152 THE BOOK OF FORESTRY 



depth of an inch to one and a half inches, it is con- 

 sidered sufficient. 



Piles and paving blocks are being treated extensively 

 at present but the preservation of railroad ties consti- 

 tutes ninety per cent of this industry in the United 

 States. Railroad ties are treated only by the pressure 

 method. The plant needed for such an operation con- 

 sists of several long cylinders from six to eight feet 

 in diameter and about one hundred to one hundred 

 and fifty feet long. The ties to be treated are first 

 thoroughly seasoned so as to remove practically all 

 available moisture, they are then loaded on cars which 

 are run into the cylinder and the doors tightly 

 closed. Live steam is turned into the cylinders under 

 high pressure and after five or six hours the steam 

 and air is pumped out and a vacuum is created 

 which draws air and moisture from the cells of the 

 timber. The preservative, which may be either creo- 

 sote or zinc chloride, is forced into the cylinder and 

 maintained under approximately one hundred pounds 

 pressure until the required amount has been forced into 

 the wood gauges in the tank show how much has 

 been absorbed. Ordinarily for complete impregnation 

 twenty-five to thirty pounds of creosote is needed for 

 each railroad tie. This makes the treatment com- 

 paratively expensive for it may run up to forty-eight 

 cents for each piece, but ordinarily less creosote is used, 

 as there is little extra benefit obtained from the extra 

 material. The average cost of treating a tie is thirty- 

 five cents, while poles and fence posts average one dollar 

 and fifty cents and ten cents respectively. 



After removal of the treated timbers from the 

 cylinders, test holes are generally bored to learn the 

 depth of penetration. Cut surfaces are repainted, as 



