186 FOREST UTILIZATION TIMBER PRESERVATION 



COMMODITY CUBIC FEET TREATED, 1933 



Cross ties 68,089,695 



Switch ties 5,430,278 



Poles 30,120,834 



Cross arms 313,440 



Paving blocks 388,537 



Constructions timbers 12,156,752 



Miscellaneous material 3,219,326 



All timbers used for bridges, mines, jetties, wharves, mill construc- 

 tion, foundations, and other forms of exterior construction should be 

 treated to increase the length of service. 



Approximately 90% of all cross ties and 80% of all poles purchased 

 are treated with preservatives. 



3. REQUIREMENTS OF A GOOD PRESERVATIVE 



The requirements of a good and effective preservative are as fol- 

 lows: 



1. Available in large quantities, and therefore reasonably inex- 

 pensive. 



2. Toxic or poisonous to wood-decaying fungi. 



3. Must not readily evaporate or leach out of the wood. 



4. It must not affect the strength of timber in service. 



Creosote fulfils these purposes as the best medium. Zinc chloride 

 is also widely used in arid regions where it is not likely to leach out 

 of the wood. Painting also serves as a good superficial treatment. 

 Copper sulphate, mercuric chloride, and several patent chemicals have 

 also been used. Creosote oil is the product of the distillation of either 

 coal, wood, or water-gas tars. 



4. PRINCIPAL METHODS OF PRESERVATION 



The principal methods used in timber preservation are: 



1. Pressure or cylinder process. 



2. Open tank method. 



3. Brush or spray treatment. 



4. Dipping process. 



Cross ties, cross arms, poles, piling, and many posts as well as 

 structural timbers are generally treated by the pressure method. This 

 means the impregnation of the wood by forcing creosote or other 

 chemical preservative into the wood under high pressure. This is 

 accomplished generally in large, long cylinders. The materials to be 

 treated are loaded on cars and hauled directly into the cylinder, which 

 may be from 50 to 175 feet or more in length and from 4 to 9% feet in 

 diameter. Various forms of the pressure treatment are the Bethel, 



