Next it is subjected to a high vacuum, in order to draw out the air, moisture, and solids. Oil is then allowed to flow into the cylinder, and is forced into the wood under heavy pressure until the required amount has been injected. Whether long steaming is of any great advantage, even in the treatment of woods of dense structure, and whether the vacuum really draws out the supposed moisture and solids is a much-debated question. It has been shown by experiment that unless the wood is very green or water-soaked at the time of treatment it is increased in weight after the steam and vacuum process. Hence it would seem that by such treatment moisture is absorbed instead of lost. The Forest Service has made a special study to devise a cheap and efficient method for the preservative treatment of timber. As a result of many experiments the "open tank" method has been evolved. In the case of piling and other large timbers which are to be impregnated throughout their length this consists in subjecting them to alternate baths of hot and cold creosote in a horizontal closed cylinder. (See fig. 5.) The piling should be air-seasoned for varying periods, depending upon the species of timber and the climate. It is then placed upon iron cars and is run into the cylinder. The door is closed, and the cylinder is filled with creosote from a large storage tank. By means of steam coils arranged along the bottom of the cylinder the creosote is heated to a temperature of from 200° to 230° F. and is held at that point for a period depending upon the species and condition of the timber. If the timber is thoroughly airseasoned, from three to four hours is sufficient, but if it has a higher moisture content a longer period may be necessary. At the conclusion of the hot bath the creosote is allowed to run from the cylinder into an underground receiving tank. At the same time a fresh supply of cold oil is admitted into the cylinder from the storage tank. The timber is allowed to remain in this for a short period, and after the oil is allowed to run into the receiving tank, from which the hot oil has meanwhile been pumped back into the large storage tank, the cylinder is opened and the cars of treated piling withdrawn by means of a cable operated by an engine placed a short distance from the cylinder. The cylinder is then available for another charge. During the hot bath the temperature of the outer layers of wood is raised sufficiently to expand the air in the wood cells and intercellular spaces. This escapes through the creosote. It is probable that in imperfectly seasoned timber some moisture is expanded and driven out, but wood is such a poor conductor of heat that appreciable evaporation of moisture takes place only in the extreme outer layers when the wood is subjected to safe temperatures. For every degree of heat below its boiling point the expansion of water is much less than that of air, consequently a slight increase in temperature in the interior of the stick causes the air to be expanded and expelled to a far greater extent than the moisture. During the cold bath the temperature of the timber is lowered, with a consequent contraction of the air and moisture it contains. A partial vacuum is thus formed which permits the preservative to be forced into the timber under atmospheric pressure. In this way are secured results similar to those obtained by the artificial pressure of other processes. By a proper application of this method all pile timbers having wide sapwood, such as loblolly and shortleaf pines and the gums, can successfully be preserved against the attacks of marine borers. A very thorough and even penetration of the creosote can be secured without danger of exposing the timber to excessive temperatures which might affect its strength. The necessary apparatus can be installed at a cost less than one-fourth of that of a pressure-cylinder plant of the same daily capacity. In the treatment of the denser timbers, such as Douglas fir and longleaf pine, in which there is a comparatively small amount of sapwood, it is a question whether the atmospheric pressure developed in the open tank is sufficient to secure a deep and uniform penetration of creosote. But with the class of timber mentioned above there is no doubt. CONCLUSION. The use of properly creosoted material for piling should be extended. The denser timbers should never be treated for piling, because of the difficulty of securing a satisfactory penetration of the oil. On the other hand, timbers of open grain, like loblolly pine, are easily penetrated, and embody all of the characteristics of an ideal pile timber. Their use should therefore be encouraged. A uniform and satisfactory penetration can be secured with such timbers if properly air-seasoned before treatment. Since this can be accomplished with an entire elimination of the expensive "pressure-cylinder" process, 50 per cent or more is saved in the time required for treatment, and thus, even with the expense of holding the material for air seasoning, the ultimate cost is much reduced. Approved: JAMES WILSON, Secretary of Agriculture. WASHINGTON, D. C., October 2, 1907. [Cir. 128] Ο |