208 FORESTRY AND TIMBER 



hardly satisfactory, owing to the ease with which the protective 

 covering may be broken ; moisture is absorbed after treatment ; and 

 causes the wood to expand and crack the cement. 



Cold-bath treatment. 



This differs from dipping because penetration of the wood is se- 

 cured by leaving the post in the bath for ten hours or more. As a rule, 

 only the cheaper preservatives can profitably be used in the cold-bath 

 treatment. 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 impreg- 

 nated. 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, therefore, is 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 investigated. 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, so designated to distinguish 

 it from the "closed " or " pressure " cylinder process which is often 

 employed in creosoting ties and piling. This consists of heating 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 por- 

 tion of this air and water is forced out. The rest contracts as the sub- 

 sequent 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 treatment 

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

 creosote maintained at a temperature of 220 F. If a single tank is 



