UNITED STATES DEPARTMENT OF AGRICULTURE · FOREST SERVICE · FOREST PRODUCTS LABORATORY · MADISON, WIS.

October 1963 FPL-013

HOW TO TREAT FENCE POSTS BY DOUBLE DIFFUSION

B y

R. H. BAECHLER, Chemist

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Preservative treatment has long been recognized as of great value in extending

the average servicelife of fence posts and thereby greatly reducing the labor

and other costs of fence maintenance --especially in farming areas where

naturally decay-resistant species are not readily available.

Pressure-treated posts usually give excellent service and can be purchased at

reasonable prices in many areas. They offer a very satisfactory answer to the

question asked by many farmers: "How can I cut down on the time needed each

year to keep my fences up?"

In areas where the cost of pressure-treated posts is prohibitive, for one reason or another, the farmer himself can apply preservatives that will add

greatly to the life of the posts.

"Preservative Treatment of Fence Posts and Farm Timbers," which may be

A number of methods may be used for treating posts on a farm. They are described in the U. S. Department of Agriculture Farmers' Bulletin No. 2049,

obtained without cost by writing to the U.S. Forest Products Laboratory,

Madison, Wisconsin 53705. A number of State forestry departments and forestry colleges also issue pamphlets on the preservative treatment of posts.

The Forestry Division of the Tennessee Valley Authority, Norris, Tenn., is

a further source of information on this subject.

One of the newer methods of treating wood by means of simple equipment is

the double-diffusion process developed at the U.S. Forest Products Laboratory.

This is a slight revision of FPL Report No. 1955, originally dated October

1953 and revised July 1961.
This process consists of soaking green wood first in one chemical solution and

then in a second solution. The two chemicals diffuse into the wet wood and then

react with each other to form a compound that is poisonous to termites and to

fungi that cause rot. Moreover, the compound formed in the wood is practically insoluble in water and therefore does not leach to any great extent from

wood that is used in moist soil.

One way in which the treatment may be applied to posts, with excellent results,

is to immerse them completely in solutions held in tanks. One hundred treated

pine posts in a Forest Products Laboratory test plot in Mississippi have shown

only five failures after about 22 years. Untreated pine posts gave an average

life of about 3 years. This appears to be a practical method for commercial

operations, especially with mechanical peeling and handling of the wood. The

method is used at a number of small post-treating plants. Because of the

initial cost of the tanks, it does not appear practical for meeting the fence

post requirements of a small farm. Anyone interested in more information

about this method is invited to write to the Forest Products Laboratory.

The farmer who wishes to treat a limited number of posts--say 100 to several

hundred--for his own use can treat them upright in barrels, thereby limiting

his equipment costs.

Field tests on posts treated in barrels by this method are not yet old enough to

show the average service life that can be obtained by following some particular

treating schedule. Indications are, however, that the service life of pine posts

can be increased at least five times by the procedure to be described in this

pamphlet. Some pine posts treated in a similar manner and installed in the

Mississippi test plot have shown no failures after 12 years. In tests made so

far, the results on hardwood posts have been less favorable than on pine; some

hardwood species are quite receptive to this treatment, but others treat poorly.

Chemicals and Equipment Needed

Several combinations of chemicals may be used in this treatment. This report

gives instructions for using two chemicals that are more commonly carried in

stock by chemical dealers than are the other chemicals that have been used in

experiments on this method.

About 25 pounds of technical copper sulfate and 12-1/2 pounds of technical

sodium fluoride will be needed for each 100 posts, depending upon the size of

the posts. Local farm or hardware stores may not have the necessary chemicals, but can order them from chemical companies. A partial list of companies handling these chemicals may be obtained from the Forest Products

Laboratory.

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The cost of chemicals per post treated varies with the size of the post, and

depends upon shipping costs and the amount ordered. The chemicals, in

powder or crystal form, come in paper bags or cartons, which minimize shipping costs. In some localities, when sufficient chemicals are purchased to

treat several hundred posts, the cost of chemicals for treating 4-inch posts

6-1/2 feet long may be as low as 10 cents per post. In other areas and with

smaller purchases, the cost may be three times as high. Posts of small

diameter cost less to treat and are easier to handle. The treatment of posts

that are just large enough for strength requirements is therefore recommended.

The chemicals can be weighed on a scale or measured with a l-pound coffee

can, as explained later. A 10-quart pail is convenient for measuring water.

Two containers are needed to hold the separate solutions. A wood barrel is

best for the copper sulfate solution since this solution is corrosive to iron. If

a steel drum is used, such as an empty oil drum with the head removed, it

should first be coated inside and out with some corrosion-resistant material,

such as roofing pitch. Care must be taken to avoid damaging the coating when

the posts are placed in or taken out of the drum. Either a wood barrel or an

empty oil drum without a protective coating is suitable for the sodium fluoride

solution.

Because a certain amount of preparation is needed and some solution is wasted,

it is generally not practical to treat less than 100 posts by this method. Farmers

who do not need many posts might plan to treat enough posts to take care of

their needs for several years, since the treated posts can be stored without

danger of decay. Another plan is to cooperate

jointly.

Precautions

Because the solutions used in double-diffusion

with neighbors in treating posts

treatments are harmful when

taken internally, they should be handled with care. Special care should be taken

to see that children do not eat the chemicals or drink the solutions. Animals

should be kept from solutions. Poultry may pick up crystals or particles soaked

with the solutions.

The solutions, especially the copper sulfate solution, are somewhat irritating

to the skin and can cause sores where the skin is broken. It is best to wear

rubber gloves when working with these solutions and to wash off any solution

or dry chemical that gets on the hands (fig. 2). The solutions do not give off

poisonous vapors, but dust arising from handling the dry chemicals should not

be inhaled. Goggles or cheap sun glasses may be worn to protect the eyes

(fig. 2). If solution is splashed into the eyes, they should at once be washed

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with plenty of water for several minutes. To prevent possible permanent

injury to the eyes, see a doctor as soon as possible.

If the chemicals or their solutions get on clothes, they should be rinsed off

with clear water. The clothes should not be washed with the regular laundry,

because the chemicals may affect the color of other garments.

Preparation of Posts

Treatment should be started as soon as possible after the posts are cut--preferably within a week. Unless treatment is started within several days of

cutting, the posts should be cut about 6 inches longer than needed to allow for

trimming just before they are treated. They should then be hauled to the

place where they will be treated and piled as close together as possible (like

cordwood) to keep them from drying out.

Just before they are placed in the first solution, the posts should be peeled.

Then, if the posts were cut more than several days earlier, about 6 inches

of wood should be cut from the butt (large) end of each post to expose a fresh

surface.

Preparation of Solutions

The sodium fluoride solution required is prepared as follows: Add approximately 26 gallons of water to the container. In a standard 55-gallon oil drum,

the water will be about 15 inches deep. Then add 9 pounds (about 3-1/2 coffee

cans) of sodium fluoride and stir for several minutes. A little chemical may

remain undissolved and settle to the bottom, which

The copper sulfate solution is prepared as follows:

24 gallons of water to the container. In a standard

the water will be about 16 inches deep. Next add 18

does no harm.

First add approximately

50-gallon wood barrel,

pounds (about six and

one-half l-pound coffee cans) of copper sulfate crystals. Stir until the crystals

dissolve. Less stirring is required if the crystals are added to the water the day

before the solution is to be used.

Treatment

The posts are first treated in sodium fluoride solution;

solution. Stand the peeled green posts butt end down in

then in copper sulfate

the sodium fluoride

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solution. Add enough po sts so that the solution rises nearly to the top, or

at least to within 4 inches of the top of the container. In this way the groundline area of the posts is surrounded by liquid. If crooked posts are used, fewer

will go into the barrel, and the solution may not rise high enough. In that

case, prepare extra solution by adding about one-third of a coffee can of dry

sodium fluoride to a 10-quart pail of water and use this solution to fill the

barrel.

After the posts have stood in the sodium fluoride solution for 3 days, remove

them and stand them butt down in the copper sulfate solution. If the solution

in this barrel does not rise to within 4 inches of the top, prepare extra

solution by adding about 2/3 of a coffee can of copper sulfate crystals to a

10-quart, enamel-lined pail and fill with water. After 3 days, remove the

posts and rinse them off with clear water.

Before another set of posts can be treated, more water and chemical must be

added to the containers. Add enough water to bring the sodium fluoride

solution to its original depth. Then add 1-1/2 pounds (a generously half-full

coffee can) of sodium fluoride, and stir until the chemical is dissolved. In

the same way, add water to bring the copper sulfate solution to its original

depth. Then add 3 pounds (1 heaping coffee can) of copper sulfate crystals

and stir.

The solutions should be added to in this way before each additional set of posts

is treated. When the last three sets of posts are to be treated, it is not

necessary to add more water and chemical, but the posts should be soaked

longer. For the second-to-the-last set, for instance, keep the posts 4 days in

each solution. For the next-to-the-last set, keep the posts 5 days in each

solution. Let the last set remain in each solution for at least 7 days. In this

way, most of the chemical is taken up by the posts and not much is wasted.

After Treatment Handling

It is well to rinse the posts after treatment so that they

without gloves.

can be handled safely

Although the posts may be used at once, it is better to close-pile them for

several weeks like cordwood to keep them from drying out. This "rest" period

helps to distribute the chemicals more evenly throughout the posts.

To get rid of the remaining solutions, dump them into a hole some distance

from wells or livestock ponds and, after they have seeped away, fill the hole

with dirt.

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Figure 1. -- All of the equipment needed to treat fence posts by

double diffusion is shown here.
Figure 2. -- When mixing the chemical solutions it is wise to

wear glasses and rubber gloves.