How Decay of Wood Is Prevented

HOW DECAY OF WOOD IS PREVENTED Decay of timber is prevented by treating it with antiseptics, or substances which are poisonous to bacteria and fungi. There are, of course, many such substances; but practical considerations make only a few of them suitable for commercial use. One of the first essentials of a good wood preservative is that it shall not dissolve out when the wood gets wet or is placed in water. For this purpose the best material so far discovered is creosote, a complex product of the distillation of coal tar. For comparatively dry situations, zinc chloride is a cheap and effective preservative; but it cannot be used for the treatment of timbers which are placed in water or in wet situations, because it leaches out quickly. Many experiments have been and are being made with various oils and distillation products; and, no doubt, other wood preservatives will be developed.

Paint lengthens the life of wood because it keeps out moisture and closes openings through which fungi might enter; but it is essential that wood be well seasoned before it is painted.

The rapid growth of the timber-treating industry may be judged from the fact that the first successful wood-preserving plants in the United States were built about 1870. In 1904 there were 35 such plants; and at present there are more than 90, with an annual output in excess of 125 million cubic feet of treated timber, of which by far the larger portion consists of railway ties and telegraph and telephone poles.

How Preservatives Are Applied There are three general methods of applying wood preservatives—the brush method, the pressure method, and the open-tank method.

Brush Method.

The brush method consists in applying the preservative with a' brush in the same manner as paint. It is the least effective method, because of the very slight penetration obtained. It is useful, however, in cases where the preservatives cannot be forced into the wood, in painting the joints in timbers, the bottom of barges, etc.

Pressure Method.

In the pressure process, the general features are practically the same, irrespective of the kind of preservative used.

The timber to be treated is placed upon small cars, and run into large steel cylinders that are fitted with swinging doors. When the wood is in the cylinder, the doors are bolted fast, and the whole made practically air-tight. Saturated steam is then forced into the cylinder; and the wood is heated for five to fifteen hours, depending chiefly upon the amount of moisture it contains. It is claimed that by this steaming process the sap in the wood is heated and all the germs of decay destroyed. At the conclusion of the steaming, a powerful vacuum is applied, and held for one to three hours. This vacuum draws out the moisture and sap in the wood, and leaves it in a condition ready for the reception of the preservative. As soon as the moisture has been withdrawn, a valve is opened, and the preservative material is permitted to flow in. When the cylinder is completely filled with the preservative solution, force pumps are started and pressure applied until the gages indicate that the amount of solution required has been absorbed by the wood. The liquid is then run out of the cylinder, the doors opened, and the treated material removed.

The pressure method is the one in general use throughout the country for treating timber thoroughly and on a large scale.

Open-Tank Method. A

plant for the treating of timber by the pressure process is expensive, and can be erected only by firms of considerable capital. In order to devise means whereby the smaller sizes of timber, and especially posts, can be cheaply treated, the Forest Service has for many years been experimenting with what is known as the "open-tank" method.

The theory of this method of treatment is as follows: All wood is of a more or less porous nature, and contains a considerable amount of air. When placed in hot oil, for example, and heated, a part of the air and moisture contained in the wood is driven out. If the wood, while still hot, is plunged quickly into a bath of cold liquid, the small amount of air and moisture remaining in the wood will contract, and in so doing draw in the liquid.