PRINCIPAL METHODS OF PRESERVATION 



187 



Rueping, Lowry, and Card processes.* Sometimes the materials are 

 thoroughly seasoned prior to treatment. In some methods live steam 

 is introduced into the cylinder and a pressure of 20 pounds per square 

 inch maintained for several hours, and followed by a vacuum period. 

 The wood is then seasoned or conditioned and has a greater capacity 

 for absorbing preservatives. Steaming softens the fibers and opens the 

 pores of the wood. After various forms of preliminary treatment, the 

 creosote, zinc chloride, or other preservative is run into the cylinder at 

 definite temperatures. Pressure 

 is then forced upon the preserva- 

 tive so as to drive the fluid into 

 the wood fibers. The amount 

 injected depends upon the 

 species, condition, character, 

 and size of the materials and 

 the amount desired in the wood. 

 After this pressure is released a 

 vacuum is sometimes applied to 

 draw out the excessive fluid and 

 hasten the drying process. 



The open-tank method is 

 used with fence posts, grape 

 stakes, highway posts, and for 

 the treatment of chestnut and 

 cedar poles. After the bark is 

 removed and the wood entirely 

 seasoned, the posts or poles are 

 immersed in a hot bath of 

 creosote of temperature not 

 exceeding 215 F. This heat- 

 ing process continues for 2 to 

 6 hours, depending upon the 



species, size, and condition of the material. During the heating 

 the moisture and air expand and a large portion passes out, 

 appearing as steam or air bubbles at the surface. The posts and poles 

 are then quickly removed to a cold bath of creosote. The contraction 

 of the air and moisture in the wood due to the changing temperatures 

 creates a partial vacuum which is destroyed by the entrance of the 

 preservative fluid. In this manner atmospheric pressure due to quick 

 changes in temperature accomplish the impregnation of the wood at 

 least to a partial degree. Sapwood is much more readily subjected to 



*For further information regarding these processes, see Annual Proceedings 

 of the American Wood Preservers' Association, Washington, D. C. 



FIG. 97. Moving poles into a small ex- 

 perimental cylinder preliminary to pre- 

 servative treatment by pressure process 

 to increase the length of life in service. 

 Photo taken at U. S. Forest Products 

 Laboratory at Madison, Wisconsin. 



