is the best drying weather

in the location considered. If piled in early

November the effective air drying days would

be: November 10, December 5, January 5,

February 5, March 10, April 20—total 55. Drying to 20 percent probably would be completed

by May 5 to 8. This would give a total of 182

days. The research by Peck (1959) indicates 20

days would be saved by taking down the piles

at 25 percent moisture content. This value

would be reached in 162 days.

A similar calendar can be devised for any

location. Rietz did not precisely define an “effective air-drying day,” but it could be assumed to be equivalent to the average of all

the days in full months of the late spring and

summer season.

The calendar would be devised as follows:

Average the mean monthly temperature

for the late spring and summer months; also

find out the mean annual relative humidity

and the mean annual wind velocity.

Assume each summer month had 30 effective air drying days (EADD). For any one of

these months that has both mean relative

humidity more than 5 percent above the annual mean and a mean wind velocity 4 or

more miles per hour less than the annual

mean, deduct 2 days (i.e., count such a month

as having 28 EADD).

The major determinant of the EADD for

the other months is temperature. For each 10° F

that the mean temperature of any month is

below the average of the mean temperatures

of the summer months, deduct 5 days. After

this deduction is made, deduct 2 days for

mean relative humidity more than 5 percent

above the annual mean or mean wind velocity

of 4 or more miles per hour below the annual

mean. If both these latter events occur, deduct 4 days.

-19-
Mean annual relative humidities are generally between 70 and 75 percent in the eastern hardwood region and mean air velocities 6

to 7 miles per hour.

The above values are suggestions based on

general considerations and may be changed

as experience is gained in applying this approach.

Once such a calendar is set up, one could

use the minimum number of days for a certain item in the best months (from drying

time tables) in conjunction with the “effective” days of the various months to predict

when that item piled on any specific day of the

year is likely to be dry. For yards located in

the Tennessee Valley, and perhaps other

parts of the Central zone, the TVA air drying

guide should be helpful (Tennessee Valley

Authority 1974). The charts in this guide

show expected finishing dates for 1-inch and

2-inch hardwood lumber stacked on the 5th,

15th, or 25th of each month. No distinction is

made, however, as to species of wood.

The practice of holding lumber 90 days on

the air-drying yard and then shipping it with

the implication that it will safely withstand

drastic kiln drying is faulty on two counts.

Ninety days is an unnecessarily long and expensive period for some areas of the country

during much of the year. On the other hand,

when the winter is abnormally cold in the

central and southern regions, 90 days is not

long enough to bring the average moisture

content down to 25 percent.

There are advantages to keeping track of

the moisture of the lumber as it air dries. A

graph of past runs (fig. 6) can provide a good

estimate of how long air drying will take for a

given species, thickness, and time of year. The

method of using samples described in the Dry

Figure 6.—Air drying curves for 4/4 northern red oak piled at different times of year in southern Wisconsin.

-20-
Kiln Operator’s Manual, with certain modifi- The curves (fig. 6) show differences that can

cations, is suitable for use in air drying. The occur in drying time depending upon time of

sample pocket can be made two boards wide, piling in northern locations. They further inwith the sample placed in the inner space and dicate the impracticality of always waiting

a dummy board on the outer edge of the until the stock comes all the way to 20 percent

package. An alternative method places the moisture content. Safe methods for starting

sample in the bolster space between the two kiln drying of stock at 25 percent moisture

lowest packages and uses some means to pre- content, regardless of kiln type, are described

vent excessive air circulation over the sam- later.

pie.

Deterioration of Lumber While Air Drying

Losses in lumber value from defects that

develop during air drying are reflected in the

overall drying cost. Air-drying defects may be

caused by shrinkage, fungal infection, insect

infestation, or chemical action. Shrinkage

causes surface checking, end checking and

splitting, honeycomb, and warp. Fungus infection causes blue or sap stain, mold, and

decay. Insect infestation results in pith flecks,

pinholes, and grub holes left in the wood.

Chemical reactions cause brown stain and

sticker marking. Detailed descriptions are included in Air Drying of Lumber. These drying

defects can be controlled by following recommended practices. Particularly helpful are attention to proper site, yard layout, drainage,

and pile roofing.

If lumber is kept on the yard for an extended period of time, it may appear weathered because of an accumulation of sawdust,

ashes, and windborne dirt. Minor darkening

of the surface is not detrimental. Excessive

darkening may be a sign that the lumber has

been held far beyond the time necessary to

reach 20 percent moisture content. After that

period, alternate wetting and exposure to sun

or hot winds can cause serious weather damage.

Air Drying Small Quantities of Lumber

Small quantities can be dried enough for

general construction by air drying or for interior use by air drying followed by a short

period of drying by another method. While

this is relatively simple, the general procedures of commercial drying should be followed, with certain adaptations.

Good lumber cannot be produced from deteriorated material so the “first steps” described in chapter 2 are necessary. When the

logs are sawed, insure that enough of each

thickness is produced. See that each thickness is sawed to 3/8 inch over final size, plus or

minus 1/8 inch. Finished items over 2 inches

thick should be made from 1-inch lumber

glued together after drying and planing. If

polyethylene glycol treatment is to be used,

apply that right away (see chapter 7).

Select a good place for the air drying pile.

Species that dry rapidly (table 2) can be piled

in the open. Species that take the longest

times usually need protection from strong

winds to avoid checking. Small piles do not

need to be high above the ground, but the pile

foundations should be open and strong, and

should present a perfect plane for the first

course of lumber. A slope of 1 inch per foot of

pile length may be provided to aid drainage if

the pile is to be wide, but two narrow, flat

piles would be preferable. Sloped piles need to

have their fronts pitched forward 1 inch per

foot of height.

Every board in a course of lumber must be

of the same thickness to hold warping to a

minimum. Stacking should be done as soon as

possible after sawing, using the general

methods described in chapter 2. Renew the

end coatings on thick lumber that has had its

end coating damaged or trimmed off. Protect

the top of the pile as it is being built and when

it is finished by a roof that will shade the pile

and shed rain. Weighting down with concrete

blocks or other materials at the rate of 60

pounds or more per square foot will be helpful

for warp-prone species.

Material that has been thoroughly air dried

(to less than 20 percent moisture content) will

not be dry enough for interior use, but it can

-21-
be made so by using heated room, de- stock in a commercial kiln load of air-dried

humidifier, or solar drying or by inserting the hardwoods. See chapters 5 and 8.

A Quick Guide for Improving Air Drying

Efficiency of Hardwoods

The following summarizes several procedures to reduce air-drying time and increase

efficiency for greatest energy savings.

1. When green lumber arrives or is first cut,

immediately get it on stickers. Even if it is

going to the kiln soon, store it where it can air

dry (where the wind can blow through the

pile). Drying is most rapid the first 3 to 4 days:

Don’t miss this opportunity for effective early

drying.

2. Spread lumber piles out to increase their

exposure to drying winds. Spacings in Air

Drying of Lumber are minimum. Lumber

piled too closely increases relative humidity

in the surrounding area and slows drying. Put

the driest lumber on the outside edges of the

yard (especially on the edges facing into the

wind) so that it will dry even faster. However,

for species such as oak, beech, and hickory,

accelerated air drying (from too wide spacing)

can also increase the risk of degrade from

checking and honey comb.7

3. Consider using a sticker that is 25/32

inch thick or more. In contrast to thin stickers, this will permit both increased air flow

and drying. Stickers should be of uniform

thickness. As an alternative, consider narrower piles.

4. Cover the tops of the piles. Why expose

lumber that is air drying to rainfall or snow?

5. Keep the yard clear of weeds and debris

so that the bottom layers of the pile will dry

as fast as the top layers. The pile foundations

should be of sturdy, open construction and

should support the lumber at least 12 inches

above the ground.

7 Species like oak and beech (refractory hardwoods)

present special problems. Daily inspection is recommended with refractory hardwoods to discover any problems in either air or kiln drying before they can become

serious.

Literature Cited

-22-
CHAPTER 4

ACCELERATED AIR DRYING

Accelerated air drying is the use of low-cost third of the bound water from green wood to

techniques involving specially designed reduce the moisture content to the air-dried

sheds, fans, and (sometimes) heating equip- range. When heating is used, temperatures

ment to accomplish air drying in a shorter generally do not exceed 120° F. Solar drying

time. It includes yard-fan drying, shed-fan that is used to bring lumber down to 20 perdrying, forced-air drying, low-temperature cent moisture content is low-temperature

kiln drying, and controlled-air drying. It thus forced-air drying, but broader scale solar dryencompasses any process that accelerates the ing is discussed later in this handbook.

evaporation of free water and the first oneResearch Basis

Forced-air drying and other forms of accelerated air drying have been studied extensively in the United States and Canada. The

methods are technically sound, and there

have been many practical applications. Much

of the research has been of an empirical nature. Several published reports are indicated

later in this chapter. The Southeastern

Forest Experiment Station of the Forest Service, U.S. Department of Agriculture, established technical data to aid in the design and

operation of forced-air dryers. Data from one

experiment by Vick (1965b) of that Station

were used to derive figure 7.

M 144694

Figure 7.—Effect of temperature on drying of 1-inch yellow-poplar lumber in a low-temperature forced-air dryer

under 10 percent EMC conditions (after Vick 1965b).

-23-
This figure shows the influence of temperature on drying time at a constant equilibrium

moisture content (EMC) condition. The air

velocity was 550 feet per minute, and the load

width 8 feet. Yellow-poplar has fast-drying

heartwood but, as can be seen, the sapwood

dried somewhat faster. At the 10 percent

EMC used, the effect of temperature is significant. A 100° F temperature dries 4/4 sapwood to 20 percent moisture content (MC) in

three-fifths of the time required at 80° F. The

original paper also shows the effects of different EMC’s. Degrade was very limited

(maximum of $1.87 per thousand board feet)

except when a 6 percent EMC was used at

120° F. The paper also shows the mathematical basis by which the actual results were

found to agree with theoretical expectations.

Similar data on 1-inch sweetgum lumber,

tupelo rounds, and mixed hardwood rounds

are available (Vick 1965a, 1968a, 1968b).

Information on the economic prospects of

accelerated air drying has been published by

a number of authors (Catterick 1970, Cuppett

and Craft 1971, Davenport and Wilson 1969,

Gatslick 1962, and Norton and Gatslick 1969).

In general, the data indicate that accelerated

air drying has little or no advantage over

good summer air drying. An advantage could

be anticipated during the 6 to 8 months of

poor air drying weather in the North and

Mid-North. The advantage may also be present in other areas where space for a welllaid-out air-drying yard is limited or where

rainfall is heavy for long periods. Means of

comparing economic advantage or disadvantage are described in the last chapter of this

handbook.

Types of Dryers and Procedures

The shed-fan dryer has a permanent roof

and canvas baffles that can be let down at the

ends of the dryer (fig. 8). One side wall is permanent and furnished with a large number of

fans. The fans always turn in one direction, to

pull the air through the lumber. The

entering-air side of the dryer is open to the

yard. The heat in the ambient air is an important factor in drying the lumber. The

shed-fan dryer works very well in the South.

One-inch elm, sap gum, hackberry, and

yellow-poplar have been dried from green to

29 percent MC and lower in 6 to 7 days (Helmers 1959). It also is very effective for soft

maple. While this type of dryer has been installed in a few places in the North, it is really

only effective there from mid-April until

mid-October.

The yard-fan dryer is similar in construction, operation, and applicability to the shedfan dryer except that different fan wall

configurations can be used and the fans may

be portable. The roof also is temporary and

portable and may be made of canvas, plywood,

or sheet metal panels. One mistake in use of

some yard-fan dryers is to pile additional

stacks of lumber in front of the fans. This

greatly reduces their effectiveness.

The next improvement that can be made in

drying lumber with forced air is to add heat. A

solar-heated lumber dryer developed by the

Forest Service in Puerto Rico with the assistance of the U.S. Forest Products Laboratory

takes advantage of heat energy coming from

the sun every day. This dryer dried 5/4

mahogany lumber to 20 percent MC in 12

days.

At Jacksonville, Fla., a semi-solar dryer

made use of both steam heat from a woodfired boiler and solar energy to predry magnolia lumber (Rucker and Smith 1961). The

rapid predrying avoided the extensive oxidative graying that had been a costly cause of

degrade of magnolia. Such a predryer could

be expected to prevent oxidative discoloration

in tupelo sapwood and hackberry.

The steam coils provided heat from 5 until

10 a.m. to bring the temperature up. From 10

a.m. on, the large, almost flat, black-painted

roof absorbed enough solar energy to keep the

temperature at about 110° F until about 8

p.m. The fans were left running overnight.

The next morning the steam heat would be

used again to bring the temperature up. This

dryer brought 4/4 magnolia down to 20 percent MC or less in 6 days and 8/4 in 14 days.

A combination shed-fan and recirculating

forced-air dryer has been installed in a

limited number of instances. Heating the air

to lower the EMC and increase the drying

rate is not economical unless the air is recirculated. During very good outdoor drying

weather, the louvers of this dryer are opened

by hydraulic cylinders actuated by humidity

controls. The overhead fans force the outside

-24-
M 143 544

Figure 8.—Shed-fan track-type accelerated-air dryer.

air through the lumber and out through the

other louvers. When the weather is damp or

cold, the louvers are closed, a small amount of

heat is used, and the air within the building is

recirculated.

Recirculating dryers that use some heat

are of two types: The forced-air dryer that

only partially controls relative humidity by

control of venting, and the low-temperature

kiln that fully controls relative humidity. A

schematic diagram of a typical Appalachian

forced-air dryer with some provision for supplemental solar heating is shown in figure 9.

Both the forced-air dryer and the low temperature kiln usually consist of an inexpensive structure (fig. 10) plus the necessary

heating and controlling equipment. An earlier design (Gatslick 1962) had longer air

travel, an undesirable feature unless very

powerful air-circulating equipment is used.

Temperatures in both types of dryers range

from 70° to 120° F. Both may have vents to

exhaust excessive moisture, but the vents

generally are operated in the closed position.

They are opened when the drying schedule

prescribes lowered relative humidity. In the

forced-air dryer, temperature is controlled by

a thermostat. Both the dry-bulb temperature

and the wet-bulb temperature are observed

with a hygrometer. In the low-temperature

kiln, a steam spray line supplies humidity

when needed, and a kiln-type recordercontroller both records the dry- and wet-bulb

temperatures and regulates the heating coils

and the steam spray.

The controlled-air dryer (fig. 11) is a large

warehouse-type building of tight design in

which large quantities of green lumber can be

dried to 20 percent moisture content. A common size would hold 200,000 board feet. A

convenient constant temperature, such as 80°

or 85° F, is maintained throughout the building. A single large door at one end allows the

green lumber to be brought in, and a similar

door at the other end permits taking out loaded

kiln trucks. These doors can be left open during operations without upsetting interior

conditions. The relative humidity in each

quarter, or 50,000-board foot bay of the building, can be controlled at the desired level (for

example, 50 percent) by power intake and

exhaust vents. These vents open when the

-25-
humidity gets too high. This procedure is, essentially, a one-step drying schedule.

Unit heaters or heating coils located at or

near each intake vent are turned on when the

vents open so that incoming air is heated to

the dryer temperature. Air circulation designs evolved over a 5-year period currently

provide about 100 feet per minute air velocity

through the lumber. User-furnished data indicate that drying times for common items

are about 40 percent longer than those of

forced-air dryers. The controlled-air dryers

have been able to satisfactorily predry

mineral-streaked sugar maple. They have

found application in the United States in

northernmost States as well as in Canada.

Care must be used in piling lumber in a

package-loaded dryer so that packages are

not tight against each other or air will not

flow freely through the sticker spaces.

Two major types of schedule s for

accelerated-air drying have been tested extensively. For forced-air dryers, with only

partial control of relative humidity, two-step

temperature schedules are used. For lowtemperature kilns with steam spray and automatic recorder-controllers, multiple-step

schedules are more appropriate.

Dry-bulb and wet-bulb temperature control

of drying conditions is discussed in the Dry

Kiln Operator’s Manual, and the reasons for

careful relative humidity control in drying

green lumber are given in Air Drying of

Lumber.

Forced-air drying schedules adapted from

research at the Forest Products Marketing

Laboratory, Princeton, W. Va. (Cuppett and

Craft 1975) are listed in table 3. In this type of

procedure, the desired dry-bulb temperature

for the first step of the schedule is somewhere

within a specific temperature range, depending on the species of wood (table 4). If the

dryer has vents, they are kept closed as the

dryer is heated up. Both dry-bulb and wetbulb temperature are observed on the

entering-air side of the load. When the differM 144692

Figure 9.—Cross section of a typical forced-air dryer with fans located overhead. (After Cuppett and Craft 1975.)

–26-
M 140 966-7

Figure 10.—Low-temperature kiln located at Allegany, N.Y.

ence between the two (the wet-bulb de- the moisture content of the wettest half of the

pression) becomes great enough to give the drying

desired EMC condition, the thermostat is set samples (chapter 6, Dry Kiln

Operator’s Manual) falls to the moisture conat the dry-bulb temperature prevailing at

that time. This temperature is maintained

tent for starting the second step of the

schedule, the thermostat is raised to the temthroughout the first schedule step. The wetbulb depression tends to remain about the

perature prescribed for that step. Generally

same for the first day or two because of moisthe vents could be opened for a few days at

this time to lower the EMC and accelerate

ture coming from the wood. Thereafter, the drying. When the wet-bulb temperature setwet-bulb temperature falls gradually. When tles to a nearly constant level, the vents

Table 3. —Forced-air dryer schedules 1 for selected eastern hardwoods

Schedule designation

Moisture

content at

start of step

Percent

1 Above MC of

step 2

2a 45

b 40

FA-1 FA-2 FA-3

Dry-bulb Wet-bulb Dry-bulb Wet-bulb Dry-bulb Wet-bulb

temperature depression temperature depression temperature depression

° F ° F ° F ° F ° F ° F

70-80 3 75-85 4 80-90 7

100 (

2

)

90 (

2

) 95 (

2

)

1

 Developed by Forest Products Marketing Laboratory, Princeton, W. Va. (Cuppett and Craft 1975). All operate at an

air velocity of 450-600 feet per minute except for 6/4 and 8/4 oak and 8/4 beech and hickory, which use 250-350 feet per

minute. 2

 No control of wet-bulb depression.

-27-
Table 4. —Index of forced-air drying schedules

Size

Species

4/4 5/4 3/4 3/4 ---------------------------------------------------------

Ash FA-3 FA-3 FA-2 FA-1

Basswood FA-3 FA-3 FA-3 FA-2

Beech FA-2 FA-2 FA-1 FA-11

Birch FA-3 FA-3 FA-2 FA-1

Buckeye FA-3 FA-3 FA-3 FA-2

Butternut FA-3 FA-3 FA-3 FA-2

Cherry FA3 FA-3 FA-2 FA-1

Hickory FA-2 FA-2 FA-1 FA-1 1

Maple; red, silver FA-3 FA-3 FA-2 FA-1

Maple, sugar FA-3 FA-3 FA-2 FA-1

Oak, red FA-2 FA-1 FA-1 1 FA-11

Oak, white FA-2 FA-1 FA-11

 FA-11

Walnut, black FA-3 FA-2 FA-1 FA-1

Yellow-poplar FA-3 FA3 FA-2 FA-2

1

 Air velocity through load, 250-350 feet per minute.

would be closed again to conserve energy.

Drying is continued until the moisture content of the wettest sample is down to the desired average moisture content for the whole

dryer charge. Vents might be used earlier in

the schedule when drying basswood, buckeye,

butternut, or yellow-poplar. Cuppett and

Craft (1975) suggest hygrostat-controlled

power venting for these species.

A final schedule consideration for this type

of dryer is air velocity. Two-inch oak, beech,

and hickory requires a relatively low air

velocity—250 to 350 feet per minute. The

same low air velocity probably should be used

for 2-inch stock of other species for which no

schedule has been established, until gradual

increases in velocity show the higher rate can

be used without causing drying defects. All

4/4 and 5/4 stock, plus the 6/4 of all species

except the oaks, can use a higher air

velocity-450 to 600 feet per minute.

M 140 966-23

Figure 11.-Inside of a controlled-air dryer.

-28-
Items and species having the same

schedule designation (table 4) can be dried

together in the same dryer load, but samples

should be selected for the thickest, slowest

drying item to determine when to remove the

stock from the dryer.

During the first 6 months of development of

a schedule for forced-air drying of a particular item of a particular species, a moisture

meter test (James 1975) should be made of 5

percent of the material in packages of lumber

from various parts of the dryer. This should

show that all the boards have core moisture

content values at or below 30 percent. If not, a

longer drying time should be used the next

time this item is dried.

The schedules are not ironclad rules. A user

should gradually approach a more severe

schedule for items that develop no defects

when drying by the above. For warmer climates, slightly higher dry-bulb temperatures

could be used if the desired wet-bulb depressions can be maintained during the first step.

Solar radiation may cause the dry-bulb temperature to rise above the set point for 6 to 8

hours. This should be satisfactory unless the

dry-bulb temperature exceeds the thermostat

setting by more than 5° F during step 1. If

that should occur, lower the thermostat setting 5° F throughout step 1.

Schedules developed industrially for lowtemperature kilns are listed in table 5. In this

type of procedure, the desired wet-bulb temperature is controlled for all steps. Partial

control is maintained of the dry-bulb temperature. As with the forced-air dryer, the vents

are kept closed as the kiln is heated up. When

both the dry-bulb and the wet-bulb temperatures reach the scheduled set points, the recorder controller maintains the wet-bulb

temperature at its prescribed level and sees

to it that the dry-bulb temperature does not

go below the set point. If solar radiation

causes the dry-bulb temperature to rise

slightly above the set point during the day, no

change in settings is made.

These schedules were developed with air

velocity through the load of 350 feet per minute and presumably would work equally well

with air velocities up to 450 feet per minute.

The vents would be controlled automatically

by the recorder-controller and presumably

would stay closed until 10° F or larger wetbulb depressions are called for. Then the

vents might be opened a short time at the

start of each drying step but would automatically close most of the time and conserve

energy. Drying condition changes would be

made on the basis of the wettest half of the

drying samples (chapter 6, Dry Kiln

Operator’s Manual). Determination of when

to terminate a run would be on the same basis

as for the forced air dryer.

Table 6 is an index of schedules for the lowtemperature kiln (table 5). Items and species

having the same schedule designation can be

dried together, but samples should be

selected for the thickest, slowest drying item

to determine when to remove the stock from

the dryer. Skillful dry kiln operators can

Table 5. —Low-temperature kiln schedules 1 for selected eastern hardwoods

1 Developed by Robert G. Potter, Potter Lumber Co., Allegany, N. Y. (personal correspondence).

-29-
select the schedule most appropriate for

species not listed in table 6 and insert the

designations in the blank places left in the

table for this purpose.

Table 6. —Index of low-temperature kiln

schedules

Size

Species

4/4 5/4 6/4 8/4

---------------------------------------------------------

Beech LT-2 LT-2

Cherry LT-3 1

Maple; red, silver LT-3 1

Maple; sugar LT- 3

1

Oak, red LT-2 LT-2 LT-12 LT-13

1 Minimum time on each step 24 hours.

2 Minimum time on first step 36 to 48 hours.

3 Part-time drying (fans on and kiln under control only

50 percent of time) during step 1.

The Eastern Forest Products Laboratory,

Ottawa, Canada, has experimented extensively with predrying of sugar maple and yellow birch prior to final drying at 212° F. The

procedure was essentially low-temperature

kiln drying. With 4/4 sugar maple, Huffman

and Cech (1969) found that least degrade

coincided with the procedure that gave the

shortest drying time. This involved a 600-

foot-per-minute air velocity through the load.

Although no steam spray was used, the kiln

was tight enough so the schedule shown in

table 7 prevailed.

In similar research done with yellow birch

(Cech and Huffman 1968) no EMC data were

presented. The dryer was run with no supplemental humidification. Presumably wetbulb depressions were similar to those

indicated in table 7 for maple. Some degrade

was observed with the table 7 conditions as

Table 7. —Approximate Canadian lowtemperature kiln schedule 1 for 4/4 maple and

birch

Stap Moisture content Dry-bulb Wet-bulb

No. at start of step temperature depression

_________________________________________________________

Percent °F °F

1 Above 60 100 13

2 60 100 17

3 50 100 20

4 40 100 26

1 Developed by Eastern Forest Products Laboratory,

Ottawa, Canada (Huffman and Cech 1969).

well as when using no humidification at 80° F

and with 200 feet per minute air velocities.

Similar results were obtained when humidification was used to provide a constant 14.6

percent EMC. Although the least degrade

was obtained by the 14.6 percent EMC in

combination with 100° F and 600 feet per minute, Cech (1973) later concluded that the

schedule using 100° F, 600 feet per minute,

and no humidification was satisfactory for

both maple and birch.

In studying the benefits of presurfacing oak

before drying, McMillen (1969, 1972) used

low-temperature kiln drying schedules that

increased temperature stepwise from 85° F to

105° F. Satisfactory results were obtained

with white oak and red oak using an air velocity of 575 to 600 feet per minute, Cherrybark

oak, however, surface checked excessively

even though the air velocity was only 375 to

400 feet per minute. For drying rough-sawed

oak, smaller wet-bulb depressions would be

needed. The schedule in table 8 is suggested

for cautious trial on central and northern region rough-sawed 4/4 red and white oak.

Table 8. —Suggested low-temperature kiln schedule 1 for rough-sawed 4/4 red and white oak

Moisture content at

start of step Dry-bulb Wet-bulb

Step No. temperature depression

Red oak White oak

------------------------------------------------------------------------------------------------------------------------------------------------------------------- -

Percent °F °F 

1

2

3

4

Percent

Above 50 Above 40 85 5

5 0

3 5

40 90 8

33 95 13

2 8 28 105 25

1 Postulated from results with presurfaced oak (McMillen 1969, 1972) and rough-sawed northern red oak (Gatslick

1962).

-30-
Advantages and Disadvantages of Accelerated-Air Drying

The main advantage of accelerated-air

dryers over yard drying is in getting lumber

out of the weather and providing closer to

ideal air-drying conditions for longer periods.

Such dryers have special advantages in the

northern part of the country where the airdrying weather is not good throughout the

entire year. Drying rate in northern areas is

likely to be two to four times as fast as outdoor drying. The air velocity through the load

is 50 to 300 feet per minute if the lumber is

piled relatively open. If the lumber is tightly

placed so most of the air has to go through the

lumber pile, velocities can be over 300 feet per

minute.

Accelerated-air drying has these advantages over ordinary air drying:

1. Drying is faster because of protection

from rain and faster air circulation. This

speeds up drying, making it possible to reduce

the lumber inventory, with very important

savings in rent, interest, risk, and insurance.

2. The lumber supplier is able to respond

quicker to a change in the market because he

does not have a long period of air drying to get

the lumber dry enough so it can go through

his dry kilns quickly.

3. Accelerated-air drying gives the operator a chance to reduce shipping weight and

thus the shipping charges.

4. It increases the productivity of the present kilns used only for drying green lumber,

and the capital investment is less than building new kilns to do the same job.

5. Accelerated-air drying prevents sticker

stain and oxidative or chemical stain in the

lumber and protects it from the weather, preventing surface discoloration and producing a

brighter colored stock with greater marketability. It also reduces surface checking and

can reduce warping.

Disadvantages may be:

1. Moisture content may not be uniform if

the dryer is not properly designed and

properly operated.

2. Sticker staining or other discoloration

may occur in light-colored woods if the relative humidity is allowed to remain high while

the temperature is in a range where discoloration goes on at a rapid rate.

3. Costs may be higher than those for air

drying when suitable land is readily available

with low rent or taxes and when the producer

is willing to assume a large share of the risk.

Drying Times

Drying times in forced-air dryers and lowtemperature kilns still depend somewhat on

the weather outside and the temperature

selected by the operator. Equally important

are the dryer design and the quality of stacking and loading. The drying times (table 9) are

based on actual forced-air dryers and lowtemperature kilns, but should be considered

only as rough estimates. No two dryers will

give the same performance. Gatslick (1962)

and Vick (1965b, 1968a) give extensive drying

data for low-temperature kilns that completely control EMC during the early part of

the drying period. Cuppett and Craft (1971,

1975) give results in drying Appalachian oak

and several other eastern hardwood species

in a dryer where the only EMC control is by

the dry-bulb temperature. Drying times in

controlled air dryers are about 40 percent

longer than those shown in table 9.

-31-
Table 9. —Accelerated air drying times for common hardwood items in slightly heated forced-air

dryers (low-temperature kilns)

Final

Species Thickness Temperature moisture Drying

Aspen

Beech

Birch

Cherry

Elm, American

Hickory

Magnolia

Maple, hard

Maple, soft

Oak, Appalachian red

Oak, northern red

Oak, white

Sweetgum, sapwood

Walnut

Yellow-poplar, sapwood

Yellow-poplar, heartwood

4/4

4/4

5/4

4/4

4/4

4/4

4/4

8/4

4/4

5/4

4/4

8/4

4/4

5/4

4/4

5/4

5/4

4/4

4/4

4/4

5/4

6/4

8/4

8/4

—

4/4

4/4

6/4

4/4

4/4

4/4

4/4

° F

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

70

85

—

—

—

—

—

—

80

100

—

80

100

80

100

content

Percent

20

25

25

25

25

18

20

35

20

20

20

20

25

25

20

20

12

20

20

20

25

25

25

20

20

20

20

17

20

20

20

20

time

Days

7

8-9

10-12

14

7-8

10

7

7

16-18

22-24

8-9

15-18

8-10

12–14

6-7

7-8

11

31

19

18-23

14-35

15-40

40-62

55

1 or 2 days less

than for red oak

7

41⁄2

14

6

4

9

61⁄2

-32-
Literature Cited

-33-
CHAPTER 5

CONVENTIONAL KILN DRYING

Kiln drying is carried out in a closed

chamber or building in which air is rapidly

circulated over the surface of the wood being

dried. Conventional dry kilns use initial drying temperatures from 100° to 170° F and final

temperatures from 150° to 200° F. These

higher air temperatures and faster circulations are the principal means of accelerating

drying greatly beyond the rates of air drying

and accelerated-air drying. Control of relative

humidity or EMC is necessary to avoid

shrinkage-associated defects and to equalize

and condition the wood to the degree of precision needed. Air velocities through the load in

drying hardwoods generally are between 200

and 450 feet per minute. Temperature and

relative humidity are controlled by semiautomatic dry- and wet-bulb temperature

recorder-controllers. A typical recordercontroller chart is shown in figure 12.

Two types of conventional kilns are in general use for hardwoods—package-loaded compartment and track-loaded compartment

kilns. There are two basic heating systems,

steam and hot air (or direct-fired). Well over

three-fourths of the kilns designed to dry

hardwoods are steam-heated, internal fan

forced circulation, track- and package-loaded

kilns. In recent years, the direct-fired kiln,

with supplemental steam or water spray for

humidification, has been used occasionally for

hardwoods. Considerable modification of kiln

M 88590F

Figure 12.—Instrument chart of conditions used in kiln drying a small load of air-dried

black cherry. (Assumed to have undergone surface moisture regain.)

-34-
temperature schedules would be needed if

such kilns, without humidification, were to be

used for green hardwoods intended for furniture or other high-class uses. More information about all types of kilns is given in chapter

2 of the Dry Kiln Operator’s Manual (1961).

Many technical factors, however, are involved in dry kiln design; it is recommended

that individuals considering installation of

kilns for moderate- to large-scale commercial

production obtain them from regular dry kiln

manufacturers.

The length of air travel through the load in

a package-loaded dry kiln and reheating the

air between the tracks in track-type kilns are

critical factors in drying green hardwoods. An

air travel of more than 8 feet without reheating requires undue lengthening of the drying

time to avoid collapse and honeycombing of

the wood in the middle of the load. The length

of drying time is one of the most important

cost factors. “Booster” reheat steam coils between tracks overcome this difficulty, but

they must be correctly designed and operated

to avoid surface checking of refractory woods

early in drying. This hazard occurs from increasing dry-bulb temperature too much by

direct radiation. Drying defects cause degrade, which is another very significant cost

factor. Kiln drying defects and methods of

minimizing them are illustrated and discussed extensively in the Dry Kiln Operator’s

Manual.

The best practices for kiln drying

hardwoods now are essentially the same as

those described in the Manual. They involve

the use of recommended kiln schedules based

on the moisture content of the lumber being

dried. This initially involves use of kiln samples, short end-coated boards by which the

changing moisture content of the kiln load is

estimated. The details of selecting, preparing,

and using kiln samples are fully covered in

the Manual, so they will not be repeated in

this publication. Basic kiln schedules, which

combine the recommended kiln schedules

with other recommendations to save energy,

to equalize board moisture content, and to

relieve drying stress, are outlined later in this

chapter.

Improvements in kiln schedules have always been sought to shorten drying time and

decrease cost without sacrificing quality. This

quest has been intensified now that energy

and other costs have risen sharply. At the

time the recommended schedules were devised, there was a great deal of variability in

the performance of dry kilns and in their operation and care. The schedules, therefore,

were purposely conservative. In many cases

they can be accelerated and considerable savings will result. Examples of how the drying

of oak can be accelerated are discussed following the section on basic kiln schedules. Similar schedule accelerations can be applied to

other woods, Two other groups o f

schedules—simplified schedules (applicable to

4/4 thickness of a limited number of woods)

and special schedules for special purposesalso are discussed under Drying Procedures.

This section includes suggestions for predicting drying time.

Equalizing and conditioning (stress relief)

are two quality-control measures necessary

to complete the seasoning of fine purpose

hardwoods. The previously recommended

methods are excessively high in energy demands. This chapter contains suggestions

that considerably reduce these demands.

A number of suggestions for tightening up

kiln structures and for carrying out the prescribed procedures are discussed under

Operational Considerations. As many or more

savings can be made by following these

suggestions as by adopting the new procedures themselves.

-35-
Drying Procedures

Basic Kiln Schedules

The recommended kiln schedules for

hardwoods in the Dry Kiln Operator’s Manual

were carefully worked out combinations of

dry-bulb and wet-bulb temperatures based on

research and experience. They will dry any

hardwood item at a generally satisfactory

rate without causing objectionable drying defects. Drying defects are caused by

shrinkage-related stresses that are related to

the average moisture content of the stock;

changes in kiln conditions during the drying

run are based on the average moisture content of the wettest half of the kiln samples.

The recommended schedules were presented as guides from which the well-trained,

experienced kiln operator was expected to derive optimum schedules best suited for his

material, equipment, and operations. In all,

the Manual indexed 77 hardwood schedules,

but the operator had a total array of 672

schedules which he could use for changing

schedules as much as circumstances and his

experience dictated.

Because much of the future practice in

kiln-drying hardwoods probably will involve

air-dried or partly air-dried stock, it becomes

feasible to group eastern hardwood schedules

into 11 pairs of basic schedules. All species in

each schedule group originally had identical

or very similar dry-bulb temperatures. Also,

since the final stages of all hardwood

schedules involve very large wet-bulb depressions, all species in each group had similar

intermediate and final wet-bulb temperatures. The basic schedules, therefore, are essentially the same as the recommended

schedules, so far as air-dried and partly airdried hardwoods are concerned.

There should be little or no increase in drying time for the basic schedules beyond that

experienced with the recommended schedule.

In a few cases, the basic schedules will result

in longer times than the recommended

schedules when drying green stock. The basic

schedules also may be slightly too severe for

green stock of a few species (see footnotes in

the basic schedule tables). When drying large

quantities of green stock, the kiln operator is

encouraged to revert to the originally recommended schedule from the Manual.

For the basic kiln schedules, species are

grouped in table 10. An alphabetical index of

Table 10. -Grouping species for basic kiln schedules

Temperature schedule

Table for: Severity

No. of Species in group

4/4 8/4 schedule

12 T2 Irregular Mild Red and white oak, southern lowland

13 T4 T3 Mild Red oak, northern or upland

14 T4 T3 Mild White oak, northern or upland

15 T6 T3 Mild Apple, dogwood, rock elm, hophornbeam, black locust, osage-orange,

persimmon, sycamore

16 T6 T3 Moderate American elm, slippery elm, holly,

mahogany, black walnut

17 T8 T5 Mild Beech, sugar maple, hickory, pecan

18 T8 T5 Moderate Black ash, green ash, white ash, yellow

birch, cherry, cottonwood (wetstreak), hackberry, red maple,

silver maple, sassafras, sweetgum

heartwood (redgum)

19 T10 T8 Moderate Basswood (light color), paper birch,

buckeye, butternut, chestnut, cottonwood (normal), magnolia, swamp tupelo

(swamp blackgum), black willow

20 T11 T10 Moderate Yellow-poplar, cucumbertree

21 T12 T11 Moderate Black tupelo (blackgum), sweetgum sap22

wood (sapgum)

T12 T10 Severe Aspen (sapwood or box), basswood

-36-
the species and their basic schedule table

number is presented in table 11. The basic

schedules themselves are given in tables 12

through 22. The schedule code numbers used

in the Dry Kiln Operator’s Manual for each

species are given at the bottom of the basic

schedule tables. Recommendations and

schedule code numbers for 1- and 2-inch

squares and thick hardwoods are given in tables 12 and 13 of the Manual.

Each of the basic schedulel tables shows

specific temperature and wet-bulb depressions during the intermediate and final

stages of drying; these temperatures and

depressions are slightly different from the

original Manual recommendations. Specific

temperature and wet-bulb depression values

are also included for equalizing and conditioning. All of these specifics are designed to

conserve energy and are essentially what a

prudent kiln operator would already be doing

to accomplish this end.

The intermediate and final stage wet-bulb

temperatures are close to those that are naturally attained when the vents are kept

closed and the steam spray is turned off.

Operators who do not know how to achieve

this mode of operation with their equipment

should check with their kiln manufacturer.

Close control of wet-bulb temperature is not

necessary during the final drying stage when

the schedule shows depression values of 45° F

or larger. Just keep the vents closed and the

steam spray off. During equalizing and conditioning, however, close control is necessary,

and a clean wet-bulb wick is necessary to attain this close control. An explanation of the

equalizing and conditioning specifics is given

later in this chapter.

Specific procedures for starting up the kiln

and the first day or two of the run when drying air-dried or partly air-dried stock are

given below. The average moisture content of

air-dried stock should be 25 percent or lower

with no material over 30 percent. For partly

air-dried stock, no material should be over 50

percent moisture content. Specific procedures

for starting up and drying green lumber are

given in chapter 10 of the Dry Kiln Operator’s

Manual. Although the same schedule generally is prescribed for 4/4, 5/4, and 6/4 of each

species, the 6/4 will take considerably longer

to dry than the 4/4, and therefore the best

practice is to dry each size separately. If 4/4

and 5/4 must be dried together, most kiln

samples should be taken from the 5/4 stock.

Table 11. —Alphabetical index of species

and basic kiln schedule table numbers

Species Table No.

Apple

Ash: black, green, white

Aspen

Basswood (slight browning)

Basswood (light color)

Beech

Birch, paper

Birch, yellow

Blackgum

Buckeye

Butternut

Cherry, black

Chestnut

Cottonwood, normal

Cottonwood, wet-streak

Cucumbertree

Dogwood

Elm: American, slippery

Elm, rock

Gum

Hackberry

Hickory

Holly, American

Hophornbeam (ironwood)

Locust, black

Magnolia

Mahogany

Maple: red, silver

Maple: sugar

Oak: red, northern or upland

Oak: white, northern or

upland

Oak: red or white, southern

lowland

Osage-orange

Pecan

Persimmon

Poplar

Sassafras

Sweetbay

Sweetgum, heartwood

(redgum)

Sweetgum, sapwood (sapgum)

Sycamore, American

Tupelo, black (blackgum)

Tupelo, swamp (swamp

blackgum)

Tupelo, water (tupelo)

Walnut, black

Willow, black

Yellow-poplar

15

1 18

1 22

22

19

17

19

18

(see tupelo)

19

19

1 18

19

19

18

20

15

16

15

(see sweetgum)

18

17

16

1 15

1 15

19

16

18

17

1 13

1 14

1 12

1 15

1 17

15

(see yellow-poplar)

18

(see magnolia)

18

1 21

1 15

21

1 19

(see Dry Kiln

Operator’s Manual)

16

19

20

1 See special note in table indicated

-37-
Table 12. —Basic kiln schedules for red and white oak, southern lowland 1

Moisture 4/4, 5/4 (T2-C1) 2

6/4, 8/4 (Irregular)

content

at start Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

of step temper- depres- temper- temper- depres- temperature sion ature ature sion ature

Percent ° F ° F ° F ° F ° F ° F

Above 40 100 3 97 (Air dry to 25 pct MC)

40 100 4 96

35 100 6 94

30 110 10 100 105 8 97

25 120 25 95 110

20

11 99

130 40 90 120 15 105

15 150 45 105 130 30 100

11 160 50 110 160 50 110

Equalize 173 43 130 173 43 130

Condition 180 10 170 180 10 170

1 For all oak species, 6/4 stock usually is dried by the 8/4 schedule.

2 The recommended green stock code number is the same.

Table 13. —Basic kiln schedules for red oak, northern or upland 1

ature sion ature

Moisture 4/4, 5/4 (T4-D2) 2

6/4, 8/4 (T3-D1) 2

content

at start Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

of step temper- depres- temper- temper- depres- temperPercent ° F ° F ° F ° F ° F ° F

Above 50 110 4 106 110 3 107

50 110 5 105 110 4 106

40 110 8 102 110 6 104

35 110 14 96 110 10 100

30 120 30 90 120 25 95

25 130 40 90 130 40 90

20 140 45 95 140 45 95

15 180 50 130 160 50 110

Equalize 173 43 130 173 43

Condition

130

180 10 170 180 10 170

1 For all oak species, 6/4 stock usually is dried by the 8/4 schedule.

2 The recommended green stock code numbers are the same.

Table 14. —Basic kiln schedules for white oak, northern or upland 1

atrue sion ature

Moisture

content

at start

of step

Percent

Above 40

40

35

30

25

20

15

Equalize

Condition

4/4, 5/4 (T4-C2) 2

6/4, 8/4 (T3-C1) 2

Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

tempera- depres- tempera- tempera- depres- temperature sion ture ture sion ture

° F ° F ° F ° F ° F ° F

110 4 106 110 4 107

110 5 105 110 4 106

110 8 102 110 6 104

120 14 106 120 10 110

130 30 100 130 25 105

140 45 95 140 40 100

180 50 130 160 50 110

173 43 130 173 43 130

180 10 170 180 10 170

1 For all oak species, 6/4 stock usually is dried by the 8/4 schedule.

2 The recommended green stock code numbers are the same.

-38-
Table 15 —Basic kiln schedules for rock elm, dogwood, persimmon, and similar woods 1

Moisture

content at

start of step

4/4, 5/4, 6/4 (T6-B3) 8/4 (T3-B2)

Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature

Percent ° F ° F ° F ° F ° F ° F

Above 35 120 5 115 110 4 106

35 120 7 113 110 5 105

30 130 11 119 120 8 112

25 140 19 121 130 14 116

20 150 35 115 140 30 110

15 180 50 130 160 50 110

Equalize 173 43 130 173 43 130

Condition 180 10 170 180 10 170

1

 Species included and recommended green stock code numbers:

Species Code No. for:

4/4, 5/4, 6/14 8/4

Apple T6-C3 T3-C2

Dogwood T6-C3 T3-C2

Elm, rock T6-B3 T3-B2

Hophornbeam T6-B3 T3-B1

(ironwood) 2

Locust, black 2T6-A3 T3-A1

Osage-orange 2

T6-A2 T3-A1

Persimmon T6-C3 T3-C2

Sycamore 2

T6-D2 Te-D1

2

 For 4/4 and 8/4 locust and osage-orange and 8/4 hophornbeam

moisture content, use the green stock schedule.

(ironwood) and sycamore stock over 35 percent

Table 16. —Basic kiln schedules for American and slippery elm, holly, and black walnut 1

Moisture 4/4, 5/4, 6/4 (T6-C4) 8/4 (T3-C3)

content at

start of step Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature

------------------------------------------------------------------------------------------------------------------------

Percent ° F ° F ° F ° F ° F ° F

Above 40 120 7 113 110 5 105

40 120 10 110 110 7 103

35 120 15 105 110 10 100

30 130 25 105 120 19 101

25 140 35 105 130 35 95

20 150 40 110 140 40 100

15 180 50 130 160 50 110

Equalize 173 43 130 173 43 130

Condition 180 10 170 180 10 170

1

 Species included and recommended green stock code numbers:

Species Code No. for

4/4, 5/4, 6/4 8/4

Elm, American T6-D4 T5-D3

Elm, slippery T6-D4 T5-D3

Holly T6-D4 T4-C3

Mahogany T6-C4 T4-C3

Walnut, black T6-D4 T3-D3

-39-
Table 17. —Basic kiln schedules for beech, sugar maple, and pecan 1

Moisture 4/4, 5/4, 6/4 (T8-C2) 8/4 (T5-C1)

content at

start of step Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature

Percent ° F ° F ° F ° F ° F ° F

Above 40 130 4 126 120 3

40

117

130 5 125 120 4

35

116

130 8 122 120 6

30 140

114

14 126 130

25

10 120

150 30 120 140 25

20

115

160 40 120 150 35

15

115

180 50 130 160 50

Equalize

110

173 43 130 173 43 130

Condition 180 10 170 180 10 170

1 Species included and recommended green stock code numbers.

Species Code No. for

4/4, 5/4, 6/4 8/4

Beech T8-C2 T5-C1

H ickory T8-D3 T6-D1

Maple, sugar T8-C3 T5-C2

Pecan 2

T8-D3 T6-D1 2 Bitter pecan (water hickory) heartwood is very difficult to kiln dry from green. Stock should be thoroughly air dried

first.

Table 18. —Basic kiln schedules for white ash, yellow birch, cherry, sweetgum, and similar

woods 1

Moisture 4/4, 5/4 6/4 (T8-C4) 8/4 (T5-C3)

content at

start of step Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature ______________________________________________________________________________________________________________________

Percent ° F ° F ° F ° F ° F ° F

Above 40 130 7 123 120 5

40

115

130 10 120 120 7

35

113

130 15 115 120 11 109

30 140 25 115 130

25

19 111

150 35 115 140 30

20

110

160 45 115 150 40

15

110

180 50 130 160 50 110

Equalize 173 43 130 173 43 130

Condition 180 10 170 180 10 170

1 Species included and recommended green stock code numbers:

Species Code No. for:

4/4, 5/4, 6/4 8/4

Ash, black T8-D4 T5-D3

Ash: green, white 2

T8-B4 T5-B3

Birch, yellow T8-C4 T5-C3

Cherry, black 2

T8-B4 T5-B3

Cottonwood (wet-streak) T8-D5 T6-C4

Hackberry T8-C4 T6-C3

Maple: red, silver T8-D4 T6-C3

Sassafras T8-D4 T5-C3

Sweetgum (heartwood) T8-C4 T5-C3 2 For green or white ash and cherry stock over 35 percent moisture content, use the green stock schedules.

-40-
Table 19. —Basic kiln schedules for magnolia, paper birch, butternut, normal cottonwood, and

similar woods 1

Moisture 4/4, 5/4 6/4 (T10-D4) 8/4 (T8-D3)

content at Dry-bulb start of step Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature

Percent

Above 50

50

40

35

30

25

20

15

Equalize

Condition

° F

140

140

140

140

150

160

170

180

173

180

° F

7

10

15

25

35

40

45

50

43

10

° F

133

130

125

115

115

120

125

130

130

170

° F

130

130

130

130

140

150

160

180

173

180

° F

5

7

11

19

35

40

45

50

43

10

° F

125

123

119

111

105

110

115

130

130

170

1 Species included and recommended green stock code numbers:

Species Code No. for:

4/4, 5/4, 6/4 8/4

Basswood (light color) T9-E7 T7-E6

Birch, paper T10-C4 T8-C3

Buckeye T10-F4 T8-F3

Butternut T10-E4 T8-E3

Chestnut T10-E4 T8-E3

Cottonwood (normal) T10-F5 T8-F4

Magnolia; sweetbay T10-D4 T8-D3

Tupelo, swamp (swamp blackgum) 2

T10-E3 T8-D2

Willow, black T10-F4 T8-F3

2 For swamp tupelo over 40 percent moisture content, use the green stock schedule. For water tupelo, see Dry Kiln

Operator’s Manual.

Table 20. —Basic kiln schedules for yellow-poplar and cucumbertree 1

Moisture

content at

start of step

4/4, 5/4, 6/4 (T11-D4)

Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature

8/4 (T10-D3)

Dry-bulb Wet-bulb Wet-bulb

temperature depression

Percent

Above 50

50

40

35

30

25

20

15

Equalize

Condition

° F

150

150

150

150

160

160

170

180

173

180

° F

7

10

15

25

35

40

45

50

43

10

° F

143

140

135

125

125

120

125

130

130

170

° F

140

140

140

140

150

160

170

180

173

180

° F

5

7

11

19

35

40

45

50

43

10

temperature

° F

135

133

129

121

115

120

125

130

130

170

1 Cucumbertree, although really one of the magnolias, is usually dried with yellow-poplar.

-41-
Table 21. –Basic kiln schedules for black tupelo (black gum) and sweetgum sapwood (sap gum) 1,2

Moisture 4/4, 5/4, 6/4 (T12-E5) 8/4 (T11-D3)

content at

start of step

Percent

Above 60

60

50

40

35

30

25

20

15

Equalize

Condition

Dry-bulb Wet-bulb

temperature depression

° F ° F

160 10

160 14

160 20

160 30

160 40

170 45

170 50

180 50

180 50

173 43

180 10

Wet-bulb Dry-bulb

temperature temperature

° F ° F

150 150

146 150

140 150

130 150

120 150

125 160

120 160

130 170

130 180

130 173

170 180

Wet-bulb Wet-bulb

depression temperature

° F ° F

5 145

5 145

7 143

11 139

19 131

35 125

40 120

45 125

50 130

43 130

10 170

1

 Species included and recommended green stock code numbers:

Species Code No. for:

4/4, 5/4, 6/4 8/4

Sweetgum sapwood (sap gum) 2

T12-F5 T11-D4

Black tupelo (black gum) T12-E5 T11-D3

2

 Lower grade sweetgum heartwood (red gum) often is included with sap gum sorts. If stock is more than 15 percent

red gum, use schedules in table 18.

Table 22. —Basic kiln schedules for basswood, aspen (sapwood or box lumber) 1

Moisture 4/4, 5/4, 6/4 (T12-E7) 8/4 (T10-E6)

content at

start of step Dry-bulb

temperature

Wet-bulb

depression

Percent

Above 60

60

50

40

35

30

25

20

15

Equalize

Condition

° F ° F

160 20

160 30

160 40

160 45

160 45

170 50

170 50

180 50

180 50

173 43

180 10

Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature temperature depression temperature

° F ° F ° F ° F

140 140 15 125

130 140 20 120

120 140 30 110

115 140 40 100

115 140 45 95

120 150 45 105

120 160 50 110

130 170 50 120

130 180 50 130

130 173 43 130

170 180 10 170

1

 Species included and recommended green stock code numbers:

Species Code No. for:

4/4, 5/4, 6/4 8/4

Aspen (some collapse) 2

T12-E7 T10-E6

Basswood (slight browning) T12-E7 T10-E6

2

 See special schedules in Appendix C.

-42-
Specific Procedure for Air-Dried Stock

4/4, 5/4, most 6/4

(1) Bring dry-bulb temperature up to the

value prescribed by schedule for the average

moisture content (MC) of the controlling 8 kiln

samples, keeping the vents closed and the

steam spray turned off.

(2) After prescribed dry bulb temperature

has been reached:

(a) If the air-dried stock had not undergone surface wetting or been exposed

for a considerable period to high relative humidity, just before it was placed

in the kiln, set the wet-bulb controller at

the prescribed wet-bulb temperature.

Turn on the steam spray only if necessary

to start equalizing.

(b) If there has been surface moisture regain, set the wet-bulb controller for a

10° F wet-bulb depression and turn on

the steam spray. Let the kiln run 12 to

18 hours at this wet-bulb setting, then

change to the dry- and wet-bulb settings prescribed by the schedule.

8/4 (plus 6/4 oak)

(1) Bring dry-bulb temperature up to the

value prescribed by the schedule for the average MC of the controlling kiln samples,

keeping the vents closed. Use steam spray

only as needed to keep wet-bulb depression

from exceeding 12° F.

(2) After prescribed dry-bulb temperature

has been reached:

(a) If there has been no surface moisture

regain, set the wet-bulb controller at

the prescribed wet-bulb temperature.

Turn on the steam spray only if necessary.

(b) If there has been surface moisture regain, set the wet-bulb controller for an

8° F wet-bulb depression and turn on

the steam spray. Let the kiln run 18 to

24 hours at this setting. Then set for a

12° F depression and run for 18 to 24

hours more before changing to the conditions prescribed by the schedule.

8 The controlling samples usually are the wettest half

of the entire group of carefully selected kiln samples

(chapter 6, Dry Kiln Operator’s Manual).

Specific Procedure for Partly Air-Dried

Stock

414, 5/4, most 6/4

(1) Bring dry-bulb temperature up to the

value prescribed by the schedule for the average MC of the controlling kiln samples.

Keep the vents closed. Use steam spray only

as needed to keep wet-bulb depression from

exceeding 10° F. Do not, however, allow the

depression to become less than 5° F or moisture will condense on the lumber.

(2) After the prescribed dry-bulb temperature has been reached, run a minimum of 12

hours on each of the first three wet-bulb depression steps of the whole schedule, but still

observe the 5° F minimum wet-bulb depression. Then change to the conditions prescribed for the MC of the controlling samples.

8/4 (plus 6/4 oak)

(1) Bring dry-bulb temperature up to the

value prescribed by the schedule for the average MC of the controlling kiln samples.

Keep the vents closed. Use steam spray only

as needed to keep wet-bulb depression from

exceeding 8° F. Do not, however, allow depression to become less than 5° F.

(2) After the prescribed dry-bulb temperature has been reached, run a minimum of 18

hours on each of the first three wet-bulb

depression steps of the schedule, but still observing the 5° F minimum wet-bulb depression. When the kiln conditions coincide with

those prescribed by the schedule for the average MC of the controlling samples, change to

the moisture content basis of operation.

Procedure for Including Small

Amounts of One Species in

Large Kiln Loads of Other Hardwoods

Individuals who have lumber from a log or

two of their own often wish to use this lumber

for paneling, cabinetry, or fine furniture.

Such wood should be carefully air dried first

under a good pile roof or in an unheated shed.

Then, if possible, it should be kiln dried so as

to bring the moisture content to about 7 percent and relieve drying stress. Such air-dried

lumber can be kiln dried with the same thickness of air-dried stock of another species in

the same basic kiln schedule group (table 10).

The kiln schedule will have to be operated on

the basis of kiln samples representing the

major kind of wood in the kiln, but final mois-

-43-
ture content and stress condition should be

satisfactory if the kiln operator uses a proper

equalizing and conditioning treatment.

Small amounts of fully air-dried lumber (20

percent moisture content or less) can be kiln

dried with air-dried stock of species that take

other basic kiln schedules, but final moisture

content and stress relief may not be as satisfactory as when dried with a species of its own

group. If the only hardwood kiln available is

drying green or partly air-dried stock, arrangements should be made to put the small

amount in the kiln sometime after it is

started, when the major stock and the airdried small amount of material have about

the same moisture content.

Small amounts of air-dried stock also can

have their moisture content reduced to the

proper level by heated room drying, which is

described later. Such drying may leave the

wood with unrelieved drying stresses, which

can cause warping problems unless special

care is exercised in the woodworking operations.

Kiln Schedule Acceleration

By shortening drying time considerably,

significant energy and cost savings can be

made—a possibility that should be explored

by every commercial kiln operator. A stepby-step approach to kiln schedule acceleration for green stock is given on pages 124–126,

Dry Kiln Operator’s Manual. Under certain

circumstances, several steps can be combined. For instance, the recommended

schedule for red oak (4/4 thickness) is T4–D2.

This is essentially the same as the basic

schedule for 4/4 in table 13.

If the green moisture content is very high,

the first acceleration is to change from T4-D2

to schedule T4–E2. Thus, changes in the wetbulb depression are accelerated. Although

the initial depression is the same (4° F), the 5°

F depression of the second step is started at

60 percent moisture content instead of 50 percent. Then the depression goes to 8° F at 50

percent, and so on. (The dry bulb value is unchanged.)

If T4–E2 works well, for several charges,

with no surface checking, the next change

should be to schedule T4–E3. In this change

from E2 to E3, the initial wet-bulb depression

is increased. The starting depression is 5° F

rather than 4° F. Subsequent wet-bulb depressions also are increased. In a kiln with

well-calibrated instruments and good construction, T4–E3 should work well, but a

slight amount of surface checking could occur

on the entering air edges of the load.

The 50° F depression prescribed in the

Manual for the latter stages of drying is only

a guide. Generally the kiln operator should

turn off the steam spray by a hand shut-off

valve and set the wet-bulb controller so that

the vents stay closed during the latter half of

the drying schedule. If the wet-bulb temperature does not come down to the value shown

for each step in the basic schedules, the kiln

operator may want to open the vents for short

periods only. When dry-bulb temperatures of

160° F or higher are reached, however, the

vents should be kept closed.

Finally, one temperature acceleration is

suggested for this 4/4 thickness only; when all

the samples are below 20 percent, go to the

final 180° F temperature. The results of all the

modifications outlined above are summarized

in table 23. If the oak cited above is from stock

that normally is very wet when green, but has

had some slight air drying, it may come down

to 60 percent moisture content very rapidly.

In this situation, extend the time on the first

schedule step to a total of 2 days.

Not all the generally recommended kiln

schedules are so conservative that they can

be modified as much as the oak schedule. See

Appendix A for a listing of schedules and

comments for each species. A general modification that applies to all species when drying

8/4 stock is to use a final dry-bulb temperature of 180° F when the average moisture content of the controlling samples reaches 11

percent.

The question is sometimes asked: “What

can be done when drying seems to stop in the

middle of the kiln run?” Drying will never

actually stop if the EMC of the kiln atmosphere is lower than the core moisture

content of the lumber. Slight increases of

dry-bulb temperature during the intermediate stages of drying can be made, but

generally they are not recommended. An increase of 5° F can be tolerated, however, by

some woods. In table 23, it might be satisfactory to use 115° F dry-bulb temperature at 35

percent moisture content. A temperature of

-44-
125° F dry-bulb has been used at 35 percent

moisture content for 8/4 hickory (table 17, Dry

Kiln Operator’s Manual). When a decision is

made to try such increases of temperature at

moisture contents above 30 percent, the kiln

operator should make sure that his kiln

recorder-controller is properly calibrated.

Table 23. —Accelerated kiln schedule for

northern or upland red oak 4/4, 5/4 with a

high initial moisture content (some risk of

slight surface checking)

Moisture Dry-bulb Wet-bulb Deprescontent temper- temper- sion

ature ature

Percent ° F ° F ° F

Above 60 110 105 5

60-50 110 107 7

50-40 110 99 11

40-35 110 91 19

35-30 110 (

1

)

1 3 0

30-25 120 (

1

)

1 40

25-20 130 (

1

)

1 50

20-18 140 (

1

)

1 50

18–(F–3)2

180 (

1

)

1 50

1 Vents closed, steam spray shut off, accept whatever

depression occurs.

2 Equalize and condition as in table 13. F is desired

final moisture content.

Experimentally, some acceleration of drying of oak has been obtained by automation in

connection with presurfacing and an accelerated kiln schedule (Wengert and Baltes 1971).

Automation has not been adopted commercially. Pilot tests have been made, however,

on presurfacing and the accelerated kiln

schedule it permits (Cuppett and Craft 1972,

Rice 1971). Drying time savings were estimated to be 24 percent or higher and kiln

capacity was increased 8 to 12 percent. Presurfacing does not fit in with current

hardwood processing practice, and there

would, of course, be some added costs for the

presurfacing. Further details are given under

Special Kiln Schedules for Special Purposes

(app. C).

Mixed Species and Simplified

Schedules

When kiln drying large quantities of valuable species from the green or partly air-dried

condition, the recommended practice is to dry

each species, in each thickness, separately.

When smaller amounts of individual species

are available, as when clearing southern pine

sites for planting, drying groups of species

with similar characteristics together is often

expedient. General suggestions for drying

mixed kiln charges are given in chapter 10 of

the Dry Kiln Operator’s Manual.

Frequently, when using easy-drying woods

for demonstration material in kiln drying

short courses, several steps of the recommended schedule were omitted without causing degrade. This led to development of

simplified schedules that primarily save some

time for the kiln operator, but they also may

reduce kiln residence time slightly. The ease

of drying and the simplicity of the schedules

suggest that several species can be mixed,

and substantial industry experience backs

this up. A classification of southern pine-site

hardwoods by similarity of schedules is given

in Appendix B, along with suggested

simplified schedules I through VI (table B1).

This classification can also be used with the

same species grown on other upland eastern

hardwood sites, but the kiln operator must

use judgment in applying it. For instance,

commercial sapgum that contains more than

15 percent sweetgum heartwood should be

dried by the red gum schedules (table 18).

Tupelo from flooded or swampy sites should

be dried by the water tupelo or swamp tupelo

schedules. In commercial practice, these more

difficult-to-dry tupelos generally are air-dried

before kiln drying. Bitter pecan heartwood

also is air-dried before kiln drying. Where

only small quantities of rock elm are available, it can be combined with open-grown

(hard type) American elm and dried by the

warp-reducing schedule II. Large quantities,

however, should be dried separately using the

rock elm schedule (table 15). Likewise, large

quantities of forest-grown soft-type American

elm can be dried by schedule I, but small

quantities can be mixed with hard-type elm

using schedule II.

Special Schedules for Special Purposes

Aspen

Now that aspen is being used for a wider

array of products than crating and rough

lumber, consideration must be given to

minimizing collapse. This defect is a principal

problem in drying aspen. Special schedules

for 4/4 and 8/4 No. 2 and 3A Common lumber

are given in Appendix C. The uppermost

grades of lumber sawed from the outside of

–45-
larger logs, as well as crating lumber, can still

be dried by the schedules in table 22. A Canadian schedule for faster drying of studs is

mentioned in chapter 6 on High-Temperature

Drying.

Sugar Maple

Schedules are sometimes needed to keep

maple sapwood the whitest color possible and

to dry maple containing mineral streak with

the least amount of honeycombing. The special schedule for white maple given in appendix C has been used successfully on 4/4 and 5/4

stock (MCMillen 1976). When drying 4/4 maple

that contains mineral streak or other character marks, many kiln operators satisfactorily

use the regular wet-bulb depression schedule,

C3, with a lower-than-customary temperature schedule, either T5 or T3. See page 118,

Dry Kiln Operator’s Manual for these separate schedules and the method of their assembly.

For 6/4 and 8/4 mineral-streak stock, see the

special procedure suggested in appendix C.

Fine internal hairline checks that do not

show up in 8/4 and thicker maple until manufacture or use have sometimes been a costly

problem in maple. These are believed to be

caused by stresses resulting from surface

moisture regain and improper redrying of

previously kiln-dried stock. The first preventative is to store kiln-dried maple in a manner

to prevent regain (see chapter 9 on Storage of

Dried Lumber). If this has not been done, see

the special schedules for adjusting moisture

content of kiln-dried wood.

Bacterially Infected Oak

Lumber from northern red oak and black

oak infected with anaerobic heartwood bacteria is highly susceptible to internal checking when dried by normal or accelerated oak

schedules. Both honeycombing and ring separation occur (Ward 1972; Ward et al. 1972).

Research has shown that material in the advanced stage of infection is especially subject

to surface checking and honeycombing. Such

wood should be sawed into 4/4 rather than

thicker lumber. Good results were obtained

when such material was forced air dried to 20

percent MC by an 8/4 oak procedure (Cuppett

and Craft 1975), and then kiln dried by the

balance of the schedule given in appendix C.

Almost as good results were obtained when

kiln drying was started at 25 percent MC. Low

air velocity was used, and the fans were run

only half the time for the first 11 days of

forced air drying. Material in an early stage

of infection was successfully kiln dried from

the green condition by the schedule in appendix C.

Presurfaced 1-Inch Upland

Red and White Oak

The suitability of an accelerated kiln

schedule for fully presurfaced upland red and

white oak was confirmed by research at the

U.S. Forest Products Laboratory (MCMillen

1969, MCMillen and Baltes 1972, Wengert and

Baltes 1971). Drying time can be reduced 25 to

50 percent by the fully accelerated schedule

and kiln capacity is increased 8 to 12 percent

by the presurfacing. Slight additional benefits are obtainable by automation, but full

benefits of the accelerated schedule are not

available without automation. The schedule

is not applicable to presurfaced green cherrybark oak, a common southern oak, but can

be applied to such stock carefully dried by

other means to 40 percent moisture content.

Successful field tests of the presurfaced oak

schedule have been carried out with northern

red and mixed Appalachian red oak in Massachusetts and West Virginia (Rice 1971,

Cuppett and Craft 1972).

The accelerated schedule, modified for kiln

sample operation, is given in Appendix C.

This is for oak fully surfaced in the strictly

green condition to remove all saw marks, not

to material that is merely blanked or skip

dressed to obtain uniform size. The thickness

after complete surfacing probably should be

33/32 inch to insure kiln dry dimension cuttings 13/16 inch thick. Rough sawing would

have to be to a l-5/32-inch thickness (see

MCMillen (1969) for further suggestions).

Warp and Shrinkage Reduction

A number of measures for warp reduction

are indicated in chapter 2. A major one is to

use rapid natural air drying or acceleratedair drying of properly piled lumber. Research

on red oak has shown that shrinkage is reduced by using lower temperatures and rapid

reduction of relative humidity (MCMillen

1963). The effect is uniform from 140° F down

to 95° F. From 95° F down to 80° F the effect is

greater because more of the wood is affected

by tension set. Tension set tends to resist

shrinkage of the board and thus reduces

warping. This is a major part of the reason

why air drying gives the least shrinkage and

-46-
warping. Compression set in the interior of

the wood is also important, as it tends to increase board shrinkage and warping. Thus,

the drying of green oak with an initial temperature above 110° or 115° F would be expected to produce more than normal warping.

The effects of uniform thickness, good

stacking, and restraint may overcome the effects of temperature differences with 4/4

stock. No significant difference was detected

in the warp of 1-inch green sugar maple with

initial temperatures of 110° to 160° F when the

same EMC was used in all runs (Rietz 1969).

For thicker stock, in which compression set

probably has more influence, use of lower

than customary initial temperature and relative humidity probably is helpful.

No series of low-shrinkage, low-warp

schedules have been developed. The kiln

operator who has the kiln time available to

take slightly longer in drying can experiment

with lower relative humidity schedules by

using slightly larger initial wet-bulb depressions. The lower the wet-bulb temperature,

the easier it is to achieve a lower dry-bulb

temperature in the kiln. Changes from recommended or basic schedules should be slight

at first, and the kiln operator will need to

observe the stock in the kiln frequently to see

that surface checking is not developing. Any

such experimentation, of course, should be

done only with the advice and consent of the

management.

Adjusting Moisture Content of

Kiln-Dried Wood

Once wood has been kiln dried to a moisture

content suitable for interior purposes, it

should be stored in a heated or dehumidified

shed or room (see Chapter 9, Storage of Dried

Lumber). Situations occur, however, when

the moisture content of the lumber must be

changed: (a) the moisture content is too low

for use in steam bending, boat construction,

or the like, or (b) the wood has not been

properly stored and must be redried.

If the moisture content is too low, a twostep procedure is advised. The lumber must

be stickered in properly built piles and the

void spaces of the kiln baffled. The exact

schedule will depend on the moisture content

desired and the time available. Allow 2 days

for each step for 4/4 stock, longer for thicker

material or large moisture differences, For

the first half of the time required, use a kiln

EMC equal to the desired moisture content.

For the second half use an EMC 3 percent

higher. If enough time is available, use a kiln

temperature of 130° or 140° F. For quicker

results, 160° or 180° F can be used. In either

case, the vents should be closed during kiln

warm-up. Steam spray should be used intermittently to avoid EMC’s lower than the

present moisture content or higher than the

desired moisture content during the warm-up

period. Do not condense moisture on t h e

lumber. Use the kiln sample procedure to

monitor the moisture pick-up. If the rate is

too slow, use a higher temperature with the

same EMC’s. The moisture level to which

hardwoods can be easily raised is limited to

about 13 percent, for it is difficult to hold an

EMC higher than 16 percent in most kilns.

To redry kiln-dried lumber that has been

kept in uncontrolled storage, great care is

needed. Otherwise surface checks that were

tightly closed may become permanently

opened, or internal hairline checks can occur.

If the storage period has been short, tightly

bundled lumber can be redried in the bundles

because most of the moisture pick-up will

have been on the board ends and the surfaces

of exposed boards. For longer storage, or for

lumber that has been kept outdoors on stickers, the lumber must be stickered for redrying.

Two temperature steps are suggested for

the redrying operation; the first about 1 day

long at 130° or 140° F. The second should be at

the final temperature of the drying phase of

the basic schedule (tables 12 to 22) for the

species and size involved. Do not use steam

spray during kiln warm-up. Surface checks

already present may open up, but will close

again as the wood dries. When the first kiln

temperature is reached, set the wet-bulb controller to achieve an EMC somewhere between the current moisture content of the

stock and the moisture content desired. For

the final step, set the controller to give an

EMC 2 percent below the desired moisture

content. When the wettest kiln sample

reaches the desired moisture content, stop

the drying, No conditioning should be needed.

-47-
Predicting Drying Time

Frequently management is faced with determining how long it will take to dry a load or

when the next change in kiln conditions can

be made.

“How long before the next change” can be

easily answered if the operator will graph the

moisture content of the samples as they dry

(fig. 13). To make this job easy, special graph

paper can be obtained which has horizontal

markings in tenths of an inch and vertical

markings in twelfths. With such paper, it is

easy to show drying time to days and hours.

By graphing the moisture content of the

samples as they are weighed, a smooth drying

curve is obtained. Any portion of this curve

can be extended 24 hours ahead with a

straight line to predict the approximate moisture content the next day. In fact, when the

same species and thickness is dried repeatedly, the total drying time can be reasonably well estimated.

Early in a kiln operator’s career at a given

location, he may find it helpful to develop

basic drying curves. He should use his own

sample data for green or nearly green stock of

various species and thicknesses. For any subsequent charge of lumber dried from a lower

initial moisture content, the drying curve of

the first day or two would generally parallel

the line for the first day or two of the green

curve. After that the drying of air-dried or

partly air-dried stock will follow fairly closely

the curve for the green stock.

Since it will not be practical to establish

green stock drying curves for all thicknesses

of lumber right away, approximate curves

can be estimated. These can be set up using

drying time factors for the most common

thicknesses. One such set of factors was developed by Higgins to predict total drying

time. Such factors have a theoretical base related to diffusion and other aspects of wood

drying, but these were empirically developed

after several years of experience in commercial drying of foreign and domestic woods.

They are roughly corroborated by other

commercial drying time data. The drying time

factor for 8/4 stock was set at 1.00 because the

best approximations for other thicknesses

were obtained when the 8/4 drying time data

M 143471

Figure 13.—Black walnut drying curve.

-48-
M 144693

Figure 14.—Method for postulating a drying time scale for a new thickness of a given

species to be dried from a known drying curve.

were used as a base. Actual drying times will

vary, of course, depending on actual thickness, width, percentage of heartwood, and

quality of stock.

Until more precise factors become available, the ones developed by Higgins can be

used in setting up the preliminary curves to

estimate drying time from various moisture

levels. These factors are as follows:

Thickness Drying time factor

3/4 0.25

4/4 .40

5/4 .55

6/4 .70

8/4 1.00

10/4 1.35

12/4 1.75

14/4 2.25

16/4 2.85

The factors for 4/4 and 8/4 indicate that 21⁄2

times as long will be required to dry 8/4 material as 4/4.

To employ these factors in developing a set

of drying curves for a species, the kiln

operator should use either his own 8/4 curve

or a 4/4 curve obtained when a conservative

schedule was used. Time and moisture content scales should be selected so that the

slope of the curve is between 30° and 48° F. The

first step in using a 4/4 curve as a base would

be to list the 8/4 drying times directly below

the 4/4 times. Then compute the drying times

at the same moisture levels for the other

thicknesses. From figure 13, some of the times

would be as follows:

Thickness Factor Drying times-days

4/4 (Given) 2 6 10 14

8/4 (

9

) 5 15 25 35

5/4 0.55 2.75 8.25 13.75 19.25

6/4 .70 3.50 10.50 17.50 24.50

10/4 1.35 6.75 20.25 33.75 47.25

9 Time for 8/4 = 1.00 ÷ (factor for given thickness)

× given time; in this example, 8/4 time

× given time = 2.5 (given time).

At the start, a separate graph should be

used for each thickness. An example for walnut 6/4 is shown in figure 14. Write the computed drying times for 8/4 and for the selected

thickness on the graph in pencil. Then interpolate whole number days for the selected

thickness with a pen and erase the pencil

values. As successive charges are dried and

-49–
drying time improved by satisfactory accelerations, new lines can be drawn on this

graph. When sufficient experience has been

gained, and any program of schedule improvement completed, a new graph can be

made using the average of the actual curves.

Ultimately the kiln operator can make up one

curve and a composite time scale for each

thickness on one graph. A crucial point, however, is the fact that hardwoods of varying

quality cannot be dried by time schedules.

The kiln operator must revert to the moisture

content schedule when he is in doubt about

the drying characteristics of a particular

charge.

A side benefit of such a finalized drying

graph is that any subsequent charge of normal stock having a slower drying rate will

indicate the kiln may not be operating properly. It may have a malfunctioning instrument,

a water- or air-bound steam line, an inoperative fan, or some similar fault, and may need

maintenance. A comparison of graphs over a

period of time should also show any slow loss

of efficiency in the kiln.

Equalizing and Conditioning

Frequently in kiln drying, and especially

with mixed species, sizes, or initial moisture

levels, some lumber in a charge is as dry as

Figure 15.—Method of cutting final moisture content and drying stress tests.

-50-
required while other lumber is still too wet.

An equalizing treatment reduces the spread

between the wettest and driest boards. Drying stresses and sets (often called casehardening) are always present at the end of

kiln drying and equalizing. Any lumber that

will be resawed, ripped, or machined

nonuniformly should be conditioned to relieve

stresses. Failure to do so will result in warping (cupping, crooking, or bowing) during o r

immediately after machining and will cause

difficulty in boring.

The normal procedures for equalizing and

drying stress relief are described in the last

parts of chapters 8 and 10 of the Dry Kiln

Operator’s Manual. Figure 15 shows a method

of cutting final moisture content and longitudinal as well as transverse stress tests.

The kiln operator’s attention is especially

drawn to the method of evaluation of

casehardening in chapter 10 of the Manual. A

final analysis for freedom from stress cannot

be made until the test prongs have air dried

for 16 to 24 hours, but a significant turning

out of the transverse test prongs immediately

after they are cut often indicates that the

transverse stresses have been relieved.

It was formerly considered that dense

hardwood 4/4 lumber required 16 to 24 hours

to condition. Conditioning has high energy

demands, so the time should be no longer

than needed to get stress relief. Several lowdensity imported woods have been successfully conditioned by equalizing at 1 percent

lower moisture content than the recommended procedures in the Manual, then conditioning 6 hours at the regular EMC

(McMillen and Boone 1974). This short-cut

has been successfully used on sugar maple

sapwood in kiln-drying demonstrations, taking about 8 hours. Red oak 4/4 reportedly has

been conditioned in 13 hours.

If the average moisture content determinations are made immediately after the conditioning treatment, the moisture content

obtained will be about 1 to 1.5 percent above

the desired value because of the surface moisture regain. After cooling, the average moisture content of the lumber should be close to

that desired. The general rules of the conditioning procedure do not apply to moisture

content values above 11 percent. Conditioning is hard to accomplish at the higher values.

The important factor in conditioning is to

obtain the specified EMC. The simplest way is

to use an equalizing dry-bulb temperature

that is lower than the dry-bulb temperature

for conditioning. At the end of equalizing, the

heating coils are shut off. The wet-bulb controller is then set up for the conditioning

wet-bulb temperature. As the steam spray

raises the wet-bulb temperature, the dry-bulb

temperature also will rise some. When the

wet-bulb temperature reaches the desired

level, the dry-bulb controller is set to its prescribed value. Only one heating coil should be

turned on. Both the dry-bulb and the wetbulb temperatures should reach and remain

at the desired levels for conditioning.

The equalizing and conditioning temperatures shown in the basic kiln schedules (tables 12–22) are selected to utilize the above

procedure and give stress-free wood at a final

7 percent moisture content. For other desired

final moisture content levels, use temperatures to equalize at an EMC 3 percent below

the desired moisture content and to conditions

at 4 percent above the desired moisture content. If equalizing is expected to be prolonged,

use the reduced dry-bulb temperature only

the last 12 to 18 hours of equalizing.

The conditioning time could be unduly

lengthened if high-pressure steam is used for

conditioning and no special measures were

taken to reach the correct wet-bulb depression. A desuperheater should be used in the

steam line to remove the excess heat. (See

Operational Considerations.)

Occasionally, the transverse prong test will

show no stress, but the lumber will bow when

resawed. The cause of the bowing is longitudinal stress resulting from either longitudinal tension set in the surface zones or

longitudinal shrinkage differentials due to

reaction wood (tension wood in hardwoods).

These stresses are most likely to be unrelieved

when conditioning temperature or EMC is too

low or when conditioning time is too short. The

longitudinal stress sticks in figure 15 will

show whether such stresses are present. If

stresses are a problem, conditioning should be

at 180° F or higher. The lumber must have

been equalized, and the recording instrument

must be in calibration. If longitudinal

stresses are still a problem, the wet-bulb setting can be raised 1° F over the recommended

-51-
value. Also, the conditioning period can be If tension wood stresses are very severe, they

extended about 4 hours per inch of thickness. may not yield to any conditioning treatment.

Operational Considerations

An extensive discussion of considerations

for operating a dry kiln is given in chapter 10

of the Dry Kiln Operator’s Manual (1961).

That chapter presents several aspects of kiln

operation, other than selection of the

schedule to use, to aid the operator in exercising good judgment. Of particular interest, in

view of the greater need to conserve energy

and material now than heretofore, are the

suggestions on starting the kiln, restricted

use of steam spray during kiln warm-up, and

operating the kiln after warm-up.

Additional information is given in this

handbook on the important heating, venting,

and air circulation equipment, on humidity

control with high-pressure steam, and parttime drying.

Heating, Venting, and Air Circulation

Many older kilns were designed for drying

air-dried hardwoods. Usually these kilns

cannot dry green hardwoods very efficiently—they have inadequate heating,

humidification, and circulation systems.

Many older kilns have inadequate venting for

the rapid drying of green stock of faster drying species.

It is easy to determine if a kiln is short of

heating capacity—it will take more than 4

hours to reach the desired elevated temperature. If there is adequate circulation and

other equipment is functioning properly, the

kiln manufacturer can add more heating pipe

or ducts.

A kiln with inadequate humidification will

be unable to condition lumber adequately—

the steam or water spray will be running all

the time but a 10° F depression cannot be

obtained. If there is little or no superheat, if

the vents are closed tightly, if there are no

leaks in the structure, and if the spray line

and holes or nozzles are not plugged, a larger

steam supply is needed. Increasing only the

pressure will increase the superheat and may

not solve the problem.

It is important to have adequate venting

capacity. Failure to exhaust moist air rapidly

will cause the wet-bulb temperature to be

higher than the schedule requires. The result

is slow drying. For some species, like hard

maple, the risk of stain and discoloration is

increased. Restricted use of steam spray during kiln warm-up is very helpful, but vents are

inadequate if desired depressions cannot be

easily obtained a few hours after they are set

on the instrument. Venting can be increased

by increasing vent size or by power venting.

Power venting is the discharging of humid

air from or the injection of air into a kiln by

fans or blowers and suitable duct work installed by the kiln manufacturer. One aim is

to increase the venting rate of a kiln when the

regular vents cannot conveniently be enlarged. Another aim is to conserve energy. To

conserve energy, care must be taken to place

the power vents so that the humid air is discharged before the kiln air passes over the

heating coils.

A poor air circulating system is evident by a

wide range of final moisture content values in

the charge and uneven drying. If air velocity

measuring instruments are available, poor

circulation will be indicated by velocities

below 200 feet per minute (ft/min) or by differences between the highest and lowest velocities greater than 150 ft/min (measured on

the leaving-air side of the load). The recommended kiln schedules are based on a velocity

through the load of 200 to 400 ft/min, and

probably are as satisfactory with velocities up

to 450 ft/min. Any improvement in circulation

will improve heating and venting, however.

Increasing the velocities in the early stages of

drying to 500 ft/min or so will accelerate drying, but will increase the risk of surface

checking with check-prone species like oak or

beech. This can be compensated for by decreasing wet-bulb depressions 1° F. Thorough

baffling to prevent “short circuiting” of air

will greatly improve circulation. Changes in

fan speed and design usually require advice of

a kiln engineer.

Humidity Control With High-Pressure

Steam

Some hardwood dry kilns are heated with

high-pressure steam (80 to 100 pounds per

square inch (lb/in2

) or higher). High-pressure

steam is excellent for economy in heating

–52-
coils because of lower initial cost, less fin

radiation area required, and smaller steam

feed lines and heat control valves than a

low-pressure kiln operating at 10 lb/in2

. However, the heat in high-pressure steam makes

it unsuitable for use in the humidity spray

system. The steam is so hot and so very dry

that it raises the dry-bulb temperature in the

kiln excessively when the wet-bulb temperature is raised; consequently, it is impossible to

carry a 4° or 5° F wet-bulb depression during

the initial stages of some schedules, or to get a

10° F wet-bulb depression for conditioning and

stress relief at the end of a charge. Modifying

the conditioning procedure, as described in a

preceding section, is helpful if the steam pressure is not too high. For steam pressures of 100

lb/in2 and over, however, a desuperheater is

needed.

A desuperheater unit supplies low-pressure

wet steam, cooled to the saturation point, for

the humidity spray system. A suitable

pressure-reducing valve brings steam pressure down to approximately 5 lb/in2

. This very

hot low-pressure steam is then mixed with

water spray in a suitable mixing chamber,

and the extra superheat is absorbed by converting some of the water into steam.

Part-Time Operation

In past years some mills and factories generated their own electricity with steam developed by burning wood waste. Often they

used the exhaust steam to operate the dry

kilns. The steam was available only during

working hours, so the kilns were operated on

a part-time basis, that is, 8 or 9 hours per day,

5 days per week. Other firms burned wood

waste to generate ordinary steam for their

kilns and frequently used coal, oil, or gas during nonworking hours. Now that fossil fuel

costs have risen greatly, such firms are considering part-time operation for periods of

slack product demand. Such operation also

could be considered for periods when demand

is higher, if a greater proportion of the wood

were dried from the air-dried instead of the

green or partly air-dried condition.

Part-time drying is technically feasible o n

air-dried stock and can be done successfully

with green or partly air-dried stock when

proper care is used in schedule application

and operating procedures. Rasmussen (1961)

recommended full-time drying during the

first stages of drying refractory hardwoods

when excessive checking occurs during parttime operation.

Part-time drying takes almost twice as long

to dry an item as does full-time operation.

Under the experimental conditions used,

Rasmussen and Avanzado (1961) found that

full-time operation required slightly less total

energy (4,510 British thermal units per pound

of water (Btu/lb H2O) evaporated) than parttime operation, either with fans running all

the time (5,490 Btu/lb H2O) or only during

heat and spray control periods (5,560 Btu/lb

H2O). The material dried was rough 4/4 red

oak lumber. Drying quality was the same by

all methods. Drying times, from 70 to 7 percent moisture content, were 18 days for fulltime, 351⁄2 for part-time with fans continually

running, and 461⁄2 days for part-time with

restricted fan use.

In the report on the above described experiment, the authors stated that making the

kiln schedule more severe for part-time operation would not be expected to reduce drying

time greatly.

Wolfe (1962, 1963) used temperatures during the “operating” hours as much as 15° F

above established kiln schedule temperature.

He also used the fans and some venting

during “off’ hours. He concluded part-time

operation was economical under regular production circumstances.

The technology of part-time drying has not

been well enough established to include

schedules and drying time in this publication.

-53-
Drying Time

The time required to kiln dry a given

species and thickness depends upon the

character of the wood, the type of kiln, and

the kiln schedule used. The time estimates

given in table 24 are generally minimum

times that can be obtained in well-maintained

commercial kilns with relatively short air

travel. They also are based on the assumption

that the operator will take some steps to increase drying rate. The times will vary a half

to a full day from kiln to kiln. Kilns with

longer air travel, with less than 200 ft/min air

velocity, or in a poor state of maintenance will

take longer in drying green stock. They may

not take much longer than the table values

for airdried stock.

By drying to 6 percent average moisture

content or slightly lower, moisture content

will finalize at about 7 percent after equalizing and conditioning. If a greatly different

moisture content is desired, proper adjustments in conditions must be made, and total

times will differ from the table values. The

time estimates in table 24 are for the drying

of stock for high-quality uses. If considerably

more severe conditions than the basic

schedules are used, time will be shortened,

but quality may be decreased.

The drying times in the table are for precisely sawed rough green material 11/8 inches

thick, plus or minus 1/8 inch. Miscut lumber

with some of greater thickness will, of course,

take longer to dry. For estimates of the effect

of other thicknesses on drying time, see the

subsection on Predicting Drying Time under

Operational Considerations.

Table 24 .—Approximate kiln-drying times 1 for 4/4 hardwood lumber

in conventional internal fan kilns

Species

Apple

Ash, black

Ash: green, white

Aspen

Basswood (slight browning)

Basswood (light color)

Beech

Birch, paper

Birch, yellow

Buckeye

Butternut

Cherry

Chestnut

Cottonwood, normal

Cottonwood, wet-streak

Dogwood

Elm: American, slippery

Elm: cedar, rock

Hackberry

Hickory

Holly

Hophornbeam, eastern

Locust, black

Time required to kiln dry

from

Air-dried 2 Green

condition condition

Days Days

4 10

4 7

4 10

3 9

3 6

4 9

5 12

2 4

5 12

3 6

5 10

5 10

4 8

4 8

4 10

5 12

4 9

5 13

4 7

4 10

5 12

6 14

6 14

Time required to kiln dry

from

Species

Air-dried 2 Green

condition condition

Magnolia

Mahogany

Maple: red, silver (soft)

Maple: sugar (hard)

Oak: red, northern or upland

Oak: white, northern or

upland

Oak: red or white, southern

lowland

Osage-orange

Pecan

Persimmon, common

Sweetgum, heartwood (red

gum)

Sweetgum, sapwood (sap

gum)

Sycamore

Tupelo, black (black gum)

Tupelo, swamp

Tupelo, water

Walnut, black

Willow

Yellow-poplar

Days Days

4 8

4 10

4 7

5 11

5 21

5 23

6 (

3

)

6 14

4 (

3

)

5 12

6 15

4 10

4 8

4 8

5 10

5 (

3

)

5 11

4 10

3 6

1 Approximate times to dry to 6 percent moisture content, prior to equalizing and conditioning, in kilns having air

velocities through the load of 200 to 450 feet per minute.

2

20 percent moisture content for most woods; 25 percent slow-drying woods like oak, pecan, hickory.

3 These items should be air dried before kiln drying,

-54-
Literature Cited

-55-
CHAPTER 6

HIGH-TEMPERATURE DRYING

High-temperature kiln drying of wood has

been explored for many years. It is similar to

conventional kiln drying, but operates at

temperatures of 212° F or higher. Technically,

high-temperature drying includes two processes. In superheated steam drying, the

wet-bulb temperature is maintained at 212° F

and air is excluded. In the other, a mixture of

air and steam is used, and the wet-bulb temperature is below 212° F, often with no precise

control. Current use of high-temperature drying in the United States involves only the

latter process.

In the last 20 years, high-temperature drying has become acceptable to accelerate the

drying of many softwoods. Most of the new

kilns being installed for southern pine lumber

in the United States are high-temperature

kilns. What are the prospects for hightemperature drying of hardwoods?

Hardwood high-temperature drying

technology has not advanced enough to permit recommendations, but the prospects are

probably good enough that anyone planning

to install new kilns should consider equipment suitable for ultimate use of the process.

Drying rates two to five times those of conventional methods make high-temperature

drying very attractive. Because of increased

drying speed, dry kilns could be smaller than

conventional kilns and still handle the same

annual volume of lumber. Smaller kilns would

permit more flexibility in drying (less mixing

of species, faster loading and unloading) and

require less space. Due to faster turnover, inventory could be reduced, resulting in savings

on interest, insurance, and taxes. Last, but

not least, high-temperature drying would require 25 to 60 percent less energy than conventional kiln drying. Because there would be

some added capital investment for extra insulation, kiln tightness, a larger capacity heating system, and larger capacity fans, direct

costs of drying per thousand board feet would

not be cut in half, but it has been estimated

they would decrease by 20 percent or more

(Wengert 1972).

High-temperature drying has been commercially used in Europe for hardwoods previously air dried. Research in Eastern

Canada and the United States has indicated

that the process would be technically feasible

for air-dried hardwoods of many U.S. species.

It is technically applicable to green hardwoods of very permeable species.

Most of the 20 species of hardwoods listed in

of high-temperature drying

(Wengert 1972) developed more drying defects

under high-temperature drying than under

conventional temperature drying. The principal defects were collapse, end checking, and

honeycombing. If dried from the air-dried

condition, such defects were absent or almost

so. Another defect that is common to hightemperature drying is discoloration. This is

usually a toast-brown color which, with some

species, is a very thin layer, and with others is

not. While these defects may largely prevent

the high-temperature drying from being used

for highest quality hardwood uses, they

should not preclude the use of the process for

hardwoods suitable for construction purposes

and other nonexacting uses.

Basic Concepts

A. In theory the high-temperature kiln dry- sapwoods of low- to medium-density harding process has three distinct stages when woods are likely to be permeable enough

used on green wood: (1) the constant drying to sustain the first stage very long.

rate; (2) the first falling rate; and (3) the sec- When the wood can no longer supply free

ond falling rate (Lowery, Krier, and Harm water to the entire surface area, the first fal1968). In the first stage, moisture moves to- ling rate starts. It continues until the free

ward the wood surface predominately by water is gone. The temperature of the wood is

mass flow. Rapid evaporation of moisture at between the wet-bulb and the dry-bulb temor near the surface keeps the wood tempera- peratures of the kiln. The length of both the

ture at the wet-bulb temperature. Only the constant and first falling rate periods are

–56–
govered by the permeability of the wood,

while the final stage (as well as normal drying) is more influenced by the diffusivity.

When the free water is gone, the second falling rate starts. The drying slows considerably more, and the wood temperature

rises approximately to the dry-bulb temperature.

B. Both the rate of moisture movement

and the rate of evaporation are greatly increased by the high temperatures. During the

first two stages, the rate of drying is determined almost solely by the rate of heat transferred to the wood. Very high air velocities

are needed to maintain this high rate of heat.

transfer as well as to remove the evaporated

water from the surface.

C. No mater what the relative himidity is

in the high-temperature kiln, the kiln conditions are severe (EMC cannot exceed 7

percent at 230° F). As a result, a hightemperature kiln, after being heated to 212° F

at the start, can be operated without concern

for the wet-bulb temperature until the conditioning. Unless the wood being dried is very

wet and very permeable, venting is unnecessary. This is one of the major energy-saving

aspects of the process.

D. After the first stage of drying, steep

moisture gradients can develop; these gradients can, in some cases, cause severe drying

stresses and checking. Stresses in unchecked

high-temperature-dried wood can be relieved

easily, but stress relief must be done below

212° F and with properly equalized material,

as is normally required.

E. The equilibrium moisture content of

high-temperature-dried wood is somewhat

lower than that of conventionally dried wood.

F. The exposure of wet wood to high temperature causes both temporary and permanent reductions in strength of the wood. The

shortness of the drying time achieved by the

use of high-air velocities in presentday kilns

is an alleviating factor and makes obsolete

results of some older studies on strength. This

area needs further evaluation.

Research and Practice to Date

The first research on high-temperature drying of hardwoods in the United States, by

Tiemann (1918), was done with superheated

steam. Basswood and sweetgum sapwood

were dried successfully, but other hardwoods

were not. Commercial superheated steam

kilns were used for softwoods on the West

Coast about 1918, but they deteriorated so

badly under the severe drying conditions that

their use was discontinued. Subsequent research by Richards (1958) and John L. Hill10

indicated good results when drying a variety

of air-dried southern hardwoods but poor results when drying the same hardwoods from

the green condition. Their drying equipment

had no humidity supply. These results and

the likelihood of discoloration, strength

losses, and other degrading factors discouraged further research on high-temperature

drying of hardwoods in the United States at

that time.

Research went on elsewhere, confirming

the general suitability of the method for airdried hardwoods and bringing out other favorable aspects of the process. Noteworthy

10 Presented at Forest Products Research Society Annual Meeting, Madison, Wis., June 1958.

reviews of this research were prepared by

Kollmann (1961); Lowery, Krier, and Harm

(1968); Wengert (1972); and Cech (1973).

Kollman (1961) reported use of hightemperature dryers in many German

woodworking plants. These generally used

air-steam mixtures rather than superheated

steam.

The commercial use of high-temperature

drying for softwoods began slowly in the

United States (Kimball and Lowery 1967,

Lowery and Kimball 1966). It developed

rapidly after tight prefabricated kilns became

available and the manufacture of housing

studs became big business. During this development, rapid heating of the kiln to operating temperature was found advantageous

both to save time and minimize degrade.

Meanwhile, research by various investigators, as reviewed by Lowery, Krier, and

Harm (1968), indicated some hardwoods could

be high-temperature-dried from the green

condition if heating up was rapid under saturated steam conditions.

A commercial trial of high-temperature

drying of hardwoods confirmed the applicability of the process to air-dried sapwood of

sweetgum in which the redgum (heartwood)

-57-
portions are below 25 percent MC.ll The Eastern Forest Products Laboratory of Canada

had done considerable research on hightemperature drying of birch and maple over

the years; Cech (1973) reported a most favorable procedure was to force-air dry the stock

to 20 percent MC, then kiln dry at 212° F.

Similar results were obtained with red oak

when the high-temperature portion was kept

at 200° F. Some of the research on birch, however, had shown possibilities for hightemperature drying after very brief airdrying

or predrying periods.

In a screening study to determine at what

moisture level a wide variety of U.S.

hardwoods could safely start hightemperature drying, Wengert (1974) heated

the kiln very rapidly to 200° F. To do this, the

steam spray was used continuously and the

heating coils intermittently. Then the temperature was increased almost immediately

to 230° F. Somewhat surprisingly, most of the

wood tested tolerated high-temperature drying satisfactorily from the green condition.

Red and white oak and sweetgum heartwood

required air drying to 25 percent MC, or conventional kiln drying to 20 percent MC, before

high temperature could be used without causing honeycombing and collapse.

Mackay (1974) designed a high-temperature

drying procedure for mixed aspen and balsam

poplar studs involving a mid-process condi11 Presentation by P. Deverick, Fairchild Chair Co.,

Lenoir, N. C., to Southeastern Dry Kiln Club, Clyde, N.C.,

Nov. 1972.

tioning period. The total time was 4 days and

the studs were dry enough in the outer zones

to pass the 19 percent maximum moisture

content test using ordinary moisture meter

techniques. Follow-up research, however,

showed that wet spots still present continued

to dry, producing delayed shrinkage and collapse (Mackay 1976).

To solve this problem of delayed shrinkage,

several industries drying aspen in the Lake

States have high-temperature-dried for 2

days and then gone to an equalization setting

at lower temperatures (160° F dry-bulb, 140° F

wet-bulb temperature) until the required

moisture contents are obtained. This appears

to be a possible procedure for hardwoods that

dry readily at first but have persistent wet

spots.

The success of the experimental hightemperature drying of hardwoods suggests

that high-temperature drying has potential

for commercial use in the United States with

great benefits. Data under semi-commercial

conditions are needed, both to determine

maximum safe initial moisture content conditions to avoid degrade and to determine procedures acceptable for commercial operation.

Also, some method of determining the moisture content during drying must be developed. Without such a method, it is likely

that some overdrying or underdrying will result. There also is some need for research to

answer questions on color, strength, and

other aspects of quality for the various uses

to which hardwoods can be put.

Drying Times and Species Potentialities

Table 25 lists the U.S. hardwoods for which timates of drying time to 8 percent moisture

high-temperature information is available, content with minimum equalizing and conditheir potential for drying from the green or tioning, and at least one reference for each

the air-dried condition, some very rough es- wood.

-58-
Table 25 –Eastern hardwood species for which high-temperature kiln-drying potential

has been indicated, and estimated drying time for 1-inch lumber

Apparent Process Requirements

If high-temperature drying is to be applied

commercially, the process requirements are

somewhat different from those of softwoods.

These requirements should be kept in mind

when planning to install a high-temperature

kiln for softwoods now and for hardwoods

later on or when installing a hardwood kiln

for conventional drying now which eventually

may be converted to high-temperature drying.

A. Target dry-bulb temperature should be

rapidly attained. The usual target is 230° F

although higher temperatures have been

tried.

B. The initial heating medium would depend on the material being dried:

(1) Green—steam spray and heated air.

(2) Partly air-dried—undetermined at

present.

(3) Air-dried—heated air. Dry-bulb and

wet-bulb temperatures should be brought up

as close together as possible, with the wetbulb temperatures raised to 200° F on green

stock. To attain the high wet-bulb temperatures needed, vents, wall panels, and doors

must seal very well.

C. Air temperature should be uniform

when at high temperatures, with no area

below boiling, especially on the leaving air

side of the load.

D. Air velocity should be high through the

load—a minimum of approximately 600 feet

per minute with 4-foot-wide loads and of 900

feet per minute for wider loads. Research still

in progress is using velocity up to 2,000 feet

per minute.

E. Equalizing and conditioning, when

necessary, must be done below 212° F. In the

high-temperature drying of aspen studs in 96

hours (Mackay 1974), an 18-hour period of

high humidity at 204° F dry-bulb temperature

was used before the final 15 hours of drying at

250° F. The reader is referred to the chapter

on Conventional Kiln Drying for general

equalizing and conditioning procedures.

In view of these process requirements, special consideration must be given to the kiln.

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The kiln must be able to operate at both normal and high temperatures. Due to the high

heat and humidity, masonry structures may

have a short average life expectancy.

Masonry structures would have to be well insulated, and the interior wall and ceiling

would have to be well sealed. Cracks will have

to be repaired immediately. Presently available prefabricated kilns with insulated

aluminum-paneled walls and roofs are expected to have long service lives. All corners,

joints, floor sills, and door frames must be

well designed and properly installed to prevent leakage and undue heat loss. Particular

attention should be paid to having tight,

well-insulated roofs. Some thought should be

given to a lightweight aggregate to provide

more thermally efficient concrete floors.

There must be sufficient heating capacity

to rapidly attain target temperatures. Since

the kiln will require steam for humidification,

heat for major operations should be provided

by fin-type steam coils. In experimental

high-temperature drying of softwoods, extra

heat for the heating up period has been provided by a direct firing equipment. Steam

pressure should be 150 pounds per square

inch.

Air travel length should be 8 feet or less, or

16 feet or less in a double-track kiln with

booster coils midway between the loads. In

package-loaded kilns, 10 to 12 feet would be

the maximum air travel unless the kiln can be

loaded from both sides, with booster coils

along the middle. To obtain the high air velocities necessary, a direct-connected fan system

(rather than a line shaft) is required. Twospeed fan motor operation is desirable when

both normal and high-temperature drying

will be done, to save electrical energy. Commonly available in-kiln fans are designed to

operate safely up to 250° F. Special attention

must be paid to baffling.

Equal attention should be paid to the

humidification system. Many hightemperature softwood kilns are not equipped

with any humidification system at all, so they

are not suitable for hardwoods without adding this feature. A proper system should be

able to deliver large quantities of saturated

steam at low pressure. Steam at high pressure suitable for the heating system must go

through a desuperheater before being used

for humidification.

If a high-temperature kiln is to be designed

for both hardwoods and softwoods, provisions

can be made for opening the vents on just the

high-pressure side of the kiln. This helps to

maintain uniform drying conditions. Otherwise, the venting that is necessary when drying a very wet permeable wood will take place

through leaks, tending to cause the kiln to

deteriorate.

Literature Cited

-60-
CHAPTER 7

SPECIAL PREDRYING TREATMENTS

Special treatments may be used before or treatments. Under steaming, the postdrying

early in drying to accelerate drying rate, to treatment of collapsed woods also is dismodify color, or to prevent checks and other

defects. This chapter deals with steaming, cussed. Presurfacing was discussed earlier

chemical seasoning, polyethylene glycol under acceleration of conventional kiln dry-

(PEG) treatment, and surface and other ing.

Steaming

To Accelerate Drying

Early in the study of wood drying at the

U.S. Forest Products Laboratory, observations were made that steaming hardwoods before drying sometimes resulted in reduction

of drying time. Detrimental effects were

noted, however, when air-dried stock containing surface checks was steamed. The checks

deepened and widened and sometimes became bottleneck or honeycomb checks. The

steaming used at that time was relatively

long. The conclusion was drawn that such

steaming represented a delay during which

no drying occurred and general use of the

practice was stopped.

More recent studies in a number of schools

and laboratories have shown a number of

benefits. The permeability of both softwoods

and hardwoods is increased by short periods

of steaming. Moisture migration rates are increased significantly, and drying times are

reduced. The development of prefabricated

aluminum kilns has made possible the use of

steaming in commercial drying operations.

Simpson (1975) reviewed the most significant research results and investigated the

acceleration of drying small specimens of

wood. He tried several species of wood in the

green condition and steamed at 212° F. Drying rates were increased for northern red oak,

cherrybark oak, and sweetgum heartwood.

The drying rate at 50 percent moisture content for these small specimens increased 34 to

75 percent for the oaks and 11 to 36 percent

for sweetgum heartwood. Steaming times

were in the range of 1⁄2 to 5 hours. For sweetgum heartwood, the 5-hour period was best.

The drying rate of sweetgum sapwood was

slightly reduced by steaming.

In another study with 1-inch-thick oak,

Simpson (1976a) found that the moisture gradients, during drying after steaming 4 hours

at 212° F, were smooth curves. The natural

moisture gradients of the unsteamed controls

had inflections that indicated free water

movement was restricted. Simpson’s work

showed that free water migration from the

center toward the surface was enhanced by

steaming.

The above results were achieved with saturated steam at 212° F, a condition difficult to

obtain in commercial kilns. A larger scale

study used both green and partly air-dried

rough 4/4 northern red oak (Simpson 1976b).

The lumber was pretreated with nearly saturated steam at 185° F for 4 hours. Reductions

in drying time for both classes of lumber were

about 17 percent. No defects occurred in the

green lumber nor in one batch of the partly

air-dried material. The other partly air-dried

batch, however, had been severely surface

checked during air drying. The steaming appeared to deepen the surface checks and

change them into bottleneck or honeycomb

checks. The change to honeycomb checks

confirms the admonition not to use steam

spray during warmup of a kiln charge of fully

air-dried oak. Surface checks may be present

in such oak but not visible.

While the above studies are only exploratory, they give some prospect for practically

accelerating the drying of eastern hardwoods

by presteaming treatments. Additional

studies are needed to confirm benefits and

determine limitations, comparative energy

demands, and economics.

-61-
To Modify Color

Walnut 12

The most common use of presteaming

treatments is to modify the color of the sapwood of black walnut. Steaming darkens the

sapwood, toning down the contrast between it

and the rich brown-colored heartwood and

facilitating the uniform finishing of the wood.

It also improves the color of the heartwood,

making it and the sapwood more uniform.

There is some extraction of coloring matter

from the heartwood during the process. These

extractives do not penetrate the sapwood beneath the surface.

The best steaming results are achieved by

treating green lumber with wet steam in as

tight a structure as possible at temperatures

that give the most color in the least time.

12 This section has been prepared by K. C. compton,

U.S. Forest Service, State and Private Forestry, Madison, Wis.

Nonpressure steaming of walnut is done in

special vats or buildings with provisions for

wet steam from 180° to 215° F. Any structure

is suitable so long as it is made of materials

that will stand up under wet heat up to 215° F

and the acid corrosion of the wood extractives. Steaming times are 24 to 96 hours. Walnut should not be steamed in the dry kiln

because of the time required and the corrosive effects of steam and volatile extractives.

Three steaming vats or chambers are

shown in figures 16 to 18. In figure 16, the

floor is reinforced concrete and the doors are

typical dry-kiln type. The lumber is stacked

by forklift trucks on wooden bolsters. Low- to

moderate-pressure steam is introduced by

perforated steam pipes in the troughs, which

are filled with water. An alternative is to circulate steam in closed pipes submerged in the

water in the troughs. The steam traps, in this

case, discharge their condensate into the

troughs to keep them full. The capacity of a

Figure 16.—Steaming chamber constructed of concrete block and poured, reinforced concrete: A, Roof is poured

concrete or prestressed concrete sections; B, apron or loading ramp; C, trough for perforated or imperforated steam

pipes in water; and D, walls of concrete blocks or poured concrete.

-62-
M 130314

Figure 17.—Steaming vat of wood and sheet aluminum construction (Details courtesy of Hartzell Industries, Inc.,

Piqua, Ohio): A, Sheet aluminum lining, with joints sealed with asphaltic materials; B, roof sections of creosoted

frame and planks, lined with sheet aluminum; sections are placed by forklift truck as vat is filled; C, trough for

perforated steam pipes in water; D, slanted jamb causes door sections to fit securely by weight alone; E, creosoted

posts set in ground; and F, creosoted 2- by 8-inch plank wall.

M 130316

Figure 18.—Steaming chamber of poured concrete walls and roof (Details courtesy of Iowa-Missouri Walnut Co., St.

Joseph, Mo.): A, drain for condensate; B, steam pipe of 2-inch diameter with 1⁄4-inch perforations; C, concrete blocks

turned on sides and imbedded in concrete to hold lumber packages off of floor; and D, apron or loading ramp.

-63-
chamber of the dimensions shown is 40 to 50

thousand board feet (M bm) of lumber in

solid-pile packages.

In figure 17, the lumber is stacked in the vat

in the same manner as in figure 16. The sections of the vat roof are put in place by the

forklift as the vat is filled. The front door sections are the same construction as the roof

sections. Joints between sections are covered

with sawdust to reduce steam losses. The capacity of this vat is 16 M bm solid-piled

lumber packages.

The walls of the chamber in figure 18 are 10

inches thick. The permanent roof is made of

hollow, precast concrete sections covered

with insulation and built-up roofing. Dry-kiln

type insulated doors with double surfaces of

aluminum are used. Steam from a highpressure boiler is injected directly into the

chamber. The capacity of this unit also is

16,000 board feet.

In figure 17, all joints of the flashing-weight

aluminum sheets were sealed with a highmelt asphalt. Other materials of construction

are possible, including solid wood cribbing,

aluminum frame and sheathing, and tile. All

types of steam vats or chambers should be

protected by a coating of asphalt or one of the

materials supplied by dry kiln companies for

kiln coating. Some walnut steam chambers

have trowelled mortar-type coatings similar

to those used in patching boiler masonry. In

any event, iron and steel fittings should not

be exposed directly to the steam. Provision

should be made for gasketing or sealing doors

at tops, sides, and bottoms.

Wet steam is preferred for coloring black

walnut. Green walnut wood darkens more

rapidly and to a greater degree than dry walnut, and the more saturated the atmosphere,

the less chance for the wood to dry during

steaming. In some installations, however,

high-pressure steam, which is dry and has a

strong superheat, is injected directly into the

chamber. Apparently drying does not take

place fast enough to offset the rapid coloring

effect from the higher temperature. Drying

defects customarily do not develop. Under

this procedure, temperatures in the range of

215° to 225° F are attained.

Walnut steaming chambers preferably are

equipped with recording thermometers, and

some have temperature controllers also.

There are no fans in steaming chambers.

Steaming has also been done commercially

under pressure. Brauner and Conway (1964)

developed the optimum conditions experimentally. Then they settled on steaming at 6

pounds per square inch pressure and 230° F

for 5 hours. A longer time is needed in the

winter. This procedure not only darkens the

sapwood; the heartwood loses its purplish

cast and become chocolate brown. Although

the coloration is rapid and time saving, the

lumber must be cooled in the pressure vessel

or end checking and honeycombing occur. An

alternative is to take the load out of the retort

and cover the wood with a tarp until cool.

Millions of board feet of walnut have been

steamed at the Conway plant using a 7- by 8-

by 20-foot pressure vessel. At another location a suitable pressure steamer was made by

adapting a preservative treating cylinder

door to the end of an old tank car.

Generally, 25 to 35 boiler horsepower are

needed for a walnut steaming installation.

Steaming walnut may increase its drying

rate, but there is no conclusive proof of this at

this time.

Other Woods

European beech lumber and dimension

items are frequently steamed to give them a

reddish-brown tone that enhances their color

when finished appropriately. Beech steaming

is not generally done in the United States, but

presumably it could be, if desired. Steam has

been used, however, to give oak a gothic

brown color. Ordinary kiln-drying conditions

can be manipulated to give sugar maple a

reddish brown color (McMillen 1976) but presumably the same could be done by a short

steaming period followed by customary drying. Sweetgum sapwood was steamed before

air drying, at one time, to sterilize the wood,

promote rapid air drying, and avoid blue

staining. No record was made of color effects

from such steaming.

To Recover Collapsed Wood

Normal eastern hardwoods ordinarily do

not collapse badly during kiln drying. Some of

the western hardwoods do. There is some evidence, however, that excessive shrinkage in

some oaks involves collapse. Collapse is technically excessive shrinkage and distortion of

individual wood cells in various zones of the

-64-
wood. In a collapse-prone wood, the external at about 18 percent moisture content, steameffect of collapse is a “washboard-type” sur- ing them for 12 to 24 hours in a chamber like

face. that used for walnut, and then finishing the

In Australia, where collapse-prone species drying in a kiln with noncollapsed wood.

are common, recovery is accomplished by tak- Steaming is done with saturated steam at

ing the collapsed boards out of a kiln charge 212° F.

Chemical Seasoning

Chemical seasoning consists of treating

green wood with a hydroscopic chemical and

air drying or kiln drying the treated material.

The chemical reduces surface checking during seasoning, rather than speeding the drying.

The objective is to impregnate the outer

zone of lumber with chemicals to a depth of

about one-tenth of the thickness, with the

highest concentration at or near the surface.

The chemicals maintain the outer zone at a

high moisture content during early stages of

drying. This reduces the shrinkage of the

outer zone and lessens the tendency to surface check. Some chemicals additionally impart a certain degree of bulking. Kiln

schedules must be modified somewhat to

bring the initial relative humidity below the

relative humidity in equilibrium with the

saturated chemical solution.

Numerous chemicals have been used

(MCMillen 1960). Common salt is cheap and

effective in reducing surface checking. Urea,

which was effective with Douglas-fir, was not

as effective as salt on oak. Other chemicals

included invert sugar, molasses, diethylene

glycol, and a urea-formaldehyde mixture. A

proprietary salt mixture having corrosion inhibitors in it was popular for a time.

The proprietary chemical, as well as common salt, can corrode metals and damage

dry-kiln equipment, woodworking machinery,

and hardware fastened to the treated wood if

the amount of treating chemical is excessive

or the treatment time too long. Salt-treated

wood, regardless of care in treatment, will

corrode metals in contact with it in regions of

prolonged high humidity such as the Gulf of

Mexico and South Atlantic coasts. Salt also

can reduce the strength of wood and cause

problems in gluing and finishing. Although

salt has been and continues to be used successfully in the seasoning of thick southern

hardwoods, considerable care in treatment,

drying, and use is recommended.

Polyethylene Glycol Process

Green wood heavily treated with PEG

(polyethylene glycol-1000) retains its green

dimension during drying and indefinitely;

thus the wood is permanently restrained from

shrinking, swelling, or warping regardless of

atmospheric humidity. For maximum dimensional stability, PEG must be diffused deeply

into the wood in amounts of 25 to 30 percent of

the dry weight of the wood. Two solutions

commonly used are 30 and 50 percent PEG by

weight. Heating the solution during treatments speeds up the diffusion, but soaking

times range from 3 to 30 days. Kiln drying

after treatment can in many cases be much

more drastic than normal. The process is

especially helpful for hardwood tree and limb

cross sections, thick novelty items, and carvings and material with highly irregular grain.

Details of the process, which is relatively expensive, have been described by Mitchell

(1972).

-65-
Surface Treatments to Prevent Checking

In addition to presurfacing, there are certain materials that can be applied to the

wood’s surface to retard checking. Chemicals

used successfully are wax, sodium alginate,

and a salt paste. These materials either retard moisture movement or alter the vapor

pressure at the surface. No details are available on use of waxes, but a thick emulsion of

microcrystalline wax has been applied to the

sides of highly figured gunstocks in California

to prevent checking.

Sodium Alginate

In Australia, Harrison (1968) investigated

the use of very viscous sodium alginate solutions or emulsions as dip treatments on a variety of hardwoods up to 2 inches thick. When

the lumber is air dried, the alginate dries out

to form a porous skin over the surface. It is

effective in preventing checking in all of the

woods tried under severe air drying conditions. The method has not been tried in the

United States but might have some benefit

for thick oak.

Sodium alginate is a dry powder obtained

from seaweed and is used in a variety of products in the United States. Considerable care

must be used in mixing it to form the 11⁄2

percent solution found most effective in Australia. The wood must be still quite green for

the treatment to be effective in preventing

checking. The air-drying piles must be carefully roofed to keep the alginate from being

washed off by rain. For 2-inch stock, a drying

shed is necessary to protect the treating

chemical and provide mild conditions for air

drying.

Salt Paste

The U.S. Bicentennial celebration inspired

widespread interest in seasoning disks or

thick sections of large trees. The disks were

desired for exhibits or usable items on which

the chronology of important events could be

shown. It is very difficult to season such disks

successfully with even the least-difficult-todry species. The most damaging defect is the

large V-shaped check that is likely to develop

because tangential shrinkage is usually much

greater than radial shrinkage. In addition,

many small end checks tend to appear over

the entire surface. Polyethylene glycol has

promise, but is slow, costly, and the temptation always exists to terminate treatment before all wood cell walls are fully saturated

with PEG.

A simple and inexpensive method developed by W. T. Simpson and J. L. Tschernitz

of the U.S. Forest Products Laboratory is

successful enough to merit its inclusion in

this handbook. It is a thick paste modification

of the old “salt seasoning” method, combining

some bulking of the surface zone with retention of moisture at the disk surface by hydroscopic action. The bark should be left on

the disk. Before applying the paste, the surfaces of the green disk are alternatively

brushed with a concentrated table salt

solution—3 pounds salt per gallon of water,

with excess salt crystals visible in the solution after thorough mixing. Several hours are

allowed for the salt to penetrate by diffusion

and gravity.

Then the wood is treated with the paste. To

make the paste, the concentrated solution

above is mixed with enough cornstarch to get

the right consistency to build up a thick layer

on the disk surface. The addition of several

egg whites acts as a convenient binder to reduce the flaking of the paste after it dries. The

treated disks can be air dried in a room with

plenty of ventilation or kiln dried using a

moderate schedule.

Other Pretreatments

Precompression thickness compression of 7 to 8.5 percent in a

roller device resulted in greatly reduced colCech (1971) has established that dynamic lapse and honeycombing compared with untransverse compression of 2-inch yellow birch compressed material. Drying time was only 81/3

lumber, before drying by a severe schedule, days compared with a customary 18 days for

significantly improved drying behavior. The noncompressed material dried by convendrying was carried out at 215° F. Momentary tional schedules.

-66-
Chech and Pfaff (1975) also used a 7.5 percent

precom pression with both conventional and

mild accelerated kiln schedules on 4/4 red oak.

Precompression resulted in about a 5 percent

saving in drying time.

Precom pression has not been accepted

commerically, at least not on any large scale,

but experimental results to date appear to

merit more study.

Prefreezing

species for which decreased shrinkage has

been found is black tupelo (blackgum). The

best prefreezing temperature for black walnut is –100° F, but significant improvement is

found at –10° F, a temperature readily attained in commercial freezing equipment. A

freezing time of 24 hours is adequate for

thicknesses up to 3 inches. A similar length of

time has been used for thawing.

Although prefreezing would require substantial additional investment, a shortening

of drying time to half of the time required by

some of the industry has been demonstrated

for walnut gunstock blanks (Cooper, Bois, and

Erickson 1976). Using a slightly accelerated

kiln schedule, 600 prefrozen gunstock blanks

dried from green in 103 days had only 2.66

percent defective from collapse, checking, and

warp while 6.50 percent of the 400 unfrozen

blanks in the pilot test had similar defects.

Thus precompression and prefreezing are

two treatments that have some promise of

improving the drying rate of hardwoods without decreasing quality.

The freezing of green wood, followed by

thawing before the start of drying, is another

treatement that increases the drying rate and

decreases shrinkage and seasoning defects in

some species. Most research on hardwoods

has been done with black walnut (Cooper,

Erickson, and Haygreen 1970). The effect on

shrinkage appears to be a good indicator of

seasoning improvement, and favorable results have been attained with black cherry,

American elm, and white oak (Cooper and

Barham 1972). Another eastern hardwood

Literature Cited

-67-
CHAPTER 8

OTHER METHODS OF DRYING

Heated-Room Drying

In heated-room drying, a small amount of

heat is used to lower the relative humidity.

This also lowers the equilibrium moisture

content (E MC) to which the wood will come if

left in the room a long time. This method is

suitable only for wood that has been air dried

first. Green lumber may check and split by

this method. This method does not dry lumber

rapidly, but it is suitable for small amounts of

lumber.

Before air drying is started, the lumber

should be cut as close as possible to the size it

will have in the product. Allowance must be

made for some shrinkage and warping during

drying and for a small amount to be removed

during planing and machining. If it is necessary to shorten some long pieces of air-dried

lumber before heated-room drying is started,

the freshly cut ends should be end coated to

prevent end checks, splits, and honeycomb.

For reasonably fast heated-room drying,

the wood should be exposed to an EMC about

2 percent below the moisture content of use.

The wood is left in the room just long enough

to come to the desired average moisture content. Then the wood is taken out and stored in

a solid pile until it is used. The storage area

should have the same EMC as the area in

which the wood will be used.

The amount that the temperature must be

raised above the average outdoor temperature depends upon the average outdoor relative humidity. Typical values are given in

table 26. Do not attempt to use more heating

with this method.

Any ordinary room or shed can be used and

any ordinary means of heating the room

Table 26. —Amount temperature must be

raised above average outdoor temperature

for various equilibrium moisture content

values in heated-room drying

EMC value

desired

Degrees above average outdoor

temperature at—

70 pct RH 75 pct RH

Percent

4

5

6

7

8

9

10

°F

38

31

23

18

13

10

6

° F

40

33

25

20

15

12

8

80 pct RH

° F

42

35

27

23

17

14

10

should be satisfactory. A slight amount of air

circulation is desirable to achieve temperature uniformity. If the material is relatively

small-sized, it can be piled in small, stickered

piles on a strong floor. It also could be sticker

piled on carts that can be pushed in and out of

the room. Long lumber should be box piled on

strong, raised supports as shown in figure 19.

The lumber should be marked or records

kept as to when it entered the room and when

to expect to remove it. Any amount of air-dry

lumber can be put in or dried lumber removed

without upsetting conditions. The doors

should be kept closed as much as possible so

that the average temperature can be kept

close to the desired elevation above average

outdoor temperature. Variations of the average outdoor temperature from night to day

and small variations from day to day do not

affect the process.

Dehumidifier Drying

Considerable interest has been expressed in Research Laboratory (Pratt 1968) indicated

dehumidifier-type kilns developed in Eng- such dryers were not operated at temperaland, Norway, Germany, and Italy. These tures over 130° F. Drying of hardwoods to 7

dryers have been used successfully for percent moisture content requires use of

hardwoods in Europe where kiln schedules higher temperatures to be practical.

are generally conservative. In these loca- The dehumidifier dryer uses electrical

tions, the desired final moisture content refrigerator-type equipment in an arrangeusually is not below 9 percent. Technical in- ment favorable to the drying of wood and

formation from the British Forest Products other materials. In a closed building or room,

-68-
Figure 19.—Lumber piled

moist air from the lumber is drawn over refrigerated coils. The air is cooled below its

dew point and part of the moisture is condensed. The water flows to a tray in the base

of the unit and is drained out of the kiln. The

air circulates past the condenser unit of the

system and picks up the latent heat of vaporization. Then the air is blown by the fans

through the lumber. In some installations,

however, the dehumidifier is located outside

the drying chamber and the latent heat benefit is not realized.

The relative humidity generally is lower

than can be obtained at the same temperature in a conventional kiln vented to the air.

Electrical resistance heaters, hot water coils,

or steam coils can be used to augment the

heat from the condenser. Brief coil-heating

periods can be used to defrost the coils, when

necessary, and “interruptions” of drying can

be inserted in the schedule to minimize drying stresses.

M 144690

for drying in a heated room.

Advantages claimed by manufacturers, and

borne out to some extent by published data,

are:

Capital investment is relatively low.

Operating costs are not exorbitant.

Drying time to 12 percent moisture content is not unduly prolonged for woods

normally dried at low temperatures.

The equipment lends itself readily to automatic (time-based) programming so

the dryers can be operated by relatively

unskilled personnel with little danger

of overd rying.

Mixed species and thicknesses can be

dried, within certain limitations, in the

same dryer.

Quality of dried material, when proper

schedules are used, is better than that

of material air dried under somewhat

adverse conditions.

The amounts of shrinkage and warping

are low.

–69-
At the low temperatures generally used,

personnel can move in and out of the.

dryer to insert more material or remove

dried wood as needed.

Many of these advantages are also found in

heated-room drying, but heated-room drying

is best restricted to air-dried material. Dehumidifier dryers can be scheduled to safely

dry refractory woods from the green condition. In their 1976 stage of development they

apparently had no provision for conditioning

to relieve drying stresses. The relative economics of careful air drying and heated-room

economics of careful air drying and heatedroom drying compared with dehumidifier drying have not been investigated.

Solar Drying

The changed world fuel situation has renewed interest in use of solar energy to dry

lumber. Research in small experimental and

semi-commercial solar dryers showed that

hardwoods can be satisfactorily dried from

the green to the air-dried condition. Drying

times are about 50 to 75 percent of the airdrying times. Hardwoods can be dried further

to 10 percent moisture content, but the time

required is long compared with kiln drying.

Peck (1962), using a wood-framed rectangular dryer with double-layer transparent plastic film walls and roof in Madison, Wis., dried

4/4 northern red oak from 75 to 20 percent

moisture content in 23 to 105 days starting in

different months of the year. Drying times

from 20 to 10 percent MC were 25 days for a

period starting in July and 42 days starting in

September. A 24-inch fan driven by a

5/8-horsepower electric motor circulated the

air during daylight hours. The dryer roof,

which essentially was the major solar

energy-collecting surface, had 95 square feet

of surface; the south wall had 17 square feet.

The dryer held 425 board feet of lumber. Average temperatures within the dryer during

drying operations were 13° to 19° F above outdoor temperatures.

Additional research on solar drying of wood

has gone on around the world using equipment varying from very small cabinets in

India to semi-commercial units holding about

3,000 board feet in Uganda. Plumbtre (1973)

discussed his own research with the Ugandan

kilns and reviewed world literature on the

subject in a United Nations report.

Starting with a design supplied by the

Forest Products Laboratory, the Institute of

Tropical Forestry, Rio Piedras, Puerto Rico,

constructed a solar dryer initially holding

2,000 board feet of 1-inch lumber and later

expanded it to 3,000 board feet (Chudnoff,

Maldonado, and Goytia 1966). The woodframed building, 10 feet wide and 20 feet long,

was oriented east and west. The sloping roof

angled 16° from the horizontal, the south wail

was 93⁄4 feet high, and the north wall 13-1/3

feet. Originally the roof, ends, and south side

were covered with two layers of plastic film

separated by a 15/8 inch air space. The north

side, which contained the loading door, was

plywood. Due to early film deterioration, the

roof panels were replaced with single-layer

window glass.

Sixteen-inch fans circulated the air over a

black-painted heat absorber under the roof

and through the lumber. Vents, with louver

dampers, provided ventilation. The dampers

were kept closed during the early part of the

drying to prevent drying defects. Later in the

cycle the vents were opened. At the end of

drying they were closed again, and a waterspray was used to relieve drying stresses.

In this tropical location, 4/4 mahogany

lumber was dried from 50 to 20 percent moisture content in 9 days, 5/4 in 13 days, and 8/4

in 23 days. Drying from 20 to 12 percent took

9, 12, and 28 more days, respectively. Mixed

5/4 hardwoods dried from 60 to 20 percent

moisture content in 27 days and from 20 to 12

percent in 15 more days. Time of year made no

practical difference in drying time for either

the mahogany or the mixed hardwoods.

Johnson (1961) built a small homemade

solar kiln in southwestern Wisconsin to dry

small quantities of hardwood lumber for his

own use. Solar heat was collected by singlethickness windows on the south facing wall of

an A-frame structure. A ventilating slot was

built in the wall below the windows. A windpowered centrifugal blower drew air up over a

heat-absorbing metal sheet behind the glass

windows and then forced the air down

through the lumber edge-piled on a rack.

One-inch cherry and white oak were dried to

10 percent moisture content in 2 to 6 weeks.

Johnson has continued to dry lumber in a

dryer of slightly modified design (Bois 1977).

-70-
The lumber is piled flat. An electric motor is

used to drive the fan, but only when the temperature is above 85° F. The air is forced down

into a central flue, then horizontally through

the lumber pile. He estimates that it takes 400

hours of sunshine (70 “good” days) to dry

hardwood 4/4 lumber from green to 10 percent

moisture content.

While this method of drying is feasible for

small quantities of lumber for hobby use, drying times are affected greatly by season of the

year and the location. The relative suitability

for solar energy collection the year round is

only fair in Northeastern United States from

Maine to central Wisconsin, including New

England, New York, and Pennsylvania, and

only good in most of the rest of the Eastern

United States (Crowther 1976). In general, it

appears that heated-room drying is a more

practical way of drying small quantities of

hardwoods as long as fuel costs are no more

than double what they are now.

The experimental and semi-commercial

Press

Press drying (Hittmeier, Comstock, and

Harm 1968) is defined as the application of

heat to opposite faces of a board by heated

platens to evaporate moisture from the board.

Temperatures range from 250° to 450° F.

Thermal contact between the heated platens

and the board face is maintained by platen

pressure of 25 to 75 pounds per square inch.

During drying, heat is transferred from the

platens to the wood, causing air and vapor in

the wood to expand and water to vaporize. A

mixture of vapor and liquid then moves to the

surface of the board where it escapes. Ventilated cauls located above and below the board

sometimes have been used to help the vapor

escape. The platens also hold the board flat

solar dryers first described are essentially

flat-plate radiation collectors and their major

collecting surface is the roof. They are relatively inefficient (Wengert 1971). Thus the

amount of lumber that can be satisfactorily

dried in a dryer of this type is limited. Such

dryers do not appear practical for moderate to

large commercial hardwood operations in the

Eastern United States.

Solar kilns do, however, have current potential in developing tropical countries where

fuel costs are extremely high. Solar kiln design research is continuing with emphasis on

separate solar collectors and heat storage

units. Read, Choda, and Cooper (1974) describe a kiln built in Australia. The U.S.

Forest Products Laboratory is developing a

design for the Philippines. Rising fuel costs

and improvements in solar energy technology

may ultimately bring solar kilns into general

use for commercial hardwood drying in the

United States, but this appears to be something for the future.

Drying

and reduce width shrinkage; however, platen

pressure and high temperatures generally

lead to greater than normal shrinkage in

thickness. The heartwood of some species of

wood tends to darken and develop checks and

honeycombing during press drying. These defects, however, may not adversely affect the

board for many applications, and the darker

color is often more appealing than the original color.

The process is particularly suitable for

1⁄2-inch and thinner boards of species highly

permeable to moisture. It is being used for

beech for flooring in Europe and is being considered for a variety of purposes in the United

States.

High-Frequency and Microwave Heating

High-frequency heating involves both

technical and cost problems. In operation,

wood is placed in a powerful electric field oscillating at more than 1 million cycles per second and heated quickly to a temperature

above the boiling point of water. If there were

no resistance to the movement of free water

or vapor through a wood, it could be dried

rapidly. In general, however, wood is not very

permeable. Some species have fairly permeable sapwood, but others have a tight structure. Moisture movement may be so impeded

that high internal pressures and temperatures well above the boiling point of water are

built up. Local explosions or ruptures may

occur. In permeable woods, temperature

levels off slightly above the boiling point as

long as free water is present. When only

–71–
bound water remains, the temperature rises.

Prolonged high temperatures weaken wood

and lower its resistance to internal pressures.

Apart from technical difficulties, highfrequency heating is generally too expensive.

The main costs are for high-frequency

generators, power tube replacement, and

electricity. A minimum estimate of the electricity costs in drying 10,000 board feet of

green beech, birch, or maple sapwood from 70

percent to 8 percent moisture content in 24

hours follows:

To heat the lumber to drying temperature

and remove the calculated 764 pounds of

water per hour requires 263 kilowatts. Because a high-frequency generator is only

about 50 percent efficient and a certain

amount of heat would be lost, the power line

would have to deliver at least 600 kilowatts to

the generator. At 3 cents per kilowatt hour,

the cost would be $43.20 per 1,000 board feet of

lumber.

In view of the above, high-frequency dielectric heating is not practical for the general

drying of heartwood. It has been used for drying persimmon sapwood golf club heads. A

company in New England tried the method

for drying turning squares of birch and

maple, then abandoned it.

Although not for eastern hardwoods, microwave heating has been tried for the seasoning of tanoak baseball bats in Oregon. The

principles and constraints are similar to those

of high-frequency drying. Although the

tanoak was believed to be principally sapwood, degrade was high (20 to 30 pct) and

there was considerable collapse; thus the

method was abandoned.

Solvent Seasoning

The solvent seasoning process involves sub- While this method has not been applied to

jetting the wood to a spray or continuous im- eastern hardwoods, extensive research has

mersion of hot acetone, or a similar solvent been done in California on drying tanoak miscible with water, for a number of hours

until most of the water is extracted from the sapwood by this method. One-inch lumber has

wood. Then the solvent is removed bv steam- been dried in 30 hours. A few boards suffered

ing or vacuuming. Additional water is re- streaks of collapse, probably because of the

moved at the same time. presence of heartwood.

Minor Special Methods

Wood can be rapidly dried in an oily liquid

maintained at a temperature high enough to

boil off the water. A complicated variation of

the method is called azeotropic drying. Recent research (Eckelman and Galezewski

1970, and Huffman, Pfaff, and Shah 1972) has

demonstrated that the process does not have

good prospects for drying hardwoods unless a

considerably reduced pressure (partial vacuum) is used. Although the simple boiling-inoil process is not suitable for hardwoods

because of checking, a review of the details

(MCMillen 1961) would be valuable to anyone

considering such use.

Vapor drying exposes the wood to the vapors of a boiling organic chemical such as in a

pressure-treating cylinder and removed the

mixed chemical and water vapors (MCMillen

1961). This method has had considerable use

in drying oak, hickory, and other hardwood

railroad crossties before preservative treatment. It would also be suitable for crossing

plank and car decking, but is not suitable for

hardwoods to be used at low moisture content

values for fine purposes.

Infrared radiation has been proposed in the

past for drying wood, but the depth of ray

penetration is slight and the method is not

suitable for drying hardwood lumber. Vacuum drying and freeze-vacuum drying are of

no practical value for hardwoods because any

heat available is quickly used up in evaporation. Patents have been granted on combining

heated-platen or high-frequency heating with

vacuum drying, but have not been commercially applied in the United States.

-72-
Literature Cited

-73-
CHAPTER 9

STORAGE OF DRIED LUMBER

Lumber dried to a moisture content of 10

percent or less, and items manufactured from

it, will regain moisture if stored for extended

periods under conditions of high relative

humidity. Excessive regain of moisture frequently results in (1) swelling of whole pieces

or of certain parts, such as the ends of the

Air-Dried

When lumber is as dry as can be expected

on the air-drying yard and at least below 20

percent moisture content, it should be stored

where it is no longer exposed to wind, sun,

and rain. T’he deteriorating effect of weathering continues as long as lumber is left outdoors. When stickered lumber is taken down

and solid stacked, it takes up less space and a

shed can provide the needed protection. When

yard space is needed for air drying more

lumber, and no adequate shed is available,

less favorable drying yard areas may be set

aside for bulk storage of fully air-dried

lumber. Such yard-stored lumber must be

fully protected, however.

If good, rain-tight pile roofs with adequate

projections were used during air drying or if

the lumber was air dried in a shed and the air

drying space is not needed, the lumber does

not need to be solid piled. The lumber will not

go above 20 percent moisture content again

unless it is rewet by rain. Dry, stickered

lumber should be inspected regularly, however, to avoid insect attack in areas of infestation.

Kiln-Dried

After lumber has been kiln dried, it is often

placed in a storage shed or room, unstickered,

until ready for use. The ends of the boards, if

not end coated, will start to pick up moisture

immediately. If storage periods are short (up

to 1 month) and the storage building is kept

pieces; (2) warping of items, glued-up panels

for example; and (3) wood or glueline failures

in solid-piled items where the moisture regain

is confined to the ends. Hence, improper storage can negate many achievements of proper

drying.

Lumber

Dry lumber that is solid piled outdoors

should be wrapped with tarpaulins or enclosed in prefabricated waterproof-paper

shrouds. Rain wetting of any dried material is

detrimental and tends to increase the degree

of checking. Surface layers can take on a

compression set that will cause any checks to

stay open when the wood is finally kiln dried.

If the amount of rain wetting is great enough,

the moisture content can go above 20 percent

and the wood again would be subject to stain

and decay. Rain that penetrates into a solid

pile does not evaporate easily.

Foundations in the storage area should

provide good support and ground clearance

just as in the original air-drying pile. Support

is still essential to prevent pile sagging.

Ground clearance is not only needed to provide room for forklift operation but also for

ventilation to keep moisture regain in the

bottom layers to a minimum. If the water

table in the ground is high, a ground cover to

provide a barrier to moisture vapor movement out of the soil may be desirable, particularly if the storage period is extended.

Lumber

(EMC) condition. In some arid or semiarid regions of the United States, such as Arizona,

New Mexico, and southern California, unheated dry storage will hold dried lumber at a

low EMC. However, in the rest of the country,

unheated dry storage will not prevent kilnclosed most of the time, little change in mois- dried lumber from gradually increasing in

ture content will occur. However, in many moisture content.

cases dry lumber is stored for several months The major effects of different wood storage

or longer in buildings not designed to main- conditions on kiln-dried wood are discussed

tain a fixed equilibrium moisture content below.

-74-
Open Sheds

An open shed that consists of a roof with no

side or end walls, or a shed open on one or

more sides, can be a dry storage shed for airdried lumber awaiting sale, kiln drying, or use

in the air-dried condition. However, kiln-dried

and well-conditioned lumber destined for furniture or other high-grade use will deteriorate in moisture quality if stored in such a

shed.

Unheated Closed Sheds

Kiln-dried lumber stored in an unheated

closed shed will ordinarily absorb some moisture. Moisture is absorbed because the moisture content of the wood is usually lower than

the EMC corresponding to the atmosphere

within the shed. Outside weather conditions

heavily affect moisture absorption by lumber

stored in an unheated shed. While appreciable moisture gains in such sheds cannot be

tolerated for uses such as furniture, this type

of storage is being successfully used in some

areas for less demanding end uses. If storage

is in an unheated shed, use the lumber

promptly.

The unheated shed should be located on a

dry, well-drained site with a concrete or asphalt floor if possible. Also, the shed should be

ventilated by adjustable openings in the roof

and walls. Never use an earthen floor if the

shed stands on a low, damp site. Shed doors

should be closed at night and as much as possible on rainy or foggy days.

Heated Sheds

The efficiency of a closed shed in maintaining a low moisture content in lumber is enhanced if the air can be heated as required to

maintain the desired EMC. Only a small

amount of heat is needed to raise the temperature enough above the average outdoor

temperature to keep the EMC in the 6 to 11

percent range. The following temperature

crease values are typical:

Amount above

Desired EMC outside temperature

(Percent) (° F)

6 25

7 20

8 15

9 12

10 8

11 5

inHeat can be supplied to a closed storage

shed by steam coils, radiators, or unit heaters. The system need not have a large capacity, but it should be arranged so that the

temperature throughout the shed is reasonably uniform. A minimum temperature of

35° F is suggested for a steam-heated storage

area, to prevent the return lines and traps

from freezing. The heat supply may be controlled by an ordinary thermostat or by a

more precise automatic device.

To use a thermostat, estimate or employ

Weather Service data to obtain the expected

mean outdoor temperature for the next 4 to 7

days. Then set the thermostat the number of

degrees higher that is needed to maintain the

desired EMC. Good air circulation is needed to

maintain temperature and EMC uniformity.

This requires the use of fans.

If the heat is to be controlled automatically,

either a hygrostat or a differential thermostat may be used. A hygrostat maintains a

given EMC by turning the heat on or off as

needed. As the relative humidity in the shed

increases, the hydroscopic element absorbs

moisture and swells. The swelling activates a

mechanism that turns on the heat. When the

relative humidity decreases, the process is

reversed.

Differential thermostats can be preset to

maintain temperature in the shed that is a

specific amount above the outside temperature. The differential thermostat is very dependable, economical, easily installed, and

the maintenance cost is low.

Dehumidified Rooms

Electric dehumidifiers can be used in small,

well-enclosed spaces. Some temperature control should be maintained, but it does not

have to be precise. EMC tables should be used

to select the proper relative humidity for the

desired average moisture content. The dehumidifier can be turned on and off by a

properly calibrated humidistat. Dehumidifiers are not as economical as heated

storage sheds for large quantities of lumber,

but large storage sheds have been dehumidified for corrosion control on precise

metal parts during war time.

-75-
CHAPTER 10

ECONOMICS AND ENERGY

The cost of drying hardwoods normally includes the cost of stacking, handling, air drying, and kiln drying. Degrade cost should be

assigned to each of the drying phases, but

often these costs are overlooked.

As wood drying involves the evaporation of

large quantities of water, energy can be a

very important element of total cost. If fuel is

to be conserved, drying procedures have

other implications besides cost. Traditionally,

the lowest cost and the lowest energy usage

in hardwood lumber production has been

achieved by combining air drying and kiln

drying. Accelerated-air drying is an attractive alternative to air drying when production time is at a premium, when the good

air-drying season is short, when good land for

an air-drying site is not available, or when

close inventory control is necessary. The current high cost of energy makes these alternative approaches worth considering.

The information in this chapter cannot

serve as a complete analysis of the entire cost

and energy situation, but it is intended to

provide guidance for those who wish to make

a current analysis of their own operation.

Professional accountants and engineers can

provide a more thorough analysis. State

forestry personnel and extension offices are

occasionally equipped to provide assistance.

Typical Costs for Various Drying Procedures

Comparative air drying and low temperature forced-air drying costs were given by

Catterick (1970) for northeastern hardwoods,

based on a mill with a production of 2 million

board feet per year. These values have been

augmented and updated to reflect January

1976 costs and have incorporated data of

Cuppett and Craft (1971). Kiln drying costs, in

custom drying, as of 1970 also have been

augmented and updated to January 1976.

These updated comparative costs are as follows:

Air-Drying Costs

($ per M bm 13)

Average drying time

3 months 6 months

Land, taxes,

maintenance, $1.05 (16 $1.20 (11

etc. pct) pct)

Interest (8

pct) on 4.00 (63 8.00 (74

inventory pct) pct)

Taxes on .35 (5 .65 (6

lumber pct) pct)

Insurance 1.00 (16 1.00 (9

pct) pct)

TOTAL $6.40 per $10.85 per

M bm M bm

Low-Temperature-Drying Cost

($ per M bm)

Average drying time

10 days 20 days

Land, taxes,

maintenance,

building, $3.50 (21 $4.50 (23

boiler pct) pct)

Interest (8

pct) on inventory includes

10-day .90 (6 1.30 (7

storage) pct) pct)

Taxes on .05 (0 .10 (1

lumber pct) pct)

Insurance .10 (1 .15 (1

pct) pct)

Fuel and 11.00 (67 13.00 (66

electricity pct) pct)

Labor .75 (5 .75 (4

pct) pct)

TOTAL $16.30 per $19.80 per

M bm M bm

These low-temperature costs apply to

winter-time operation in north to central locations. Fuel costs would be lower during

summer operation.

13 Thousand board feet.

-76-
Kiln-Drying Cost 14

($ per M bm)

Kiln drying time

5-7 days 10-15 days

Equipment depre- $1.40 (6 $2.80 (8

ciation pct) pct)

Insurance .80 (3 .80 (2

pct) pct)

Maintenance 1.95 (8 1.95 (6

pct) pct)

Supplies .70 (3 .70 (2

pct) pct)

Sticker re- .50 (2 .50 (1

placement pct) pct)

Steam and 8.40 (35 16.80 (48

electricity pct) pct)

Labor 7.95 (33 7.95 (23

pct) pct)

Land and 1.30 (5 2.00 (6

building pct) pct)

Interest (8

pct) on inventory (includes

10 day .80 (3 1.50 (4

storage) pct) pct)

TOTAL $23.80 per $35.00 per

M bm M bm

14 Presentation by E. M. Conway, Conway Corp., Grand

Rapids, Mich., to Joint Meeting Midwest and

Wisconsin-Michigan Wood Seasoning Association, Antigo, Wis., May 1970.

General company selling and administrative expenses and degrade costs are not included in the above data. Degrade costs are

discussed in a following section.

Handling and stacking costs are often included with drying costs, but they are not

included in the estimates above. Conway has

estimated handling and stacking costs in a

hardwood air-drying and kiln-drying operation.

Handling Cost

($ per M bm)

Labor $14.10 (76 pct)

Equipment depreciation 1.50 (8 pct)

Equipment maintenance 1.25 (7 pct)

Land and building .70 (4 pct)

Insurance .40 (2 pct)

Miscellaneous .50 (3 pct)

TOTAL $18.45 per M bm

It should be emphasized that drying costs

and energy usage can be controlled, however,

by following the technical information and

suggestions listed in the various previous sections of this handbook and in the Dry Kiln

Operator’s Manual and Air Drying of

Lumber. Usually, the biggest cost in energy

savings can be made by air-drying rather

than kiln drying green lumber.

Degrade Costs and Causes

Not included in the previous costs is degrade, and degrade can be the biggest cost of

all, especially in air drying. Cuppett (1966)

studied degrade in Appalachian air drying

yards. He reported an average loss, adjusted

to 1976, of over $16 per 1,000 board feet, although at some mills the losses were controlled to less than $3 per 1,000 board feet.

Hanks and Peirsol (1975) estimated air drying

degrade losses for 10 hardwoods dried under

good conditions. Degrade values ranged from

0 to 5 percent of the lumber value. A recent

study in Pennsylvania (Bean and Spoerke,

1973) has shown that, under careful air drying procedures and terminating the drying

when the wood was at 20 percent moisture

content, degrade for 4/4 oak was between 1

and 2 percent of the lumber value and for 8/4

oak, 3 to 4 percent. With proper procedures

and attention degrade can be controlled and

almost eliminated.

In all of these studies, checks were the most

common defect, followed by splits, and then

warp. Cuppett (1965) has analyzed the major

causes of degrade with the first listing the

most common cause.

1.

2.

3.

4.

1.

2.

3.

1.

2.

3.

4.

5.

1.

2.

3.

4.

5.

6.

Checks

Lack of roofing.

Board edges exposed at bunk spaces.

Stickers not flush with ends of boards.

Too rapid drying due to excessive exposure of lumber

stacks.

splits

Too few stickers.

Lack of roofing or poor roofing.

Stickers not flush with ends of boards.

Warp

Poor sticker alinement.

Poor bunk alinement.

Lack of sufficient stickers.

Foundation out of level.

Thick and thin lumber in same course in stack.

Stain

No chemical dip.

Use of green stickers.

Use of wide stickers.

Base of piles too low.

Grass and weeds growing between stacks.

Poor yard orientation or location.

-77-
Often surface checks are not noted until

after the lumber has been air dried, kiln

dried, and undergone some manufacturing.

For a company trying to identify the cause

(did it occur in air drying or kiln drying?) a

close examination of the checks with a 10 ×

magnifying glass may show the checks to be

full of dirt particles. Such evidence is good but

not a completely reliable indication that the

checks were opened during air drying. Clean

checks are almost always attributed to kilndrying difficulties, usually too dry initial conditions. Although the surface checks in

hardwoods normally close as drying is completed, exposure to very high relative

humidities, saturated steam, or water will

cause the checks to stay open after redrying.

Dry wood that is wetted also may check as it

dries again.

A certain amount of drying degrade is unavoidable and can be attributed to inherent

characteristics of the wood itself. Cuppett

(1975) has established rough guidelines on the

minimum amount of degrade for several

hardwoods in the Appalachian region:

Species

------------------------------------------------------------------------------------ -

White and red oak

White and red oak

Poplar, basswood,

and cucumber

Poplar, basswood,

and cucumber

Maple and beech

Maple and beech

Thickness

4/4 and 5/4

6/4 and 8/4

4/4 and 5/4

6/4 and 8/4

4/4 and 5/4

6/4 and 8/4

If drying degrade exceeds allowable limits,

chapter 9 in the Dry Kiln Operator’s Manual

and chapter 6 in Air Drying of Lumber discuss the basic causes and remedies for drying

defects.

Two recent reports have looked at drying

costs in detail. Engalichev and Eddy (1970)

have prepared a computer program to evaluate the cash flow situation for purchasing a

new kiln, a low-temperature kiln, or a forcedair dryer. Goulet and Ouimet (1968) have

assembled a cost accounting approach for calculating the cost of drying-air, acceleratedair, kiln, or various combinations of these

methods. Goulet and Ouimet’s approach has

been adapted and is included in this section,

in order to permit an operator to assess drying costs for his own operation. If, in working

with this method, more wood is air dried annually than is kiln dried (or vice versa), the

analysis must be run twice, once for air drying and once for kiln drying. Note that in

using this general approach for specific cases,

not all items will have charges or costs. Also

note that in the example many charges will be

only a few pennies per thousand board feet,

and therefore it is not necessary to establish

these minor charges with a great deal of accuracy. Degrade, labor, and energy are major

expenses and care should be exercised to determine these costs accurately.

To use the method presented here, the

numerical values for the 66 items must be

ascertained and inserted in the blanks of the

Allowable

losses in percent of airdry lumber

value

1.0

2.0

.5

1.0

2.0

3.0

A Cost Accounting Approach

section, “Basis for Calculations.” Then these

numbers are added, multiplied, and divided as

shown in the section headed “Calculations” in

order to obtain the charges per thousand

board feet.

Method

Basis for Calculations

A. Direct investments (total costs)

Buildings, sheds, etc.

Kiln, auxiliary equipment

(including boiler)

Stickers

Pile roofs

Pile bases, bolsters

Total direct investment

(lines 1 thru 5)

B. Amortization period for direct

investments (years)

Buildings, sheds, etc.

Kiln, auxiliary equipment

Stickers

Pile roofs

Pile bases, bolsters

C. Quantity of wood dried annually

(M bm)

(If all air-dried wood is not

kiln-dried, two entirely

separate calculations should

be made)

D. Annual interest rate (percent as

decimal)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

-78-
E.

F.

G.

H.

I.

J.

K.

L.

M.

N.

O.

Drying yard investments (Total P .

Q.

R.

S.

T.

U.

V.

W.

X.

Y.

Z.

AA

Forklift time (M bm/hr)

Hourly forklift cost (include

machine cost, labor, and fringe

benefits) ($/hr)

(52)

costs)

Storage sheds for stickers,

dried lumber, etc.

(should not duplicate item

No. 1)

Permanent road construction,

rail access, etc.

Temporary road construction

(includes drying alleys)

Fences

Lighting systems

Drainage systems

Sprinkler systems

Total (lines 14 thru 20)

Amortization period for drying

yard investments

Storage sheds for stickers, dried

lumber, etc.

Permanent road construction,

rail access, etc.

Temporary road construction

(includes drying alleys)

Fences

Lighting systems

Drainage systems

Sprinkler systems

Maintenance and repair of yard,

percent of line 21 ($/yr)

Plus snow removal ($/yr)

Plus yard cleaning ($/yr)

Plus annual lighting expense

(bulbs and electricity) ($/yr)

Land area (ft2

)

Air drying area (include space

between the piles)

Road area (refers to line 15,

roads)

Area for buildings, kiln, boiler,

etc.

Total (lines 33 thru 35)

Land value ($/ft2

)

Taxable values ($)

Land (line 36 times line 37)

Buildings (line 1)

Kilns, equipment (line 2)

Fences, lighting, drainage,

sprinklers (line 17 + 18 + 19 +

20)

Total (lines 38 thru 41)

Insurable values ($)

Direct investments (line 6)

Storage sheds (line 14)

Fences (line 17)

Wood (line 62 times line 64)

Total (lines 43 thru 46)

Tax rate to be applied to line 42

(percent as decimal)

Insurance rate to be applied to

line 47 (percent as decimal)

Stacking time (M bm/hr)

Hourly stacking cost (include

machinery, labor, and fringe

benefits ($/hr)

(53)

(54)

(14)

Time spent each day observing

and running kilns, boilers, etc.

(hr)

Hourly wage rate and fringe

benefits for kiln operator and

(15)

(16)

(17)

(18)

(19)

(20)

(21)

auxiliary equipment including

boiler ($/hr)

Maintenance of kilns and boiler, a

percentage of line 2 (in dollars)

Annual office costs attributed to

drying (in dollars)

Energy costs (boiler costs should

be included in lines 2 and 56, not

here)

Annual fuel consumption

(gal. or 1,000 ft3

)

Fuel cost ($/gal. or 1/1,000 ft3

)

(55)

(56)

(57)

(22)

(58)

(23) (59)

Annual electrical usage

attributed to drying (kWh)

Electrical cost ($/kWh)

Average price of lumber ($/M bm)

Average drying degrade, based

on lumber value (percent as

decimal)

Average daily volume of lumber

on yard and in kilns on any

given day (M bm)

Total capacity of kilns

Average length of kiln run

(24)

(25)

(26)

(27)

(28)

(60)

(61)

(62)

(63)

(29)

(30)

(31) (64)

(65)

(32)

(include loading and unloading

time) (66)

(33)

Calculations

Values are computed in dollars per 1,000

board feet (numbers are line numbers from

the previous section):

(34)

(35)

(36)

(37)

(38)

(39)

(40)

(41)

(42)

(43)

(44)

(45)

(46)

(47)

(48)

(49)

(50)

(51)

-79-
Total of all = ($/M bm)

Example

The following represents a hypothetical

hardwood air and kiln drying operation in a

northern climate and should provide

guidelines for the expected data necessary to

perform the calculations.

Data

Calculations $M bm

TOTAL

-80-
Energy Considerations

Energy usage in kiln drying is significant.

The energy used in the kiln can be reduced by

full air drying and by efficient kiln operation.

The energy saving by air drying is illustrated

in table 27 where generalized energy usage is

given.

A recent Maine extension bulletin (Shottafer and Shuler, 1974) presented a method of

estimating the individual components of

energy usage in kiln drying. In the example

presented in this extension bulletin, energy

usage is broken down for a 25,000 board-foot

charge of 4/4 sugar maple, drying from 70 to

10 percent moisture content in 14 days in a

prefabricated, aluminum, insulated kiln.

Energy Use Million Btu’s per

25,000 board feet

Heating wood and residual water 4.3

Water evaporation 49.8

Venting 18.0

Building Heat Losses 35.7

(The building heat losses, in turn, can be broken down

into four components: through the walls, 29 ‘pct; through

the roof, 7 pct; through the door, 12 pct; and through the

floor, 52 pct.)

The energy used for heating the wood and

for evaporating the water, in the above listing, cannot be reduced (except through air

drying; air drying will actually reduce energy

use in all four items). In a properly operating

and adjusted kiln, vent losses can be reduced

somewhat, but not greatly. However, savings

can be made in building losses—for example,

the floor can be insulated or the kiln can be

enclosed in another building (but vented to

the outside). With the enclosure, the heat lost

through the kiln walls can be utilized to heat

the building that encloses the kiln.

Kiln wall insulation can also be increased to

reduce wall losses. If the insulation and wall

thickness is increased from 2 to 4 inches in an

aluminum prefab-type wall, the savings in

the above example would be about 5 million

Btu’s per 25,000 board feet, or approximately

$250 annually. However, thicker walls and

added insulation would add several thousand

dollars to the building’s cost.

The following listing (Wengert, 1974) should

be helpful in increasing the efficient use of

energy during kiln drying.

1. Use as much air drying or forced-air drying as possible—preferably drying to 25 percent moisture content or less.

2. Do not use steam spray or water spray in

the kiln except during the conditioning. Let

the moisture coming out of the wood build up

the humidity to the desired level. Steam may

have to be used, however, when very small

wet-bulb depressions are required. (Dry Kiln

Operator’s Manual, p. 160-162.)

Table 27. -Energy consumption and cost estimates in kiln drying hardwood lumber 1,2

Total energy cost 5 for kiln drying 4

Energy to Total energy used

From kiln dry per M bm 4 Per M bm Per 30 M bm kiln charge

per M bm 3

Good Intermediate Good Intermediate Good Intermediate

Million Million Million

Btu Btu Btu Dollars Dollars Dollars Dollars

Air-dried

(22-7=15 pct) 0.518 1.294 2.587 1.90 3.80 58 115

Partly air-dried

(37-7=30 pct) 1.035 2.587 5.175 3.80 7.60 115

Green

228

(47–7=40 pct) 1.380 3.450 6.900 5.05 10.10 153 305

1 Assumptions: Av. sp. gr. of wood—0.55

water/1 pct MC/M bm of rough, 4/4 lumber—33.95 lb

Energy (Btu) to evaporate 1 pct MC/M bm—34,561

2 Does not include electrical energy for fans.

3 100 pct efficiency of kiln.

4 Kiln efficiencies of 40 pct for “Good” and 20 pct for “Intermediate;” if 50 pct efficiency is maximum possible in

well-insulated, virtually perfect condition, steam-heated kiln, these estimates seem practical and are borne out

generally by industry experience.

5 Rate of $1.50 per million Btu.

-81-
3. Repair and talk all leaks, cracks, and

holes in the kiln structure and doors to prevent unnecessary venting and loss of heat.

Make sure the doors close tightly, especially

at the top. Temporarily plug any leaks around

the doors with rags, and order new gaskets,

shimming strips, or hangers if necessary. In a

track kiln, use sawdust-filled burlap bags to

plug leaks around tracks. Adjust and repair

the vents so that, when they are closed, they

close tightly.

4. For brick or cinder block kilns, maintain

the moisture vapor-resistant kiln coating in

the best possible condition. This will prevent

the walls and the roofs from absorbing water.

Dry walls conduct less heat to the outside.

5. For outdoor aluminum kilns only, paint

the exterior walls and roof a dark color to

increase the wall temperature by solar heat

and reduce heat loss from the kiln. Check to

insure that weep holes are open, not plugged.

(Painting would be disastrous on permeable

walls like brick or cinder block.)

6. In many kilns, more heat is lost through

the roof than through the walls. Much of this

loss is due to wet insulation. To reduce heat

losses, consider installing a new roof or repairing an old one. Add additional insulation

if necessary. Make sure the interior vapor

barrier or coating is intact (see suggestion 4).

7. Install or repair baffling to obtain a high,

uniform air velocity through the lumber and

prevent short circuiting the air travel. This

pays off in saving energy. Reverse air circulation only every 6 hours.

8. Research has shown that in the early

stages of drying, high air velocities (more

than 600 ft/min) can accelerate drying. In the

late stages, low velocities (250 ft/min) are as

effective as high velocities and use less

energy. Therefore, arrange to adjust fan

speeds if possible during a run.

9. Have the recorder-controller calibrated

and checked for efficient operation. The kiln

should not oscillate between periods of venting and steam spraying and should not vent

and steam at the same time. (Dry Kiln

Operator’s Manual, p. 67-73.)

10. Check the remainder of the equipment.

Are traps working? Do traps eject mostly hot

water with little, if any, steam? Do valves

close tightly? Are heating coils free of debris?

Is valve packing tight? Is there adequate

water for the wet bulb?

11. Accurately determine the moisture

content of the wood you are drying. Do not

waste energy by overdrying or by taking too

long because your samples do not represent

the load. Try to plan your loads so that, when

they are sufficiently dry, someone will be

available to shut off the kiln (and, if possible,

to unload it, reload it, and start it again). Do

not allow a kiln load of dry lumber to continue

to run overnight or through a weekend.

12. Unload and reload the kiln as fast as

possible. Avoid doing this until the air temperature has warmed up from the morning

low—do not cool the kiln unnecessarily.

13. In a battery of adjacent kilns, avoid

having one kiln being unloaded or loaded

while the adjacent kiln is at 180° F or other

high temperature.

14. During nonuse periods, close all valves

tightly and keep kiln doors closed. Use a small

amount of heat, if necessary, to prevent freezing of streamlines and waterlines.

15. Use accelerated schedules where possible. Check the chapter on Conventional Kiln

Drying for accelerating schedules with

minimum risk. The higher the temperature

for drying, the more efficiently energy is used.

16. If possible, reduce the length of time

used for conditioning; some low-density

hardwoods can be conditioned in 6 hours.

17. Finally, check with the manufacturer of

your equipment and find out if you can lower

steam pressures or reduce gas or oil flow rates

during periods of constant dry-bulb temperature. Also have the manufacturer check the

burner for top efficiency.

–82–
Literature Cited

-83-
APPENDIX A—SPECIES, THICKNESSES, DRYING SCHEDULES,

AND COMMENTS

Common name Thickness Kiln schedule1 Conservative Other

schedule 2

Apple 4/4, 5/4, 6/4 T6-C3 No

8/4 T3-C3 No

Ash, black 4/4, 5/4, 6/4 T8-D4 Yes

8/4 T5-D3 Yes

Ash: green, white 4/4, 5/4, 6/4 T8-B4 Yes

8/4

Aspen 4/4, 5/4, 6/4

Basswood

Beech

Birch, paper

Birch, yellow

Buckeye, yellow

Butternut

Cherry, black

Chestnut

Cottonwood: norm al

wet streak

Dogwood

Elm: American,

slippery

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

1-in. squares

2-in. squares

4/4, 5/4, 6/4

8/4

1-in. squares

2-in. squares

4/4, 5/4, 6/4

8/4

1-in. squares

2-in. squares

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 614

8/4

Shuttles

4/4, 5/4, 6/4

T5-B3

T12-E7

T10-E6

T12-E7

T10-E6

T8-C2

T5-C1

T8-C3

T5-C2

T10-C4

T8-C3

T10-C6

T8-C4

T8-C4

T5-C3

T8-C5

T5-C4

T10-F4

T8-F3

T10-E4

T8-E3

T8-B4

T5-B3

T10-E4

T8-F4

T10-F5

T8-F4

T8-D5

T6-C4

T6-C3

T3-C2

T3-B2

T6-D4

Yes

No

No

Yes

Yes

No

No

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

No

Yes

Yes

Yes

Yes

—

—

See appendix B

for mixed green

and black.

—

Possible acceleration for 4/4,

5/4 green ash is

T8-C4. See

appendix B.

Use for No. 1 Common and Better

and for crating

lumber; middle

grades—see

appendix C.

See appendix C.

Use T9–E7 for white

color.

—

See special report;

degrade control

important.

Do.

Do.

Do.

—

—

Use T5–C6 for white

color.

Use T5–C4 for white

color.

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

American elm,

soft type, use

T8-D4; air dry

hard type first,

then T2–D5.

-84-
APPENDIX A–SPECIES, THICKNESSES, DRYING SCHEDULES,

AND COMMENTS-Cont.

Common name Thickness Kiln schedule 1 Conservative Other

schedule 2

8/4

Elm: rock, cedar, winged

Hackberry

Hickory

T5-D3

T6-B3

T6-C3

T8-C4

T6-C3

T8-D3

Yes

Yes

Yes

Yes

Yes

Yes

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

414, 5/4

6/4 —

—

—

—

—

—

See table 17,

Dry Kiln Operator’s Manual.

Do.

Do.

—

—

—

—

—

—

For cucumbertree,

use yellow-poplar schedule,

Use T1-C5 for

whiter color or

see appendix C.

8/4

Handle stock

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

— —

—

T6-D4

T4-C3

T6-B3

T3-B1

T6-A3

T3-A1

T10-D4

T8-D3

—

Yes

Yes

No

No

No

No

Yes

Holly

Hophornbeam

(ironwood)

Locust, black

Magnolia

Maple, sugar

(hard)

4/4, 5/4, 6/4 T8-C3 Yes

8/4

1-in. squares

2-in. squares

4/4, 5/4

6/4, 8/4

T5-C2

T8-C4

T5-C3

T2-C1

Yes

Yes

Yes

Yes

—

—

See appendix B.

Air dry to 20 pct

MC.

See appendix B.

Oak, lowland

southern (red

and white)

Oak, upland

red

Oak, upland

white

Osage-orange

4/4, 5/4

6/4, 8/4

4/4, 5/4

6/4, 8/4

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

T4-D2

T3-D1

T4-C2

T3-C1

T6-A2

T3-A1

T8-D3

Yes

Yes

Yes

Yes

No

No

Yes

See appendix B.

—

—

Bitter pecan should

be air dried

first.

—

—

—

—

—

—

For sap gum lumber

containing large

amounts of heartwood, use red gum

schedules.

—

—

—

—

—

—

Pecan

8/4

4/4, 5/4, 6/4

8/4

Golf club heads

Shuttles

4/4, 5/4, 6/4

4/4, 5/4, 6/4

8/4

T6-D1

T6-C3

T3-C2

T3-C2

T3-B2

T8-D4

T12-F5

T1l-D4

Yes

Persimmon —

—

Yes

Yes

Yes

Yes

Yes

Sassafras

Sweetgum: sapwood

(sap gum)

1-in. squares

2-in. squares

4/4, 5/4, 6/4

8/4

4/4, 5/4, 6/4

8/4

T12-F6

T11-D5

T8-C4

T5-C3

T6-D2

T3-D1

Yes

Yes

Yes

Yes

Yes

Yes

heartwood

(red gum)

Sycamore

-85-
APPENDIX A—SPECIES, THICKNESSES, DRYING SCHEDULES,

AND COMMENTS—Cont.

Common name Thickness Kiln schedule1 Conservative

schedule 2

-----------------------------------------------------------------------------------------------------------------------------------------

Tupelo, black 4/4, 5/4, 6/4 T12-E5 Yes

8/4 T11-D3 Yes

Tupelo, swamp 4/4, 5/4, 6/4 T10-E3 Yes

8/4 T8-D2 Yes

Tupelo, water 4/4, 5/4, 6/4 T6-H2 No

8/4

Walnut, black 4/4, 5/4, 6/4 T6-D4 Yes

8/4 T3-D3 Yes

Gunstock T3-D4 Yes

blanks

Willow, black 4/4, 5/4, 6/4 T10-F4 Yes

8/4 T8-F3 Yes

Yellow-poplar 4/4, 5/4, 6/4 T11-D4 Yes

8/4 T10-D3 Yes

Other

See table 17, Dry

Kiln Operator’s

Manual.

Do.

See table 17, Dry

Kiln Operator’s

Manual.

Do.

See tables 16 and

17 Dry Kiln Operator’s Manual.

Air dry first.

Steam only green

stock.

—

End coating required; T5–D4 possible acceleration.

Air dry butt

material

Do.

—

1 See page 119 of Dry Kiln Operator’s Manual.

2 “Yes” indicates schedule probably can be accelerated by following procedures on pages 124–126 of Dry Kiln

Operator’s Manual; “No” suggests the recommended kiln schedule probably is close to optimum. All 8/4 thickness

conservative schedules can be modified to 180° F dry bulb at 11 pct moisture content or below.

-86-
APPENDIX B—MIXED DRYING OF SOME

SIMPLIFIED SCHEDULES

SPECIES BY

Eighteen southern hardwoods growing on

upland sites can be grouped for kiln drying by

simplified schedules:

Simplified kiln

Species schedule No. for

4/4 lumber 1

EASY-DRYING WOOD S

Ash

Black III

Green, white III

Elm

American (soft type)2

,

slippery I, III

American (hard type) 2

I

Cedar: rock, winged I

Hackberry III

Hickory: pecan 3

II, I

Maple: red, silver III

Magnolia: sweetbay IV

Sweetgum

Sapwood (not over 15

pct heart) V

Heartwood (red gum) III

Tupelo: black (black

gum), tupelo V

Yellow-poplar (poplar) IV

DIFFICULT WOODS

Oak

Red oak VI

White oak VI

In some cases the same schedule is shown

for woods that have greatly different green

moisture content values. In these cases, two

columns of moisture content schedule change

points are shown. The dry-bulb and wet-bulb

temperature settings for these woods are

governed by the average moisture content of

the wettest half of the kiln sample of the

species or group of species that are slowest in

reaching the designated moisture content

level in its own series.

The simplified kiln schedules I through VI

are listed in table B1.

1 Where a wood has two different simplified schedules

listed, see the distinctions in the schedule tables.

2 Soft type is forest grown, has narrow growth rings,

specific gravity (green volume basis) of 0.40-0.48, and

may be sold as gray elm; hard type is open growth, has

wide rings, and specific gravity of 0.44-0.52.

3 Divided into bitter pecan (water hickory, Cary a

aquatica) and sweet pecan (pecan, C. illinoensis; and

nutmeg hickory, C. myristicaeformis).

-87-
Table B1 –Simplified kiln schedules I through VI for drying 4/4 material

of mixed species

Moisture content

Dry-bulb Wet-bulb

Lower Higher temperature temperature

______________________________________________________________________________________________________________________

Pct Pct ° F ° F

I. American Elm (Hard Type), Rock Elm, Hickory

and Bitter Pecan Heartwood

Rock, elm Hard -type American

winged elm elm, hickory, pecan

Above 35 Above 50 120

35 50 120

30 35 130

25 25 140

18 18 180

II. Hickory, Sweet Pecan, and Bitter Pecan Sapwood

Above 50 125

50 125

40 130

30 150

20 180

III. Black, Green, and White Ash; American Elm

(Soft Type); Hackberry; Red and Silver Maple; and

Sweetgum Heartwood

Green and white Black ash; softash; hackberry; type American

sweetgum elm; red and

heartwood silver maple

115

113

100

105

130

120

118

114

112

130

Above 40

40

32

18

White

Above 40

40

34

30

25

20

15

Above 50 130

50 130

35 140

18 180

IV. Magnolia and Yellow-Poplar

Above 50 140

50 140

35 150

18 180

V. Black Gum, Upland Tupelo, and Sweetgum Sapwood

Above 60 160

60 160

40 160

25 170

18 180

VI. Upland Red and White Oak

Red

Above 50 110

50 110

40 110

32 120

25 130

20 140

15 180

123

118

95

130

133

128

120

130

150

145

130

130

130

106

105

102

106

100

95

130

-88-
APPENDIX C—SPECIAL KILN SCHEDULES

FOR SPECIAL PURPOSES

Most kiln operators have their own special

kiln schedules for particular circumstances.

Some of these take into account specific conditions for their unique situation. But such

special situations are almost infinite. A

limited number of special hardwood schedules

is given on pages 127 to 129 of the Dry Kiln

Operator’s Manual, Some additional special

schedules are included here for these species

and purposes:

Aspen—to minimize collapse in 4/4 to 8/4 material.

Maple-to retain the whitest color possible in 4/4 and

5/4 stock, and to dry 6/4 and 8/4 material with mineral

streak with little honeycombing.

Upland red oak—to minimize honeycomb and degrade

in bacterially infected 4/4 stock, and for presurfacing

to accelerate drying of 4/4 material.

These have been given the designation of

“C” for Appendix C, “S” for special schedule

and a number. Such a combination should set

it off from the basic and simplified schedules:

Table CS1. –Aspen, low-to moderate-collapse kiln schedules

4/4, 5/4, 6/4 stock 8/4 stock

Moisture

content

at start

of step

Dry-bulb

temperature

Wet-bulb

temperature

Dry-bulb

temperature

Wet-bulb

temperature

P c t

Above 70

70

60

50

40

30

25

20

2

1 2

3

8

 F

110

110

115

120

130

150

150

1

1 8 0

180

180

173

180

° F

100

100

100

100

105

110

110

135

130

130

130

170

° F

140

140

140

140

140

1

150

170

170

180

200

200

—

° F

133

130

125

120

110

100

120

120

130

140

150

—

Equalize 4

Condidtion

1Operate with vents closed, no steam spray until equalizing.

2For 8/4, continue until very wettest sample is 8 pct moisture content.

3For 8/4, target is 31⁄4 to 41⁄4 pct moisture content; time on this step about 5 dyas.

4For 8/4, time on this step about 2 days.

Table CS2. –Maple, whitest color 4/4 and 5/4 schedules

4/4 and 5/4 stock initial

moisture content 50 percent

or lower

4/4 and 5/4 stock, initial

moisture content 51 percent

or higher

Moisture

content

at start

of step

Dry-bulb

temperature

Wet-bulb

temperature

Moisture

content

at start

of step

Dry-bulb

temperature

Wet-bulb

temperature

---------------------------------------------------------------------------------------------------------------------- -

P c t ° F ° F Pct

Above 40

above 28

28

24

20

16

13

10

105

108

108

108

115

125

160

170

95

95

90

85

80

80

40

35

30

26

20

16

12

° F

105

108

108

108

108

115

125

160

° F

95

95

50

85

80

80

80

105

105

Conditioning 154 Conditioning 170 154

-89-
Table CS3. –Maple, 1 mineral streak stock 6/4 and 8/4 schedule for minimum honeycombing

Moisture content 2

Average for

wettest half

of kiln

samples

Darkest zone

fin wettest

sample

Dry-bulb

temperature

Wet-bulb

temperature

----------------------------------------------------------------------------------------------------- -

P c t P c t

Above 40

40

35

30

25

20

° F

110

110

110

120

125

130

140

150

160

180

° F

106

105

102

106

9 5

8 5

9 5

100

110

130

30

25

20

15

1 This schedule also should be suitable for mineral-streaked yellow birch.

2 Kiln samples should be 2 feet longer than normal so that three or four intermediate moisture content tests (page

105, Dry Kiln Operator's Manual) can be made. For green stock, start with normal kiln sample procedure. For air-dried

stock, cut both an average section and a "darkest zone" section at the start. Cut out the darkest, wettest appearing

portion of the latter section with a bandsaw. Weigh and ovendry this portion sseparately to determine when temperature of 140° F and higher cn be used. After the final drying condition has run 1 day, revert the full-size kiln sample

method to start equalizing and conditioning.

Table CS4. —Upland red oak, bacterially infected 4/4 schedule for minimizing honeycomb and

degrade

Moisture content Dry-bulb Wet-bulb

at start of step temperature temperature

------------------------------------------------------------------------------------------------------------- -

Pct ° F ° F

Above 52 105 102

52 105 101

45 105 99

40 105 97

35 110 95

30 115 85

25 120 85

20 130 85

15 150 100

11 170 120

Equalize 170 127

Condition 175 165

1 Material with advanced infection should be air dried in a sheltered location or forced air dried by a mild 8/4

procedure until the wettest half of the infected kiln samples have a moisture content of 22 percent or lower. Kiln drying

then can be started using 120° F dry-bulb and 110° F wet-bulb temperatures for 16 to 24 hours. Then the above schedule

can be followed in accordance with the moisture content of the controlling samples. Material in the early stages of

infection can be dried by this schedule from the green, partly air dried, or air dried state in accordance with the general

procedures in chapter 5.

-90-
Table CS5. —Presurfaced upland oak 4/4 schedule 1

Moisture content for: Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature

White oak 2 Red oak 2

Pct Pct ° F ° F ° F

BASIS—AVERAGE MOISTURE CONTENT, ALL SAMPLES

Above 42 Above 53 115 4 111

42 53 115 5 110

37 43 115 8 107

33 37 115 14 101

BASIS—AVERAGE MOISTURE CONTENT, WETTEST HALF OF SAMPLES

35 35 120 35 85

30 30 125 40 85

27 27 130 45 85

21 21 140 50 90

17 17 180 50 130

1 Accelerations achieved by presurfacing depend partly on 400 ft/min air velocity through the load.

2 When there is a mixture of red and white oak in the kiln, operate on the basis of the moisture content of the samples

of the predominating species. If either the red oak or the white oak is not strictly green, operate on the basis of the

samples of the species closest to the green moisture content.

-91-
APPENDIX D—LUMBER NAMES, TREE SPECIES, SEASONING

TYPES, AND BOTANICAL NAMES OF WOODS

MENTIONED IN THIS HANDBOOK

Commercial name

for lumber

Ash:

Black

W bite

Aspen (popple)

Basswood

Beech

Birch:

Soft (white)

Yellow

Blackgum

Buckeye

Butternut

Cherry

Chestnut

Cottonwood

Cucumber

Dogwood

Elm:

Rock

Soft (American)

Gum

Hackberry

Hickory

Holly

Ironwood (hophornbeam)

Locust

Magnolia

Maple:

Hard

soft

Normal

Wet streak

Seasoning Official common tree

subtype name

Apple

Black ash

Blue ash

Green ash

White ash

Bigtooth aspen

Quaking aspen

American basswood

White basswood

Beech

Gray birch

Paper birch

River birch

Sweet birch

Yellow birch

(see Tupelo)

Ohio buckeye

Yellow buckeye

Butternut

Black cherry

Chestnut

Balsam poplar

Eastern cottonwood

Swamp cottonwood

Cucumbertree

Flowering dogwood

Cedar elm

Rock elm

September elm

Winged elm

American elm

Slippery elm

(See sweetgum)

Hackberry

Sugarberry

Mockernut hickory

Pignut hickory

Shagbark hickory

Shellbark hickory

American holly

Eastern hophornbeam

Black locust

Honeylocust

Southern magnolia

Sweetbay

(Also see cucumbertree)

Black maple

Sugar maple

Red maple

Silver maple

Hard type

Soft type (gray)

Botanical name

Malus spp

Fraxinus nigra

F. quadrangulata

F. pennsylvanica

F. americana

Populus grandidentata

P. tremdoides

Tilia americana

T. heterophylla

Fagus gvandifolia

Betula populifolia

B. papyrifera

B. nigra

B. lenta

B. alleghaniensis

Aesculus glabra

A. octandra

Juglans cinerea

Prunus serotina

Castanea dentata

Populus balsamifera

P. deltoides

P. heterophylla

Magnolia acuminata

Cornus florida

Ulmus crassifolia

U. thomasii

U. serotina

U. alata

U. americana

U. rubra

Celtis occidentals

C. laevigata

Carya tomentosa

C. glabra

C. ovata

C. laciniosa

Ilex opaca

Ostrya virginiana

Robinia pseudoacacia

Gleditsia triacanthos

Magnolia grandifiora

M. virginiana

Acer nigrum

A. saccharum

A. rubrum

A. saccharinum

-92-
Commercial name Seasoning Official common tree Botanical name

for lumber subtype name

Oak:

Red Northern or upland

Southern lowland

White Northern or upland

Southern lowland

Osage-orange

Pecan:

Sweet pecan

Bitter pecan

Persimmon

Poplar

Sassafras

Sweetbay

Sweetgum Red gum

Sap gum 3

Sycamore

Tupelo, black (black

Sapwoo d

Heartwood

gum)

Tupelo

Walnut

Willow

Yellow-poplar

Black oak

Blackjack oak

Cherrybark oak1

Northern pin oak

Northern red oak

Pin oak

Scarlet oak

Shumard oak1

Southern red oak

Cherrybark oak2

Laurel oak

Nuttall oak

Shumard oak2

Water oak

Willow oak

Bur oak

Chestnut oak

Chinkapin oak

Post oak

Swamp white oak

White oak

Overcup oak

Swamp chestnut oak

Osage-orange

Pecan

Nutmeg hickory

Water hickory

Common persimmon

(See yellow-poplar)

Sassafras

Sweetbay

Sweetgum (heartwood)

Sweetgum (sapwood)

American sycamore

Black tupelo

Swamp tupelo

Water tupelo

Black walnut

Black willow

Yellow-poplar

Quercus velutina

Q. marilandica

Q. falcata var. pagodaefolia

Q. ellipsoidalis

Q. rubra

Q. palustris

Q. coccinea

Q. shumardii

Q. falcata var. falcata

Q. falcata var. pagodaefolia

Q. laurifolia

Q. nuttallii

Q. shumardii

Q. nigra

Q. phellos

Q. macrocarpa

Q. primus

Q. mushlenbergii

Q. stellata

Q. bicolor

Q. alba

Q. lyrata

Q. michanxii

Maclura pomifera

Carya illinoensis

C. myristicaeformis

C. aquatica

Diospyros virginiana

Sassafras al bidum

Magnolia virginiana

Liquidambar styraciflua

Do.

Platanus occidentals

Nyssa sylvatica

N. sylvatica var. biflora

N. aquatica

Juglans nigra

Salix nigra

Liriodendron tulipifera

1 When grown on upland sites.

2 When grown on lowland (wetter) sites.

3 If sap gum grade contains more than 15 pct heartwood, dry with red gum schedules.

-93-
Accelerated air drying. ––The use of equipment and procedures to accelerate air drying. In this handbook it includes yard fan

drying, shed fan drying, forced air drying,

low temperature kiln drying, and controlled air drying.

Active drying period. —In air drying, the

period or season of the year when conditions are most favorable for drying the

wood at the highest rate.

Air drying. —The process of drying green

lumber or other wood products by exposure

to prevailing natural atmospheric conditions outdoors or in an unheated shed.

Air drying calendar. A table showing the

number of effective air drying days each

month of the year in a specific area.

Air drying efficiency. Operation of the air

drying process to reduce green wood to

the air-dried moisture content level with

least cost in energy, time, and money.

Effective air drying day (EADD). A day

with drying potential equivalent to the

average of all the days in full months of

the late spring-summer season.

Good air drying month. Thirty consecutive

days in which daily mean temperatures

are over 45° F. This is roughly equivalent

to 22 or more EADDS.

Alleys. —In air drying, the passageways between rows or lines of piles of lumber or

other wood products on a yard.

Cross alley. The passageways that connect

main alleys and lie at right angles to the

piled lumber.

Main alley. The roads in a yard for the

transport of lumber and other wood

products.

Baffle. —In forced air or kiln drying, a canvas,

metal, or wood barrier used for deflecting,

checking, or otherwise directing the flow of

air.

Blank. —A piece of wood cut to a specified size

and shape from which a finished product or

pattern is made, e.g., gunstocks, bowling

pins.

Board. —Yard lumber that is less than 2

inches thick and 2 or more inches wide; a

term usually applied to 1-inch thick stock of

all widths and lengths.

Boiling in oil. —A special process for drying

wood; the rough green wood products are

submerged in an open hot bath of a waterrepelling liquid such as petroleum oil,

creosote, o r molten wax, o r perchloroethylene in ozeotropic drying, all of

which have a boiling point considerably

above that of water.

Bolster. ––A piece of wood, generally a nominal 4 inches in cross section, placed between stickered packages of lumber or

other wood products to provide space for

the entry and exit of the forks of a lift

truck. (The bolster should be placed in

alinement with a tier of stickers and a supporting foundation member.)

Bound water. ––In wood technology, moisture

that is intimately associated with the finer

wood elements of the cell wall by adsorption and held with sufficient force to reduce

the vapor pressure.

Bow. —A form of warp in which a board deviates from flatness lengthwise but not

across the faces.

British themal unit. —The amount of heat

required to raise the temperature of 1

pound of water at its maximum density,

1° F.

Bulk piling. —In handling lumber and other

wood items, the stacking onto dollies or pallets, into unit packages or into bins without

vertical spaces or stickers between the

layers for air circulation.

Casehardening. —A condition of stress and set

in dry wood in which the outer fibers are

under compressive stress and the inner

fibers under tensile stress, the stresses

persisting when the wood is uniformly dry.

Cell. —In wood anatomy, a general term for

the minute units of wood structure having

distinct cell walls and cell cavities including wood fibers, vessel segments, and other

elements of diverse structure and function.

In dense woods the fibers are thick walled

and make up the major part of whole zones

of wood. These fibrous zones are slow drying.

Cellulose. —The carbohydrate that is the

principal constituent of wood and forms the

framework of the wood cells.

Check. ––A separation of the wood fibers

within or on a log, timber, lumber, or other

wood product resulting from tension

stresses set up during drying, usually the

-94-
early stages of drying. Surface checks

occur on flat faces of boards; end checks on

ends of logs, boards, or dimension parts.

Chemical seasoning. —The application of a

hydroscopic chemical, (e.g., sodium

chloride) to green wood, for the purpose of

reducing defects, mainly surface checks,

during drying. The chemical may be

applied by soaking, dipping, spraying with

aqueous solutions, or by spreading with the

dry chemical and bulk piling.

Collapse. —Flattening or buckling of the wood

cells during drying resulting in excessive

or uneven shrinkage plus a corrugated surface.

Conditioning. ––In kiln drying, a process for

relieving the stresses present in the wood

at the end of drying. Consists of subjecting

the stock while still in the kiln to a fairly

high dry-bulb temperature and an equilibrium moisture content condition 3 to 4 percent above the desired average moisture

content for the stock. The process should be

of sufficient duration to eliminate

casehardening through the reduction of

compression and tension sets.

Course. —A single layer of lumber or other

wood products of the same thickness in a

stickered pile, package, or kiln truckload.

Crook. —A form of warp in which a board deviates edgewise from a straight line from

end to end.

Cup. —A form of board warp in which there is

a deviation from a straight line across the

width.

Debarking. —Removing the bark from a log.

Debris. ––In air drying, rubbish such as broken stickers, fragments of boards, broken

down foundations, dried weeds, etc., that

restrict air movement. In kiln drying, broken stickers, fragments of wood, and other

rubbish that interfere with air circulation

and increase the fire hazard.

Decay. —The softening, weakening, or total

decomposition of wood substance by fungi.

Defect. ––Any irregularity or imperfection in a

tree, log, bolt, lumber, or other wood product that reduces the volume of useable

wood or lowers its durability, strength, or

utility value. Defects may result from

knots and other growth conditions and abnormalities; from insect or fungus attack;

from milling, drying, machining, or other

processing procedures.

Degrade. —Generally, in lumber and other

forest products, the result of any process

that lowers their value for any purpose.

Drying degrade. A drop in lumber grade

and volume due to drying defects.

Machine degrade. The downward change of

grade or value of wood items due to defects developed during machining.

Density. ––The weight of wood per unit volume, usually expressed in pounds per cubic

foot or grams per cubic centimeter. As

changes in moisture content of wood affect

its weight and volume, it is necessary to

specify the conditions of wood at the time

density is determined.

Depression, wet-bulb. ––The difference between the dry- and wet-bulb temperatures.

Desuperheater. —A device for removing from

steam the heat beyond that required for

saturation at a given pressure.

Diamonding. ––A form of warp; the changing

of the cross section of a square-sawn wood

item to diamond-shaped, during drying.

This occurs where the growth rings pass

through diagonal corners and is caused by

the difference between tangential and radial shrinkage.

Diffusion. —Spontaneous movement of water

through wood from points of high concentration to points of low concentration.

Diffusivity. ––The measure of rate of moisture

movement through wood as a result of differences in moisture content.

Dimensional stabililization. —Through special

treatment, reduction in the normal swelling and shrinking of wood.

Discoloration. ––Change in the color of wood

due to fungal and chemical stains, weathering, or heat treatment.

Dryer. —Air moving and directing equipment

used to accelerate the air drying of wood.

Controlled-air dryer. A warehouse-like

building with overhead fans and layout

designed for gentle recirculation of air

through the lumber piles, having controlled vents and unit heaters to maintain a desired temperature and relative

humidity.

Forced-air dryer. An inexpensive building

with reversible fans, tight baffling, and a

source of heat, providing high air velocity

through the lumber piles, thermostatically controlled temperature in the range

of 70°–120° F, and partial relative humid-

-95-
ity control by limited venting.

Low-temperature kiln. Similar to a forcedair dryer having sources of heat and

humidity, providing high air velocity

through the piles, and dry- and wet-bulb

temperature control by a recordercontroller.

Semi-solar dryer. A low-temperature kiln in

which part of the heat is from solar

energy and part from an artificial heat

source.

Shed-fan dryer. A shed with permanent

roof, permanent or temporary end walls,

and a permanent side wall with enough

fans to draw yard air through the lumber

piles at a high velocity.

Solar dryer. A forced-air dryer or lowtemperature kiln in which only solar

energy is used for heat.

Yard-fan dryer. Similar in operation to a

shed-fan dryer, but roof and end walls

are temporary and made of canvas,

plywood, or sheet metal. Some such

dryers have portable fans used to

exhaust air from an enclosed central

plenum space between two yard piles.

Dry kiln. —A room, chamber, or tunnel in

which the temperature and relative humidity of air circulated through parcels of

lumber, veneer, and other wood products

can be controlled to govern drying conditions.

Dryer or kiln concepts. —Important ideas in

wood drying technique.

Air travel. The distance air has to move

from the entering-air side to the

leaving-air side of the load.

Air velocity. The velocity of air as it leaves

the sticker spaces on the leaving-air side

of the load.

Entering-air side of load. Where the air enters the lumber pile when the fans are

turning in a specific direction. The air on

this side of the load must be controlled in

accordance with the drying schedule to

prevent drying defects.

Leaving-air side of the load. The side of the

load from which the air is exhausted or

returned to the heat source and fans.

This air is always cooler and more humid

than the entering air because of the cooling effect of moisture evaporating from

the lumber.

Drying. —The process of removing moisture

from wood to improve its serviceability in

use.

Drying or kiln schedule. The prescribed

schedule of dry-bulb temperature and

wet-bulb temperature or relative humidity used in drying a load of lumber. The

humidity aspect is sometimes expressed

in terms of wet-bulb depression or EMC.

In kiln drying, air velocity is an important aspect.

Drying rates. The loss of moisture from

lumber or other wood products per unit of

time. Generally expressed in percentage

of moisture content lost per hour or per

day.

Drying record. A daily or weekly tabulation

of dryer or kiln operation including sample weight, MC, and temperature

readings or recorder-controller charts.

Drying shed. In air drying, an unheated

building for drying lumber and other

wood products. The building of various

designs may be open on all sides, or

closed.

Drying stress. The force per unit area that

occurs in some zones of drying wood due

to uneven shrinkage in response to normal moisture gradients and to set that

develops in wood.

Edge piling. ––In air drying, stacking of wood

products on edge, e.g., 2 by 4’s, so that the

broad face of the item is vertical; usually

done to restrain crook. In kiln drying,

stacking of lumber on edge for drying in

kilns with vertical air circulation.

End coating. ––A coating of moisture-resistant

material applied to the end-grain surfaces

of green wood such as logs, timbers, boards,

squares, etc., to retard end drying and consequent checking and splitting or to prevent moisture loss from the ends of the

drying samples.

End pile. ––In air drying, stacking of green

lumber on end, and inclined, in a long fairly

narrow row, the layers separated by stickers. In lumber storage, placement of wood

items on end in suitable bins, a practice at

mills and retail yards. In kiln drying, placing lumber on kiln trucks with their length

parallel to the length of the kiln.

End racking. —In air drying, placing of boards

on end against a support so as to form a pile

-96-
in the shape of an “X” or an inverted “V.”

Equalization and conditioning. —In kiln drying, the process of increasing the equilibrium moisture content condition in the

final stages of drying lumber and other mill

products to—(1) reduce the moisture content range between boards, (2) flatten the

moisture content gradient within boards,

and (3) relieve drying stresses. Usually

equalization and conditioning are two

separate stages in final kiln drying.

Equilibrium moisture content. —The moisture

content at which wood neither gains nor

loses moisture when surrounded by air at a

given relative humidity and temperature.

EMC is frequently used to indicate potential of an atmosphere to bring wood to a

specific MC during a drying operation.

Extractives. ––Substances in wood, not an integral part of the cellular structure, that

can be removed by solution in hot or cold

water, ether, benzene, or other solvents

that do not react chemically with wood

substances.

Fiber-saturation point. —The stage in the drying or wetting of wood at which the cell lalls

are saturated with water (bound water)

and the cell cavities are free of water. It is

usually taken as approximately 30 percent

moisture content, based on the weight

when ovendry.

Flat pile. ––In air drying and kiln drying,

stacking of stock so that the broad face of

the item is horizontal. In kiln drying, the

stickered loads are level.

Flue. —In stacking lumber or other wood

products for air drying, a vertical space, 6

inches or less in width in the center of the

pile and extending the length of the pile,

intended to facilitate circulation of air

within the pile.

Foundation. —Structural supports for an

air-drying pile, designed to prevent warping and facilitate air circulation under the

pile.

Free water. —In wood technology, water that

is held in the lumens or the grosser capillary structure of the wood.

Fungi. —Low forms of plants consisting

mostly of microscopic threads (hyphae)

that traverse wood in all directions,

dissolving materials out of the cell walls

that they use for their own growth.

Grain. —The direction, size, arrangement,

appearance, or quality of the fibers in

lumber or other wood products. When used

with qualifying adjectives, it has special

meanings concerning the direction of the

fibers or the direction or size of the growth

rings.

Across the grain. The direction (or plane) at

right angles to the length of the fibers

and other longitudinal elements of the

wood.

Along the grain. The direction (or plane)

parallel to the length of the fibers and

other longitudinal elements in the wood.

Cross grain. Wood in which the fibers deviate from a line parallel to the sides of

the piece. Cross grain may be either

diagonal or spiral grain or a combination

of the two.

Edge grain. Wood that has been sawed or

split so that the wide surfaces extend approximately at right angles to the annual

growth rings. Lumber is considered

edge-grained when the rings form an

angle of 45° to 90° with the wide surface of

the piece.

End grain. The ends of logs or timbers, dimension, boards, and other wood products that are cut perpendicular to the

fiber direction.

Flat grain. Lumber or other wood products

sawed or split in a plane approximately

perpendicular to the radius of the log.

Lumber is considered flat-grained when

the annual growth rings make an angle

of less than 45° with the surface of the

piece.

Interlocked grain. Lumber or other wood

products in which fibers are inclined in

one direction in a number of rings of annual growth, then gradually reverse, and

are inclined in an opposite direction in

succeeding growth rings. This pattern is

reversed repeatedly.

Raised grain. Lumber and other wood

products in which a roughened condition

of the surface after planing, particularly

softwood, is due to either the earlywood

or the latewood projecting above the

general level of the surface.

Slope of grain. In lumber and other wood

products, the degree of cross grain. It is

the ratio between a 1-inch deviation of

-97-
the grain from the long axis of a piece

and the distance along the edge within

which this deviation occurs.

Straight grain. Wood in which the fibers

and other longitudinal elements run

parallel to the axis of a piece.

Green volume. —Cubic content of green wood.

Growth ring. —A layer of wood (as an annual

ring) produced during a single period of

growth.

Hardwoods. —Generally one of the botanical

groups of trees that have broad leaves in

contrast to the conifers or softwoods. The

term has no reference to the actual hardness of the wood.

Heartwood. —The inner layer of a woody stem

wholly composed of nonliving cells and

usually differentiated from the outer enveloping layer (sapwood) by its darker

color. It is usually more decay resistant

than sapwood and more difficult to dry.

Heated room drying. —Drying air-dried wood

in a room or small building, using just

enough heat to raise the temperature

slightly above outdoor air temperature and

provide an EMC 2 percent below the desired final EMC. Mild air circulation promotes temperature uniformity.

High-frequency dielectric heating. ––The use of

an electric field oscillating at frequencies of

1 to 30 million cycles per second to heat

wood. The electric energy is applied to wood

between metal plates as electrodes.

High-temperature drying. —In kiln drying

wood, use of dry-bulb temperatures of 212°

F. or more.

Honey combing. —In lumber and other wood

products, separation of the fibers in the interior of the piece, usually along the wood

rays. The failures often are not visible on

the surfaces, although they can be the extensions of surface and end checks.

Humidity. —The moisture content of air.

Relative humidity. Under ordinary temperatures and pressures, it is) the ratio of

the weight of water vapor in a given unit

of air compared with the weight which

the same unit of air is capable of containing when fully saturated at the same

temperature. More generally, it is the

ratio of the vapor pressure of water in a

given space compared with the vapor

pressure at saturation for the same drybulb temperature.

Hygroscopicity. —The property of a substance, such as wood, which permits it to

absorb and lose moisture readily.

Hygrostat. —A device for automatically regulating the EMC of the air.

Hysteresis. —The tendency of dried wood

exposed to any specified temperature and

relative humidity conditions to reach

equilibrium at a lower moisture content

when absorbing moisture from a drier condition than when losing moisture from a

wetter condition.

Infrared drying. —A special process in drying

wood by direct radiation from highintensity sources such as heat lamps and

radiant gas burners.

Kiln. —A chamber or tunnel used for drying

and conditioning lumber, veneer, and other

lood products in which the temperature

and relative humidity of the circulated air

can be varied.

Conventional kiln. Such kilns use initial

temperatures from 100° to 17W F and

final temperatures from 150° to 200° F.

Control of EMC is necessary to avoid

shrinkage-associated defects and t o

equalize and condition the wood at the

end of drying. Air velocities generally are

between 200 and 450 feet per minute.

High temperature kiln. A kiln for drying

lumber and other wood products operated at dry-bulb temperatures above

212° F.

Kiln charge. In kiln drying, the total

amount of lumber or wood items to be

dried in a dry kiln.

Kiln dried. Lumber or other wood items

that were dried in a closed chamber in

which temperature and relative humidity of the circulated air can be controlled.

Kiln leakage. The undesirable loss of heat

and vapor from a kiln through and

around doors and ventilators or through

cracks in the walls and roof.

Kiln operator. In kiln drying, the supervisor

or person responsible for the performance of dry kilns and related equipment.

Kiln run. The term applied to the drying of

a single charge of lumber or other wood

product.

Kiln sample. A length cut from a sample

board and placed in the kiln charge so

that it may be removed for examination,

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weighing, or testing.

Kiln schedule. In kiln drying, the prescribed

schedule of dry-bulb and wet-bulb temperatures used in drying a kiln charge of

lumber or other wood products.

Low temperature kiln. Forced air drying in

a moderately tight building equipped to

produce air movement through the loads

and recirculate the air over heat and/or

humidity sources, with dry- and wet-bulb

controls to maintain small to moderate

wet-bulb depressions in the temperature

ranges between 85° and 120° F.

Package-loaded kiln. A trackless compartment kiln for drying packages of stickered lumber or other wood products. The

dryer usually has large doors that can be

opened so that the kiln charge can be

placed in or removed from the dryer by

forklift trucks. It is usually a forced-air

circulation kiln with fans mounted overhead or at the side.

Progressive kiln. A dry kiln in which the

total charge of lumber is not dried as a

single unit but as several units, such as

kiln-truckloads that move progressively

through the dryer. The kiln is designed so

that the temperature is lower and the

relative humidity higher at the entering

end than at the discharge end.

Reversible circulation kiln. A dry kiln in

which the direction of air circulation

through the stickered loads of lumber or

other wood products can be reversed at

desired intervals.

Track-loaded kiln. A kiln, with doors at one

or both ends, for which loads are built up

on kiln trucks outside the kiln and moved

in and out of the kiln on tracks.

Knot. —That portion of a branch or limb that

has been surrounded by subsequent

growth of the wood of the trunk or other

portions of the tree. As a knot appears on

the sawed surface, it is merely a section of

the entire knot, its shape depending upon

the direction of the cut.

Layout. —On an air-drying yard, layout refers

to arrangement and orientation of alleys,

row and line spacings, pile sizes and spacings, and foundation placement.

Losses, drying. —In drying lumber and other

wood products, the reduction in volume and

grade quality that can be attributed to the

drying process.

Lumber. —The product of the sawmill and

planing mill not further manufactured

than by sawing, resawing, passing

lengthwise through a standard planing

machine, cross cutting to length, and

mat thing.

Boards. Yard lumber less than 2 inches

thick and 1 or more inches wide.

Common lumber. A classification of medium

and low-grade hardwood lumber and/or

softwood lumber suitable for general

construction and manufacturing but not

suitable for finish grade.

Dimension lumber. As applied to hardwood

lumber, a term loosely used but generally

referring to small squares or pieces of

rectangular cross section used for furniture and like purposes (small dimension).

Dressed lumber. The dimensions of lumber

after drying and surfacing with a planing

machine.

Finish lumber. A collective term for upper

grades of lumber suitable for natural or

stained finishes.

Flooring lumber. Generally, a grade of

either hardwood or softwood boards that

have been found to produce maximum

quantity of flooring of the desired quality.

Nominal size. As applied to timber or

lumber, the size other than the actual

size, by which it is known and sold in the

market.

Rough lumber. Lumber as it comes from the

saw.

Structural lumber. Lumber that is nominally 2 or more inches thick and nominally 4 or more inches wide, intended for

use where working stresses are required.

The grading of structural lumber is

based on the strength of the piece and

the use of the entire piece, e.g., stud,

joist, beam, plank, girder, rafters, framing, etc.

Yard lumber. Lumber of all sizes and patterns that is intended for general building purposes, The grading of yard lumber

is based on the intended use of the particular grade and is applied to each piece

with reference to its size and length

when graded, without consideration to

further manufacture.

Lumen. —In wood anatomy, the cell cavity.

Microwave heating. —Heating a material

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using electromagnetic energy alternating

at a frequency from 915 megahertz to

22,125 megahertz.

Mineral streak. —An olive to greenish-black or

brown discoloration in hardwoods, particularly hard maples, due to wound-induced

bacterial, chemical, or fungal action. Narrow streaks often contain accumulations of

mineral matter.

Moisture content. —The amount of water contained in the wood, usually expressed as a

percentage of the weight of the ovendry

wood.

Average moisture content. The moisture

content, in percent, of a single section

representative of a larger piece of wood

or the average of all the moisture content

determinations made on a board or other

wood item or of a number of determinations made on a lot of lumber or other

wood products.

Shipping dry. Lumber partially dried to

prevent stain or mold in brief periods of

transit, preferably with the outer 1/8 inch

dried to 25 percent MC or below.

Moisture meter. —An instrument used for

rapid determination of the moisture content of wood by electrical means.

Moisture movement. —The transfer of moisture from one point to another within wood

or other materials.

Mold. —A fungus growth on lumber or other

wood products at or near the surface and,

therefore, not typically resulting in deep

discolorations. Mold is usually ash green to

deep green in color, although black and yellow are common.

Final moisture content. The average moisture content of the lumber or other wood

product at the end of the drying process.

Initial moisture content. The moisture content of the wood at the start of the drying

process.

Movement. —The alternate swelling and

shrinkage that occurs in dried wood due to

moisture content changes caused by variations in the surrounding atmospheric conditions.

In-use moisture content. The moisture content that wood items attain in the environmental conditions of usage.

Range. The difference in moisture content

between the driest and wettest boards in

a shipment, lot, kiln charge, etc., or between representative samples of the lot.

Moisture gradient. In lumber drying, the

distribution in moisture content within

the wood. During drying, the differences

are between the low moisture content of

the relatively dry surface layers and the

higher moisture content at the center of

the piece.

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Out-of-round. —A form or warp, the elliptical

shape assumed by turned circular green

wood items upon drying due to the difference in tangential and radial shrinkage.

Overhang. —The ends of boards that are unsupported by stickers and extend beyond

the ends of most boards in an air drying

pile, kiln truckload, or unit handling package.

Moisture content classes.-

Air dried. Wood having an average moisture content of 25 percent or lower, with

no material over 30 percent.

Green. Freshly sawed wood or wood that

essentially has received no formal drying.

Kiln dried. Dried in a kiln or by some other

refined method to an average MC

specified or understood to be suitable for

a certain use, such an average generally

being 10 percent or below for hardwoods.

Ovendry. —A term used to describe wood that

has been dried in a ventilated oven at 212°

to 221° F. (100 to 105° C) until there was no

further significant loss in weight.

Ovendry weight. —Weight of wood when all

the water has been driven off by heating in

an oven at 212 to 221° F. (100 to 105° C).

Ovendrying. —Drying wood specimens to constant weight in an oven maintained at 212°

to 221° F. (100 to 105° C).

Part-time drying. —In kiln drying, discontinuous operation of the dry kilns, usually

necessitated by an interrupted steam, fuel,

or power supply.

Permeability. —The ease with which a fluid

flows through a porous material (wood) in

response to pressure.

Pile. —In air drying, stacking lumber layer by

Kiln-dried lumber can be specified to be

free of drying stresses.

Partly air dried. Wood with an average MC

between 25 and 45 percent, with no material over 50 percent.
layer, separated by stickers or self snickering, on a supporting foundation (hand

stacked). Also, stickered unit packages by

lift truck or crane, one above the other on a

foundation and separated by bolsters.

Box pile. A method of flat stacking randomlength lumber for air drying or kiln drying. Full-length boards are placed in the

outer edges of each layer and shorter

boards in between are alternated

lengthwise to produce square-end piles,

unit packages, or kiln truckloads.

Crib pile. Stacking lumber to form a hollow

triangle with their faces in contact at the

corners.

Hand-stacked pile. Lumber and other wood

products stacked by hand, course by

course, on suitable foundations to build

the pile.

Machine-stacked pile. Unit packages of

stickered lumber or other wood products

stacked by mechanical means onto a pile

foundation and one above another to

build a pile of packages.

Open pile. Spacing the boards or other

rough-sawn wood products in stickered

layers and in wide piles or packages incorporating flues in addition to board

spacing.

Pile roof. A cover on top of the pile to protect the upper layers from exposure to

the degrading influences of sun, rain, and

snow. The sides and ends of the roof may

project beyond the pile to provide added

protection.

Pile spacing. The distance between individual piles in the row.

Random-length pile. Stacking lumber of

various lengths in the same pile or package. The pile or package is usually square

at one end with the long length at the

other end unsupported by stickers.

Self-stickered pile. Stacking in which the

stock is used as stickers to separate the

layers. In crib stacking, the boards are in

contact at the three corners. In level or

sloped stacking of softwood boards and

dimension, stock is used for stickers and

the pile width is the same as the length.

Hardwood dimension stock and railroad

ties are often self stickered.

Pit. ––In wood anatomy, a recess in the secondary wall of a cell where a thin membrane

may permit liquids to pass from one cell to

another.

Pitch. ––The mixture of rosin and turpentine

or other volatiles produced in the rosin

canals of pines and other conifers.

Predryng. —A wood drying process carried

out in special equipment before kiln drying.

Predryng treatments. ––Special measures before or early in drying to accelerate drying

rate, to modify color, or to prevent checks

and other drying defects.

Blanking. Surfacing one face of a roughsawed board to achieve uniform thickness and reduce warping.

Chemical. Impregnating the outer zone of

green lumber with hydroscopic and

sometimes bulking chemicals to about

one-tenth the thickness to reduce surface

checking during drying. Involves control

of relative humidity at a value below the

RH that is in equilibrium with the saturated chemical solution.

Polyethylene glycol. Deeply impregnating

green lumber with polyethylene glycol

1000 (PEG) to retain the green dimension

during drying and indefinitely thereafter, minimizing shrinkage, swelling, and

warp.

Precompression. Momentarily compressing

green lumber transversely about 7.5% to

permit drying by a severe schedule and

improve drying behavior.

Prefreezing. The freezing and subsequent

thawing of green wood before drying to

increase drying rate and decreas e

shrinkage and seasoning defects.

Presurfacing. Surfacing both broad faces of

green rough-sawed boards to permit drying by a schedule more severe than the

prescribed schedule for rough lumber,

achieving faster drying and fewer drying

defects.

Steaming. Subjecting green wood to saturated steam at or close to 212° F to accelerate drying, achieve a desirable color, or

both.

Surface treatment. Applying a salt or

sodium alginate paste to the surface of

green wood to help prevent checking as

the wood is dried by other means.

Press drying. —The application of heat to opposite faces of a board by heated platens to

evaporate moisture from the board, using

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temperatures between 250° and 450° F and

platen pressures between 25 and 75 pounds

per square inch.

Radial surface. —A longitudinal surface or

plane extending wholly or in part from the

pith to the bark.

Recorder-controller. —An instrument that

continuously records dry- and wet-bulb

temperatures of circulated air and regulates these temperatures in a dryer or kiln

by activating automatic heat and steam

spray valves.

Redry. ––In kiln or veneer drying, a process

whereby dried material found to be at a

moisture content level higher than desired

is returned to the dryer for additional drying.

Refractory. —In wood, implies difficulty in

processing or manufacturing by ordinary

methods; resistance to the penetration of

preservatives, difficulty in drying, or difficulty in working.

Ring failure. ––A separation of wood along the

grain and parallel to the annual rings,

either within or between the rings.

Rot.—

Brown rot. In wood, any decay that concentrates on attacking the cellulose rather

than lignin, producing a light to dark

brown friable residue.

Dry rot. A term loosely applied to any dry,

crumbly rot but especially to that which,

when in an advanced stage, permits the

wood to be crushed easily to a dry powder. The term is actually a misnomer for

any decay, since all fungi require considerable moisture for growth.

White rot. In wood, any decay attacking

both the cellulose and lignin, producing a

generally whitish residue that may be

spongy or stringy or occur in pockets.

Rounds. —Pieces of green wood dried in the

form of cylinders. For example, ash

baseball bat stock is often kiln dried as

rough turned rounds rather than squares.

Sap. ––The moisture in green wood, containing nutrients and other chemicals in solution.

Sapwood. ––In wood anatomy, the outer layers

of the stem that in the living tree contain

living cells and reserve materials, e.g.,

starch. The sapwood is generally lighter in

color than the heartwood.

Season. —To dry lumber and other wood items

to the desired final moisture content and

stress condition for its intended use.

Set. ––A semipermanent deformation in wood

caused by tensile and compressive stresses

during drying.

Compression set. Set, occurring during

compression, that tends to give the wood

a smaller than normal dimension after

drying, usually found in the interior of

wood items during the later stages of drying but sometimes in the outer layers

after extended conditioning or rewetting.

Also caused by external restraint during

rewetting of dried wood.

Tension set. Set, occurring during tension,

that tends to give wood a larger than

normal dimension after drying; usually

occurring in the outer layers during the

early stages of drying.

Shrinkage. —The contraction of wood fibers

caused by drying below the fiber saturation

point. Shrinkage—radial, tangential and

volumetric—is usually expressed as a percentage of the dimension of the wood when

green.

Longitudinal shrinkage. Shrinkage of wood

along the grain.

Radial shrinkage. Shrinkage across the

grain, in a radial-transverse direction.

Tangential shrinkage. Shrinkage across the

grain, in a tangential-transverse direction.

Volumetric shrinkage. Shrinkage of wood in

volume.

Sinker. —A log which sinks in water.

Sinker stock. —Lumber or other sawmill

products sawed from sinker logs. The green

moisture content is very high and the drying rate can be low.

Softwood. —Generally, one of the botanical

groups of trees that, in most cases, have

needlelike or scalelike leaves; the conifers;

also, the wood produced by such trees. The

term has no reference to the actual hardness of the wood.

Spacing.––

Board spacing. The spacing of boards or

other rough-sawn wood products, in a

course of a pile, kiln truckload, or stickered unit package.

Pile spacing. Spacing between the piles in

the row or line of piles in the yard.

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Row spacing. Spaces between rows of piles

in the yard.

Sticker spacing. Distance between adjacent

stickers in a pile, kiln truckload, or a

stickered unit package of lumber.

Species. —A group of individual plants of a

particular kind; that is, a group of individuals sharing many of the same characteristics. It is a category of classification

lower than the genus but higher than the

variety.

Specific gravity. ––In wood technology, the

ratio of the ovendry weight of a piece of

wood to the weight of a volume of water at

4° C. (39° F.), equal to the volume of the

wood sample. Specific gravity of wood is

usually based on the green volume and

ovendry weight.

Stacking. ––Constructing packages, piles, or

kiln truck loads of lumber whose courses or

layers are separated by stickers to facilitate

drying the lumber.

Stain. —A discoloration in wood that may be

caused by micro-organisms, metal or chemicals. The term also applies to materials

used to impart colors to wood.

Blue. A bluish or grayish discoloration in

the sapwood caused by the growth of certain dark-colored fungi.

Chemical. A general term including all

stains that are due to color changes of

the chemicals normally present in the

wood.

Iron-tannate. A surface stain, bluish-black

in color, on oak and other tannin-bearing

woods following contact of the wet wood

with iron, or with water in which iron is

dissolved.

Sticker. —A wooden strip, or its substitute,

placed between courses of lumber or other

wood products, in a pile, unit packages or

kiln truckload, at right angles to the long

axis of the stock, to permit air to circulate

between the layers.

Dry sticker. A sticker that has been made

from lumber that was dried prior to

sticker manufacture.

Green sticker. A sticker cut from green

lumber or made by processing slabs and

edgings into stickers.

Sticker marking. Indentation or compression of the lumber or other wood product

by the sticker when the superimposed

load is too great for the sticker bearing

area. Also, sometimes light areas under

the sticker as the rest of the board

browns.

Sticker alinement. The placing of stickers in

a pile, unit package, or kiln truckload of

lumber or other wood products so that

they form vertical tiers.

Stock stickers. In air drying lumber, use of

boards being piled (hand-stacked) as

stickers. Usually restricted to softwoods

being air dried in semi-arid and arid regions.

Storage. —Bulk or stickered piling of air- or

kiln-dried wood products with protection

from the weather in accordance with the

desired level of moisture content; protection might be tarpaulins, or open, closed, or

closed and heated sheds.

Sun shields. ––In air drying, plywood panels,

boards, or some other type of shield placed

to protect the ends of piles from direct sun.

The purpose is to retard end drying, thus

minimize end-checking and end-splitting.

Swelling. — Increase in the dimensions of

wood due to increased moisture content.

Swelling occurs tangentially, radially, and

to a lesser extent, longitudinally.

Tangential surface. —Surface tangent to the

growth rings; a tangential section is a longitudinal section through a log perpendicular to a radius. Flat-grained lumber is

sawed tangentially.

Temperature. —May be defined as the condition of a body which determines the transfer of heat to or from other bodies; it is a

measure of the thermal potential of a body.

Dry-bulb temperature. Temperature of air

in a yard or drying apparatus indicated

by any temperature-measuring device

with its sensitive element or bulb uncovered.

Mean monthly temperature. In air drying,

the average dry-bulb temperature over a

period of a month. Usually obtained from

Weather Service records.

Wet-bulb temperature. The temperature

indicated by any temperature-measuring

device the sensitive element of which is

covered by a water-saturated cloth

(wet-bulb wick).

Tension failure. ––The pulling apart or rupturing of wood fibers, as a result of tensile

stresses.

Tension wood. ––In wood anatomy, reaction

–103–
wood formed typically on the upper sides of

branches and leaning hardwood trees, and

characterized anatomically by little or no

signification and by the presence of an

internal gelatinous layer in the fibers. It

has an abnormally high longitudinal

shrinkage. The machined surface tends to

be fibrous or woolly, especially when green.

Tier. —In air drying or kiln drying, a stack of

packages of lumber or other wood products

in vertical alinement. Also refers to sticker

alinement from layer to layer.

Transverse. —The directions in wood at right

angles to the wood fibers or across the

grain. A transverse section is a section

through a tree or timber at right angles to

the pith.

Unit package. —Lumber or other wood products that have been assembled into a parcel

for handling by a crane, carrier, or forklift

truck.

Vapor barrier. —In kiln drying, a material

with a high resistance to vapor movement

that is applied to dry kiln surfaces to prevent moisture migration.

Vapor drying. —Drying wood by subjecting it

to the hot vapors produced by boiling an

organic chemical such as xylene.

Veneer. —A thin layer or sheet of wood produced on a lathe, slicer, or saw and is commonly referred to as rotary, sliced, or

sawed veneer.

Vent. —In kiln drying, an opening in the kiln

roof or wall that can be opened and closed

to control the wet-bulb temperature within

the kiln.

Volume, green. —The volume of wood determined from measurements made while the

wood moisture content is above the fiber

saturation point (i.e., above 30 pct moisture

content; the green state).

Wane. —Bark, or the lack of wood from any

cause, on any edge of a piece of squareedged lumber.

Warp. —Distortion in lumber and other wood

products causing departure from its original plane, usually developed during drying.

Warp includes cup, bow, crook, twist, outof-round, kinks, and diamonding, or any

combination thereof.

Warp restraint. —In drying lumber and other

wood products the application of external

loads to a pile, package, or kiln truckload of

products to prevent or reduce warp.

Weight of wood. —The weight of wood depends

on its specific gravity and its moisture content. Weight includes the ovendry wood

and the moisture it holds. It is expressed as

pounds per cubic foot at a certain moisture

content or weight per 1,000 board feet at a

specified moisture content.

Wetwood. —Wood with abnormally high water

content and a translucent or water-soaked

appearance. This condition develops only in

living trees and does not originate through

soaking logs or lumber in water.

Wood. —The tissues of the stem, branches, and

roots of a woody plant lying between the

pith and cambium, serving for water conduction, mechanical strength, and food

storage, and characterized by the presence

of tracheids or vessels.

Diffuse-porous wood. Wood in which the

pores are of fairly uniform or of only

gradually changing size and distribution

throughout the annual growth ring.

Nonporous wood. Wood devoid of pores or

vessels; characteristic of conifers.

Porous wood. Wood with vessels; typical of

hardwoods as opposed to conifers.

Reaction wood. Wood with more or less distinctive anatomical characteristics,

formed in parts of leaning or crooked

stems and in branches. In hardwoods this

consists of tension wood and in conifers of

compression wood.

Residue. In the woodworking industry, that

portion of the wood developed as bark,

slabs, edgings, trim, sawdust, and shavings.

Ring-porous wood. Wood in which the pores

of the earlywood are distinctly larger

than those of the latewood and form a

well-defined zone or ring.

✩ U.S. GOVERNMENT PRINTING OFFICE: 1978 O—258—680

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