FIGURE 5.-Chemically treated lumber used for stadium seats.

Approximately 100,000 board feet of treated wood was used in the construction of these stands

for protection against decay and insects, can not properly be classed as a fireproofing material. Its fire-retardant nature adds to its value for treatment of mine timbers, and it is being used to-day in considerable quantities by mining companies for this purpose.

Excep

FIGURE 6. This arbor and gate are constructed of chemically treated wood. tional service may be expected of out-door garden furniture, trellises, etc., given preservative treatment

Some State highway departments are using zinc chloride treated guard rails and posts which are thoroughly dried after treatment and painted white for better visibility.

Generally an absorption of three-fourths or 1 pound of zinc chloride per cubic foot of wood is considered good practice for building construction.

OTHER PRESERVATIVES

Numerous other oils and chemicals have been experimented with and highly advertised as wood preservatives. Scarcely a year passes but some new material is launched and claimed by its promoter to possess all the desirable qualities of previously tried materials, plus a goodly number of new ones. Some of these are corrosive to treating-plant equipment; some are injurious to the wood; some are too poisonous for safe handling; some are ineffective.

While a few of these new compounds show promise of possessing valuable properties as wood preservatives they have not been in use long enough to demonstrate their effectiveness over long periods of time in this country.

It is not necessary for the purposes of this pamphlet to discuss all of the compounds suggested for wood preservation purposes, but those mentioned below are being used to some extent.

NONPATENTED PRESERVATIVES

Mercuric chloride is undoubtedly very effective in prolonging the life of wood, but its extremely poisonous character and its corrosiveness have militated against it. It is used to an appreciable extent in nonpressure treatments in Europe.

Sodium fluoride in mixtures with other chemicals has been used rather extensively in Europe, especially for preserving mine timbers. In the United States it has been used alone, in experiments with railway ties and in mine ties and timbers, since 1914. The evidence thus far available indicates that it is a good wood preservative. Its advantages and disadvantages are for the most part similar to those of zinc chloride. It constitutes an important and valuable ingredient in several proprietary or patented preservatives.

PATENTED PRESERVATIVES

One proprietary compound being used in central Europe and in the United States to a limited extent and sold under the name of Aczol, consists of copper, zinc, and phenol, dissolved in ammonia and water. A 6 per cent solution of this mixture is used, and the wood is treated to refusal; that is, until no more solution can be forced into it. Its use is limited to the treatment of air-seasoned wood, since the mixture can not be heated. The relative value of this material for preserving wood as compared with other preservatives has not been definitely established. Fluoride-phenol salt mixtures are represented by Wolman salts and are also being used for treating wood. They are patented products composed of sodium fluoride to which other chemicals have been added. The large percentage of sodium fluoride contained in these products would indicate that they have preservative value. While they have not been in use for a great length of time in the United States, it is generally understood they have been used in Germany for some time, principally for the treatment of mine timber and to some extent for ties, and have given good service. An absorption of about three-tenths pound of the salt per cubic foot is recommended by the producers. Zinc-meta-arsenite is a patented preservative developed by the Western Union Telegraph Co. It has been promoted for wood preservation purposes in

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the United States within the last few years. Extensive tests are now being conducted with this material; and while it appears to possess considerable preservative value, its effectiveness compared with other proven preservatives can not be accurately determined until sufficient time has elapsed to complete the actual service records. An absorption of one-fourth pound of the salt per cubic foot is recommended by the inventors.

The following comments on the comparative value of proprietary preservatives and zinc chloride and creosote are quoted from the Forest Products Laboratory publication, Wood Preservatives, dated February, 1929:

There are many preservatives sold under trade names of various kinds. Some of them are ordinary coal-tar creosote or coal-tar creosote which has been modified somewhat by taking out some of the solid ingredients. Some of them have had the lighter fractions removed, making them higher boiling than the ordinary run of creosote. In the main, preservatives thus derived from coal-tar creosote are good preservatives and may be used with assurance. Whether it is economical to use them depends upon their convenience and their cost per year of service as compared to coal-tar creosote or zinc chloride.

There are other proprietary preservatives containing wood-tar products or other oils. The value of these is not so well established but no doubt some of them have value. In a third group are preservatives made of water soluble salts in various mixtures. Whether or not these are good depends upon the material of which they are composed. Before buying a preservative of this kind the purchaser should insist upon knowing its ingredients and their proportions. It is unnecessary and unwise to invest money in secret preservatives. The literature exploiting some of the proprietary preservatives not infrequently contains extravagant claims as to their properties and their effectiveness. Obviously, such claims should be greatly discounted. As a general rule, it may be said that there are very few, if any, proprietary preservatives, regardless of price, that have shown themselves to be better or more generally satisfactory than straight coal-tar creosote and zinc chloride in their respective classes.

METHODS OF TREATMENT

The selection of the proper method of treatment is quite as important as the selection of the proper preservative. The use of the best preservative known will not increase the life of the wood appreciably if it is not properly applied, and the increase in life obtained from its use is in proportion to the thoroughness with which the treatment has been made. Methods of treatment are classified under two distinct heads: (1) Pressure treatment and (2) nonpressure

treatment.

PRESSURE TREATMENT

The superiority of pressure-treated lumber as compared with nonpressure-treated lumber has been established definitely by experience covering a long period of years, and lumber for construction purposes should be pressure treated to obtain maximum service against decay and insect attack.

Pressure treatment consists of impregnating the wood with preservatives by means of pressure. (Fig. 7.) In this way, the preservative may be forced into the wood until all of the sapwood and as much of the heartwood as possible have been penetrated. If only the outer surface of the wood is coated with the preservative, or if it penetrates to only a very slight depth, a great increase in life will not be obtained, since a check or crack in the surface of the

wood can expose the untreated portion and permit the entrance of decay or insects. Since by the pressure process it is possible to vary both the quantity of preservative and the depth of penetration, the greatest economy in the use of materials is thus permitted.

There are two distinct methods of treating wood by pressure. They are: (1) The full-cell processes and (2) the empty-cell processes.

FULL-CELL PROCESSES

The object of the full-cell process is to fill the wood as completely as possible with the preservative. The process is divided into three classes, named for the inventors: The Bethell process, using coal-tar creosote; the Burnett process, using zinc chloride; and the Card process, using a mixture of coal-tar creosote and zinc chloride.

Bethell process.-In this process the seasoned timber is placed in the retort, a vacuum drawn, and without readmitting air the cylinder is completely filled with the preservative fluid. After the retort is filled additional preservative is forced into the cylinder by means of pressure pumps, the pressure being gradually raised to and maintained at 125 to 175 pounds per square inch, with a temperature of from 165° to 200° F., until the required amount of creosote has been forced into the wood. The pressure is then released, the preservative drawn from the cylinder, and a vacuum applied to free the timber from dripping preservative. About 12 pounds of creosote per cubic foot of wood is considered standard practice in this process, unless the wood is to be placed where it is subject to unusually severe conditions.

Burnett process. In this process after drawing a vacuum, a 2 to 5 per cent solution of zinc chloride is run into the cylinder without readmitting air, until the cylinder is filled, after which more preservative is forced into the cylinder and timber by pressure pumps. The