322 



FOREST PRODUCTS 



Probably 95 per cent or more of the poles that are treated in this 

 country are given the open-tank treatment, whereby a penetration of 

 from one-third to 3 in. or more of the preservative from the surface is 

 secured on the butt of the pole. Many pole companies have recently 

 installed open-tank plants in connection with their pole yards or dis- 

 tributing depots, where the poles are raised by means of a derrick and 

 stood on end in a hot bath of creosote at a temperature of about 215° F. 



Photoffraph by u. S. Forest Service. 



Fig. 87. — Method of treating poles in an open tank to increase their length of service. The 

 butts are treated up to a point above the ground level. Wilmington, Los Angeles Co., 

 California. 



for about two hours. The creosote oil is then permitted to cool or cold 

 oil is pumped in. The heating process causes the water and air in the 

 wood to expand. The cool bath causes a contraction in the cells and 

 intercellular spaces and the oil penetrates the partial vacuum caused 

 by change in temperature. Experiments have shown that a penetration 



