Wood now in useful service is being destroyed in this country by decay and insects at the rate of several billion board feet a year—an amount approaching the normal average used annually in the construction of dwelling units. The enormous drain upon the resources of our forests can be lessened through greater use of preservative-treated wood, one piece of which may do the work of several replacements of untreated wood.

The railroads long ago found that preservative treatments save wood and money. In the early days when most railroad ties were untreated, railroads required for replacements each year approximately 450 ties to the mile; in recent years when a large percentage of all ties in service were treated, they required less than one-third as many replacements to the mile. The same order of savings also applies to telegraph and telephone poles and to poles for other public-utility lines. Ninety percent of all poles now being set in the ground are either fully treated or butt-treated.

Many other wood products that are exposed to decay and insect attack are not so extensively treated. For example, a large part of the 600 million fence posts set yearly are not treated, although it has been shown that many species of wood in post size will last from 3 to 10 times as long when well-treated as when untreated. The serviceable life of still other products would be increased by preservative treatments.

The type of preservative used and the thoroughness with which the wood is treated have much to do with the length of service rendered by the wood. Good preservatives and poor treatments or poor preservatives and good treatments are of little value. The purpose of treating wood with preservatives is to protect it against decay organisms, insects, and marine borers.

Preservatives of various kinds are used to treat various classes of wood products.

Among the wood-preserving oils, coal-tar creosote has long been effective. It has good penetrating properties and will remain in the wood for many years; it is safe to handle, harmless to wood and metal, readily available, and reasonably cheap. It is used mainly on wood that is to be in contact with the soil and water out of doors, and where its odor will be unobjectionable and painting will be unnecessary.

For wood that is used indoors or not in contact with the ground or water outdoors, water-borne preservatives are usually favored. Among these are zinc chloride, chromated zinc chloride, and several proprietary preservatives consisting of various mixtures of compounds of arsenic, chromium, copper, or fluorine, all of which leave the wood in a paintable condition.

Still other preservatives, such as pentachlorophenol, that are soluble in volatile oils, when they are so used, provide clean, odorless, readily paintable, treated wood suitable for interior or exterior use.

Many proprietary preservatives of undisclosed composition are on the market. Some of them are good, but others have little value. A good precaution to take before accepting any proprietary preservative is to have the manufacturer state the ingredients used in his products.

PREPARING THE WOOD FOR TREATMENT is necessary for satisfactory results with any treating process.

In a few methods the wood is treated green and sometimes with the bark on, but usually it should be well peeled, and, for best results with most processes, seasoned. Because preservatives will not make weak timber strong or restore the strength of timber that has been partly destroyed by decay, only sound timber should be used. The cutting, boring, and framing of the wood should be completed before treatment, if possible, to avoid the exposure of untreated surfaces that often results when cutting is delayed until after the treatment.

Wood is treated by both pressure or nonpressure processes, although the bulk of wood treated is given a pressure treatment. For most uses, wood that has been treated under pressure gives the best service. Such treatments require closed cylinders with vacuum, pressure, and heating facilities.

A number of pressure processes differ from one another in a few details, but the general principle is the same in all. The wood, placed on steel cars, is run into a long steel cylinder, which is then tightly closed and filled with preservative. The wood may be steamed or otherwise heated to reduce its moisture content and be subjected either to a vacuum or to an initial air pressure before the preservative is admitted to the cylinder. By a proper use of pressure and heat, the preservative is forced into the wood until it has absorbed the desired amount. In most kinds of wood, this results in a relatively deep penetration. This treatment, when properly made with a good preservative, should add from 20 to 30 years to the life of untreated wood for most uses. About 200 pressure-treating plants are in operation at various points in the United States.

Of the nonpressure methods, the hot-and-cold bath method is the most effective. It involves less equipment than pressure processes and is better adapted to the small commercial and home-use treatments, such as those recommended for fence posts. This method requires either one or two open treating tanks. In the one-tank method, the wood is first heated and then cooled in the same treating solution. The wood may also be heated in air, steam, or other media, and then placed in the tank of cold preservative. In the two-tank process, the wood is heated in a tank of hot preservative and then quickly transferred to the other tank, where it is submerged in cold preservative and allowed to cool. In all cases the heating and subsequent cooling creates a partial vacuum within the wood that aids penetration of the preservative. The treatment, when properly made with a good preservative, should increase the serviceable life of the wood by many years. With equal penetration and absorption of the preservative, pressure- and nonpressuretreated wood should be equal in serviceability.

Other nonpressure methods of more recent development are the cold-soaking of seasoned timbers in oil solutions of pentachlorophenol and other oil-soluble preservatives and the steeping of green or freshly cut timbers in waterborne preservatives. A single tank or container for the preservative is sufficient, but soaking treatments usually require a longer period to get the wood well-treated than when the hot-and-cold bath process is employed. When thoroughly done, these soaking methods add appreciably to the serviceable life of wood.

Brushing or spraying a preservative on wood adds only about 1 to 3 years to its serviceable life. Applied in this way, the preservative does not penetrate the wood deeply enough to form an effective barrier to wood-destroying organisms, termites, or borers, so that only a limited degree of protection can be expected.

For many years the Forest Products Laboratory has conducted tests on preservative treatments and maintained service records on treated and untreated fence posts, poles, railroad ties, and other forms of timber subject to decay and insect attack, often in cooperation with farmers, railroads, industrial concerns, experiment stations, and the national forests.

Service records have shown the good natural durability of the heartwood of such species as cedars, baldcypress, chestnut, black locust, Osage-orange, and the redwood, and the nondurable properties of the sapwood of all species, and the heartwood of many, unless protected by a preservative treatment. They have also shown differences in the value of various preservatives and methods of treatment, which provide the basis for treating specifications on which the wood-preservation industry is largely based.