Our national forests

by Richard H. D. Boerker

A good deal of attention is given to testing the strength of woods grown in the United States, as a means of assisting users to select the species best adapted to a given purpose, or to find substitutes for species which are becoming difficult to obtain. The strength of a good many species used for structural timbers has been tested. The species most used for this class of timber are the Southern pines, Douglas fir, Norway pine, Tamarack, and Red spruce. An important discovery was made several years ago that Western hemlock, generally considered an inferior timber, showed an average strength 88 per cent. as great as that of Douglas fir, one of the best construction timbers in the United States. Strength tests have also been made on fire-killed timber and these have shown that timber killed by fire is almost as strong as green timber. Other tests have been made to determine the effect of preservative treatment upon the strength of timber. As a result of the large number and variety of strength tests carried on by this Laboratory the United States Government now has a more thorough and comprehensive collection of data on the mechanical properties of wood than any other nation.

Many studies are also conducted to determine the physical properties and the structure of the different kinds of wood grown in this country. The minute structure of the wood of many of our native species has been studied by means of microscopic slides. A study has also been made of a large number of species to determine the specific gravity of the actual wood substance. Other tests are made to determine the specific heat of woods.

The drying or seasoning of woods, more especially of certain species which have been found difficult to season, has received a good deal of attention. A new type of kiln, invented by a Forest Service man, has been devised to season such woods as the eucalyptus, which has always been very difficult to handle in drying. Western larch has been seasoned with a loss of only 5 per cent., whereas the loss in ordinary commercial kilns usually ran between 60 and 70 per cent. As a result, many manufacturers have remodeled their old kilns to embody the new Forest Service methods. A new method has also been developed for the rapid dry-kilning of Eastern hemlock, which has great commercial possibilities.

Experiments in wood preservation have to do with the kind of preservatives it is best to use, the character of the wood to be treated, and the methods of injection. Experiments have developed the best methods for treating railroad ties, mine timbers, fence posts, wood paving blocks, telephone and telegraph poles, and wharf piling. Untreated mine timbers have been found to last only from 1 to 2 years, while treated ones are usually entirely sound at the end of 4 years. Untreated railroad ties last from 5 to 10 years, while treated ones will last over 15. Such experiments as these have shown the advisability of treating all kinds of timbers with creosote or zinc chloride, or some other preservative. Many new preservatives are being proposed or marketed each year by various companies or individuals. These are all tested to determine their value to prevent the growth of fungi in the wood. Their efficiency varies greatly and many of them have been shown to have very small value.

Studies in wood distillation seek to find new woods which can be used for this industry, new and more efficient methods which can be employed, and new uses for wood waste and stumps. Charcoal, wood alcohol, acetate of lime, and tar are derived from the distillation of such woods as beech, birch, and maple, to which tar oils and turpentine are added for the pines and other resinous woods. These by-products of wood distillation have many uses, as well as the many products which are, in turn, made from these by-products. Charcoal is used in the manufacture of black powder, acetic acid is used in the manufacture of explosives, and wood alcohol is converted into formaldehyde for disinfection against contagious diseases. By means of temperature control methods developed at the Laboratory in the destructive distillation of hardwoods, the net gain per annum of one company's plant was over $17,000. About one-half of the plants of the country have adopted the new method developed by the Forest Products Laboratory.

Experiments have been conducted by the Laboratory in the distillation of the needles of coniferous trees and the distillation of the crude gum of some of the important timber trees of the South and West. The oils distilled from many trees in this way have found great use for various purposes. Shoeblacking owes its peculiar aromatic odor, faintly suggestive of the deep spruce and hemlock woods, to an oil which is distilled from these same kind of needles. Evergreen tree leaf oils are used for the perfume of soap, and in the manufacture of liniments, insecticides, and medicinal preparations.

Investigations have been carried on at the Forest Products Laboratory in making artificial silk from sawdust. The industry has already attained considerable proportions. It consists principally of converting cellulose into viscose, which, in turn, is manufactured into an almost endless number and variety of silk and other goods varying from sausage casings to silk hose and tapestries. Sawdust is used also in the manufacture of inlaid linoleum and dynamite.

Experiments in naval stores are attempting to improve the old methods of harvesting turpentine, which have proven very destructive to the forests. With the approaching exhaustion of the Southern Pinery as a field for the naval stores industry, it has become more and more important to find other species for this purpose. Consequently the Laboratory has conducted experiments with the various pines on the National Forests in California, Colorado, Arizona, and New Mexico.

Figure 32. A plank of Incense cedar affected by a disease known as "pin rot." By cutting the cedar timber when it is mature this can be largely avoided. Lassen National Forest, California. Photo by the author.