FORESTRY AND CONSERVATION 399 



The treatments thus far considered merely coat the surface 

 with a preservative. Penetration or "impregnation" of some 

 woods is secured by prolonged soaking (a ten-hour bath) in the pre- 

 servative at ordinary air temperature. This method of treatment 

 is attractive in many cases, because it is so simple. Nevertheless, far 

 betters results are obtained in all cases by the use of a hot bath as 

 well as a cold bath. As a rule only the cheaper preservatives can 

 profitably be used in the cold-bath treatment, for the same reason as 

 in dipping. Coal tar is so ropy and sticky that it will scarcely pene- 

 trate even the most easily treated woods. Crude petroleum enters 

 the wood rather readily but lacks strong antiseptic qualities. A long 

 bath in crude petroleum may, however, prove a feasible method of 

 treatment where petroleum is very cheap and the woods used are 

 readily impregnated. Creosote is usually the best preservative to 

 employ. Coal-tar creosote requires a slight heating to liquefy it. 

 Water in the wood cells resists the penetration of the oil. Thorough 

 seasoning before treatment is, therefore, necessary to allow the oil 

 to penetrate readily and to prevent checking after treatment. The 

 cold-bath method of treatment has not yet been thoroughly inves- 

 tigated. It is probable, however, that it will impregnate but few 

 woods. The woods which are likely to prove most suitable are 

 beech, cottonwood, the gums, pin and red oaks, the pines, sycamore, 

 and tulip tree. 



Impregnation With Creosote. The impregnation of fence posts 

 with creosote is best accomplished by the so-called "open-tank" 

 process. This consists of heating the wood for a certain period and 

 then cooling it in the preservative. The principle is simple : During 

 the heating the high temperature causes the air and water contained 

 in the wood cells to expand, so that a portion of this air and water 

 is forced out. The rest contracts as the subsequent cooling progresses, 

 and a partial vacuum is formed, into which atmospheric pressure 

 forces the cool preservative. 



The open-tank principle may be variously applied in the treat- 

 ment of posts. The best way to heat the posts is to immerse their 

 butts in creosote maintained at a temperature of 220 F. It a single 

 tank is used the cooling bath may be given by permitting the tem- 

 perature to fall, and in this case the preservative must, of course, be 

 used for the hot bath. It is better, however, to employ an additional 

 tank containing the cold preservative. If two tanks are used and a 

 thorough impregnation of the top of the post is desired, the cold- 

 bath tank should be large enough to permit the soaking of the entire 

 post. The top of the post will not be too heavily impregnated, 

 because it has not been immersed in the hot oil. Creosote is usually 

 the most satisfactory preservative. With two tanks crude petroleum 

 or any heavy (high-boiling) oil may be used in the hot-bath tank. 



Preparation of the Posts for Treatment. For treatment with 

 creosote some importance attaches to the size and the form of the 

 posts. The removal of the bark and the seasoning of the wood before 

 treatment are essential for success. At present many posts of large 

 size are used in order that the strength of the fence may still be sum* 



