U.S DEPARTMENT OF AGRICULTURE • FOREST SERVICE • FOREST PRODULCTS LABORARORY • MADISON, WIS. 

U.S. FOREST SERVICE 

RESEARCH NOTE 

FPL-085 

January 1965 

PRESERVATIVE TREATMENT OF WOOD FOR FARM USE1 

By 

J. OSCAR BLEW, JR, Technologist 

Forest Products Laboratory, Forest Service 

U.S. Department of Agriculture 

Introduction 

Wood has always been a highly useful material to the farmer. In the early days 

the forests had to be partially cleared to make room for his crops. Wood was the 

only readily available material to which he could turn for his shelter, his fuel, 

and his tools. While other materials are now available to the farmer, wood continues to have an important part in agriculture because of its favorable cost, 

strength, weight, beauty, working characteristics, insulation value, and availability. 

With all the favorable properties that contribute to its wide use in farming, 

wood nevertheless needs to be used intelligently, and protected from certain 

natural enemies. For example, while some species of wood are naturally 

1 This is a revision of Forest Products Laboratory Report 2098, of the same title, written in 1957 

and presented at the annual meeting of the American Society of Agriculture E n g i n e e r s 

December 16, 1957, Chicago, Ill. 
resistant to attack by decay fungi and harmful insects, most species lack 

adequate resistance when exposed to attack, This is not serious when wood can 

be kept dry and away from contact with the ground. However, for such farm uses 

as fencing, poles, bridges, culverts, irrigation structures, silos, storage sheds, 

barns, and some types of vehicles, wood must be used in contact with moisture; 

it is thereby subject to decay and, in some areas, to termite attack. Fortunately, 

this can be corrected by preservative treatment. 

The railroads, through preservative treatment, have demonstrated that the 

average life of a tie can be increased from about 5 years to more than 30 years. 

On the farm it has been shown that an untreated fence post that would normally 

last about 3 years can have its life increased to 35 years or more. When 

effectively treated, wood can therefore be considered on the same basis as other 

so-called permanent materials. 

There are a wide variety of wood preservatives and several methods by which 

preservatives can be applied. Some of these preservatives and methods are more 

effective than others. 

Wood Preservatives 

Wood preservatives are of two general types; (1) Oils, such as creosote, 

creosote solutions and pentachlorophenol, and copper naphthenate in oil carriers; 

and (2) waterborne salts applied as water solutions. 

Preservative oils are used for both commercial and on-the-job treatments. 

They generally have high resistance to leaching and are therefore suitable for 

outdoor exposures. They do not cause the wood to swell, but the wood may shrink 

if it loses moisture during the treating process. Creosote and solutions with 

the heavier, less volatile petroleum oils often help protect the wood from 

weathering but may adversely influence its cleanliness, odor, color, paintability, 

and combustibility. Preservative oils sometimes travel from treated studs or 

subflooring along nails, and discolor the adjacent plaster or finish flooring. 

Odors from creosote and petroleum oils in treatedwood are quickly absorbed by 

foods such as meats, fats, and apples. Creosote vapors and those of some 

petroleum oils are harmful to growing plants. 

Water-retardant properties can be provided for wood by using preservative 

oils to which water-repellent components are added. Such materials retard 

moisture changes in wood and thereby help to reduce swelling and shrinking. 

They do not prevent these changes, however, when the wood is exposed to moisture 

for prolonged periods. 

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Coal-tar creosote that meets the requirements of Federal Specification 

TT-W-556b and American Wood Preservers Standard P 1 is generally available 

in small or large quantities. Pentachlorophenol, in ready-to-use 5 percent 

solutions and in concentrated solutions that can be diluted with petroleum oil, 

is also available in gallon, drum, or tank-car lots. 

Where water repellency is desired, Federal Specification TT-W-572 includes 

water-repellent preservatives with either 5 percent pentachlorophenol (Composition A), or 2 percent copper as copper naphthenate (Composition B). The 

National Woodwork Manufacturers’ Association also has a standard on a waterrepellent preservative for millwork. These water-repellent preservatives are 

sold in gallon, drum, and larger lots under a wide variety of trade names, but 

the name and concentration of the preservative are given on the label of the 

container. 

For most farm uses the recommended retentions of preservative oils vary 

from 6 to 10 pounds per cubic foot. 2 

Standard wood preservatives used in water solution include chromated zinc 

chloride, copperized chromated zinc chloride, Tanalith (Wolman salts), acid 

copper chromate (Celcure), ammoniacal copper arsenite (Chemonite), chromated 

zinc arsenate (Boliden salt), chromated copper arsenate (Greensalt or Erdalith), 

and Osmosar (Osmosalts). Recommended retentions of these chemicals vary 

from 0.3 pound to 1.00 pound per cubic foot for wood used on farms.

2 These 

preservatives are employed principally in the commercial treatment of wood for 

uses where it will not be in contact with the ground or water, and where the 

treated wood requires painting. As a general rule, they are less resistant to 

leaching than the preservative oils and do not perform so satisfactorily under 

conditions favorable to leaching. The leaching resistance of some of these preservatives has been developed to the extent that good performance can be 

expected in ground contact or in other wet installations. On the other hand, 

waterborne preservatives are generally preferable to creosote for indoor use 

and can give indefinitely long life where they are not subject to leaching. 

Waterborne preservatives leave the wood surface comparatively clean, 

paintable, and free from objectionable odor. Since water is added during treatment, the wood must be dried, after treatment, to the moisture content required 

for use. 

2 American Wood-Preservers’ Association. Manual of recommended practice. Standard C 16-64, 

Wood used on farms: Preservative treatment by pressure processes. 

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Zinc chloride and chromated zinc chloride are frequently used as fire 

retardants for wood but at retentions higher than those used only for woodpreserving purposes. 

The composition and other requirements for waterborne preservatives are 

contained in federal specifications and most of these preservatives are included 

also in AWPA Standard P5. 

Treatment Processes 

Wood-preserving methods are of two general types: (1) Pressure processes, 

in which the wood is impregnated in closed vessels under pressures considerably 

above atmospheric pressure; and (2) nonpressure processes, which vary widely 

as to procedures and equipment used. Pressure processes generally provide a 

closer control over preservative retentions and penetrations, and to this extent 

they usually provide greater protection than nonpressure processes. Some of the 

nonpressure methods, however, are better than others and are occasionally as 

effective as pressure processes in providing good preservative retentions and 

penetrations. The Wood Handbook3 describes various pressure and nonpressure 

processes. 

Requirements for Effective Wood Preservation 

To obtain good and effective wood preservation, the user must observe several 

well-recognized principles as follows: 

(1) Make a careful selection of the wood species and form to obtain the 

best possible treatment. 

cation. 

(2) Select the preservative best suited to use requirements. 

(3) Have the wood properly conditioned and machined. 

(4) Select the process of treatment best suited to use requirements. 

(5) Purchase the preservative and treatment under a recognized specifi­

(6) Obtain good inspection and quality control. 

(7) Be certain of proper care and handling of material after treatment. 

3 U.S. Forest Products Laboratory. Wood handbook. U.S. Department of Agriculture, Agriculture 

Handbook 72, 1955. 

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Selection of Species 

For maximum protection with any wood preservative, good penetration of the 

wood is important. The sapwood of most species is more easily penetrated with 

preservatives than is the heartwood. Consequently, with other things equal, 

round timbers with considerable sapwood will be better penetrated than those 

with less sapwood; they will also be penetrated better than sawed timbers 

consisting mostly of heartwood. Southern pines usually have a high percentage 

of sapwood even in sawed lumber and timbers. Douglas-fir lumber, on the other 

hand, is likely to be mostly heartwood. 

Fortunately, a procedure known as “incising” can be used to improve the 

preservative penetration in the heartwood of Coast-type Douglas-fir. Incising 

will also improve this penetration in the heartwood of species such as western 

hemlock, redwood, and several of the hardwoods. The Forest Products Laboratory 

has treated a wide variety of wood species and can often furnish some indication 

of the ease or difficulty with which different woods take treatment. 

The preservative does not kill or inhibit the growth of decay fungi and harmful 

insects unless it is actually present in the wood that is subject to attack. Some 

persons assume that when wood is sterilized during treatment it will be adequately protected by a continuous, although thin, surface coating of the preservative. Investigators in Germany, however, found that fungi penetrated a 

coating of creosote. Termites have been known to nibble on the treated surface 

of superficially penetrated wood, pierce it, and damage the unpenetrated wood 

beneath. Checks, mechanical injury to the wood, and cutting after treatment are 

also likely to expose unpenetrated wood to attack by fungi and insects, particularly if the piece was not deeply penetrated by preservative initially. 

Selection of Preservative 

All preservatives are not equal in their wood-preserving properties, even 

though they may be recognized on an equal basis in treating specifications. 

Results of service tests and of experiments in field and laboratory show which 

preservatives perform best under the particular conditions used in the test. 

The Forest Products Laboratory for many years has conducted laboratory and 

field tests to determine the effectiveness of various wood preservatives and--of 

special interest to farmers--has published service records on posts of 91 species 

of wood treated by 12 processes with 73 wood preservatives. 4 

4

Blew, J. O. Jr., and Kulp, John W. Service records on treated and untreated fence posts. 

U.S. Forest Service Research Note FPL-068, 1964. Forest Products Laboratory, Madison, Wis. 

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Service tests on treated products, such as crossties, poles, and piling, are 

published from year to year in the Proceedings of the A m e r i c a n W o o dPreservers' Association andby several railroad, telephone, and power companies. 

Results of these tests on crossties should indicate the service that might be 

expected from farm timbers when used in contact with the ground under similar 

climatic conditions. In the same way, results of tests on poles and posts should 

furnish some indication of the performance to be expected of round timbers of 

similar species and similarly treated when used in pole-type buildings. Treated 

wood used away from ground contact should generally perform somewhat better 

than posts and poles, although it may be more likely to check. 

Stake tests 5 furnish another good indication of the performance to be expected 

from treated wood in contact with the ground, when results of actual service 

tests are not available. Laboratory accelerated tests on small blocks are 

useful for showing up differences in preservative effectiveness, but they cannot 

yet be accepted as a substitute for service and field tests that more closely 

simulate actual service conditions. 

Conditioning and Machining Wood Before Treatment 

Most wood-preserving companies are interested in doing their job in a way that 

will assure maximum service to their customers. These companies will not 

attempt to treat wood unless it has been seasoned sufficiently to take treatment 

satisfactorily. They will also strongly urge the user to have all machining done 

on the wood prior to treatment, since they recognize that cutting or drilling of 

the wood after it has been treated will expose unpenetrated wood to attack by 

decay fungi and insects. They know, furthermore, that brushing a preservative 

on a cut surface is a makeshift and a poor substitute for wood with the treated 

portions intact and uncut. 

The treating companies are usually equipped to air season, kiln dry, or vapor 

dry wood of different species, to Boultonize (boil under a vacuum) green 

Douglas-fir, and to steam condition southern pine that is not adequately seasoned 

for treatment. Treatment tests 6 at the Forest Products Laboratory showed that, 

when green, Boultonized Douglas-fir and steam-conditioned pine were pressure 

treated with such preservative oils as creosote and pentachlorophenol solution, 

5 Blew, J. O. Jr. Comparison of wood preservatives in stake tests. U.S. Forest Service Research 

Note FPL-02, 1964. Forest Products Laboratory, Madison, Wis. 6 

Blew, J. O. Jr. Study of the preservative treatment of 'lumber. Forest Products Laboratory 

Rpt. 2043, 1955. Reviewed and reaffirmed 1960. 

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results compared favorably with those obtained with air-dry material. In the 

case of southern yellow pine treated with a waterborne preservative such as 

chromated zinc chloride, better results were noted when the material had been 

air seasoned. In this lumber-treating investigation, incising prior to treatment 

resulted in much more uniform penetration of Douglas-fir lumber, and this 

procedure is recommended for other species that resist penetration. 

Selection of the Treating Process 

Good penetration of a wood preservative is important. When the wood is of a 

species readily penetrated, the method by which the preservative is applied will 

determine whether the penetration is to be deep or shallow. In the abovementioned investigation,6 matched unseasoned and seasoned lumber of Douglasfir and shortleaf pine were treated by pressure and by several nonpressure 

processes. In the seasoned sapwood of the shortleaf pine, good preservative 

penetrations were obtained with the more effective nonpressure processes. However, the penetrations resulting from pressure treatment were deeper than those 

from the nonpressure applications in the heartwood of the same seasoned 

material, in the sapwood and heartwood of unseasoned pine, and in the heartwood 

of seasoned and unseasoned Douglas-fir. In general, therefore, and unless the 

wood can be properly conditioned and carefully selected for sapwood, which is 

easy to penetrate, pressure treatment gives better assurance of good penetration 

and good service. This is shown in the service records on posts referred to 

earlier. 4 

There are often instances where pressure treatment may not be the most 

practical process to use. On-the-job applications of preservatives are sometimes 

desirable on the farm Even the brush and dip applications, which are the least 7 

effective, provide some protection to wood, and it may be advantageous to use 

them when more effective methods cannot be used. Such superficial treatments 

have recognized value for sash and similar millwork, which is generally not 

exposed to a high decay hazard. The open-tank or hot-and-cold-bathprocess 

has had long and successful use for western redcedar poles, but the poles must 

be incised at the critical groundline zone to assure adequate penetration of the 

preservative. 

Posts and poles of lodgepole pine have also given long service when carefully 

treated by the hot-and-cold-bathprocess. Treatment of round unseasoned wood 

members, a process developed by the Forest Products Laboratory and known as 

7

Blew, J. O. Jr., and Champion, F. J. Preservative treatment of fence posts and farm timbers. 

U.S. Department of Agriculture Farmers’ Bulletin 2049. Revised 1956. 

FPL-085 -7-
“double diffusion,’’ has been shown to provide good protection to southern pine 

posts,4 There are no shortcuts or miracles in wood preservation, however, and 

the user can expect to gain protection commensurate with the effort, expense, 

and thoroughness involved in making the treatment. 

Purchased and Inspection of Preservative Treatment 

Users of large quantities of treated wood issue their own specifications for 

proper selection and conditioning of wood before it is treated, as well as for 

penetrations and retentions of preservative. However, the user of small quantities can neither develop his own treating specifications nor inspect the treatment of the wood he purchases. To avoid inferior products, therefore, it is 

advisable for the small user to purchase his treated posts, poles, or lumber 

from an established and reputable dealer and to obtain assurance from the 

dealer that the wood is selected and treated in accordance with recognized 

standards, such as Federal Specification TT-W-571, which covers treating 

practices. The American Wood-Preservers’ Association has prepared a standard 

for pressure-treated wood for use on farms2 and approved standards covering 

preservatives, treatments, and inspection procedures. 

Inspection is necessary to determine whether the preservative treatment 

conforms to specification requirements. In cases involving premature deterioration of treated wood, failure to meet specification requirements is most 

commonly the reason. Careful and competent inspection of the material at the 

time of treatment could have resulted in rejection or retreatment of such 

material. For those who are not in a position to inspect the treatment of wood, 

commercial inspection agencies perform this service. Some users, however, 

prefer to obtain a certificate from the treater indicating that the requirements 

of the specification have been met. Reputable treating companies are willing 

to furnish that certification to dealers who purchase treated products for resale 

to farmers. It is the general practice to brand or identify poles with the name of 

the treating company and with symbols indicating the species, size, and class 

of the pole, the preservative used, and the date of treatment. Some companies 

also brand posts and lumber. Many users encourage this practice by requesting 

treated products on which the name of the treating company has been branded. 

Proper Care and Handling of Material After Treatment 

The user of treated wood should realize that complete penetration of the wood 

with preservative is generally the exception rather than the rule. The internal, 

FPL-085 -8-
unpenetrated zone is therefore subject to attack by decay fungi and insects if 

the treated protective shell is broken. It is important that all possible care 

be taken, in handling the treated wood, to avoid breaking this treated shell. The 

use of picks and other sharp-pointed tools should be avoided in handling the 

material. Dropping the material from cars during unloading or other rough 

handling is harmful. Treated material that is stored in uncovered piles exposed 

to the weather is subject to checking, which may expose unpenetrated wood. The 

objectionable results of cutting into the wood after treatment have been referred 

to. 

In farm structures, treated items that are horizontally exposed frequently show 

deep checking after several years of service. Filling these checks with mastictype materials may be more harmful than beneficial, since such materials often 

shrink and pull away from the checks and seldom maintain a tight seal. A filler 

thus shrunken not only permits water to enter but retards its evaporation, 

thereby creating a decay hazard rather than preventing it. The periodic flushing 

of the checks with an oil-type preservative, on the other hand, protects the 

exposed untreated wood. 

Farm Uses for Treated Wood 

Uses for treated wood on the farm vary widely, ranging from posts and poles 

to buildings or truck bodies. Some require the wood to be used in contact with 

the ground or water; others do not. 

Posts and Poles 

Wood fence posts are one. of the most widely used treated items found on 

farms. Pole-type barns and other farm buildings, however, are popular because 

of their low cost, ease of construction, and demonstrated resistance to hurricane 

and flood. Treated wood poles are therefore becoming an important farm item. 

Fence posts are sometimes treated on the farm7 but are more commonly 

treated under such recognized specifications as AWPA Standard C5 and Federal 

Specification TT-W-571, which recommend such preservatives as coal-tar 

creosote, creosote-coal tar solution, creosote-petroleum solution, and a solution 

of 5 percent of pentachlorophenol in a specified petroleum solvent. The recommended minimum retention for these preservatives is 6 pounds per cubic foot 

(or 0.3 pound of pentachlorophenol) with the exception of creosote-petroleum 

solution, for which the recommended minimum retention is 7 pounds per cubic 

foot. For fence posts that will be painted, these specifications recommend the 

standard waterborne preservatives, which leave wood paintable. 

FPL-085 -9-
Poles for buildings are higher in initial cost, are more costly to replace, and 

generally should be expected to give longer service than fence posts. It is therefore recommended that they be treated under specifications for telephone and 

power line poles. Service records on simple on-the-job methods generally 

indicate that they will not provide sufficient protection to poles for use in poletype structures that are expected to last 25 years or longer. Federal Specification TT-W-571 and AWPA Standards C4 and C8 for poles recommend preservative oils, coal-tar creosote, or 5 percent of pentachlorophenol in a specified 

petroleum oil. This federal specification and AWPA Standard C4, for pressure 

treatment require a minimum retention of 8 pounds per cubic foot and higher 

retentions for severe service conditions or where climatic conditions are 

particularly favorable to decay and termite attack. AWPA Standard C8 for the 

nonpressure treatment of western redcedar poles requires that the poles be 

incised and prescribes definite requirements for the retention and penetration 

of preservative. 

Lumber and Timbers for Ground or Water Contact 

In addition to poles and posts, wood is used in numerous farm structures that 

are partially in contact with the ground or water for prolonged periods. Examples 

are arbors, barn sills, bridges, cattle guards, culverts, irrigation structures, 

root cellars, silos, splash boards, troughs, and water tanks. In general such 

preservative oils as coal-tar creosote or pentachlorophenol solution are best 

suited for these uses, although for root cellars and other food storage structures 

the waterborne preservatives are safer. Federal Specification TT-W-571g in 

note 6.1.3 refers to preferred waterborne preservatives for ground and fresh 

water contact where cleanliness, paintability, and freedom from o d o r are 

important requirements. AWPA Standard C2 for lumber recommends a minimum 

retention of 8 pounds of a preservative oil per cubic foot, and Federal Specification TT-W-571 recommends a minimum of 10 pounds per cubic foot for such 

uses. The experience with creosoted silos and tests with pentachlorophenol have 

shown that these preservatives, in the quantities used to treat wood, are not 

harmful to cattle. Recommended minimum retentions for waterborne preservatives will be noted below. 

Wood Uses Not In Contact With Ground or Water 

In items such as fence rails and pickets, fruit storage houses, grain storage 

bins, manure spreaders, outdoor tables, truck bodies, milk houses, and sills 

FPL-085 -10-
and lower joists in buildings, the wood is generally exposed to a limited decay 

and termite hazard. The use of preservative oils may be objectionable because 

of their odor and because they are difficult to paint over. Waterborne preservatives can be used to treat these items, although at times there may be 

advantages in using one of the specially prepared pentachlorophenol solutions 

with a water repellent. In such cases the wood may be pressure treated; or, 

with easily penetrated wood such as dry sapwood of pine, some of the nonpressure applications (such as the vacuum process and methods involving the 

heating-and-cold-bath principle) should provide good protection. When wood is 

treated with pentachlorophenol solutions, the faster evaporating solvents should 

be used; even so, considerable time or special conditioning is required after 

treatment to obtain a paintable or dry surface. 

The pressure treatment of lumber with waterborne preservatives is covered 

by Federal Specification TT-W-571 and AWPA Standard C2. The former includes 

the following waterborne preservatives and recommended minimum retentions: 

8 Preservative Retention 

(Lb. per 

cu. ft.) 

1. Acid copper chromate (Celcure) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.50-1.00 

2. Ammoniacal copper arsenite (Chemonite) . . . . . . . . . . . . . . . . . . . . . . . . .30- .50 

3. Chromated copper arsenate (Greensalt or Erdalith) . . . . . . . . . . .35- .75 

4. Chromated zinc arsenate (Boliden salt) . . . . . . . . . . . . . . . . . . . . . . . . . . . .50-1.00 

5. Chromated zinc chloride (CZC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75-1.00 

6. Copperized chromated zinc chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75-1.00 

7. Osmosar (Osmosalts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35- .50 

8. Tanalith (Wolman salts). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35- .50 

Federal Specification TT-W-571 also provides for pressure treatment of lumber 

with a water-repellent pentachlorophenol preservative and calls for minimum 

retention of 6 pounds per cubic foot. Industry specifications, such as that of the 

Vacuum Preservers’ Institute, cover the treatment of lumber by the vacuum 

process. The treatment of window sash and millwork by dipping or vacuum is 

covered by U.S. Commercial Standard CS 262-63. 

8

The higher retentions are recommended for posts or for lumber that may be required for uses 

that involve moderate leaching. 

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Conclusion 

The experience of the farmer with fence posts has shown that the life of 

untreated wood may be increased from 5 to 10 times through effective wood 

preservation. To obtain effective wood preservation requires careful selection 

and preparation of the wood, care in the choice of the preservative and method 

of application, purchase and inspection under recognized specifications, and 

careful handling of the material after it has been treated. With effective treatment it should be possible for the farmer to reduce replacements on other wood 

structures and thus obtain better service from this material, which has such an 

important part in his operations. 

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