Untreated wood can be used effectively for temporary structures and facili- 

 ties. 



b. Pressure Processes . The most effective method of treating wood with 

 preservatives is by means of pressure. There are a number of pressure 

 processes that employ the same general principle but differ in the details 

 of application. Treatment includes loading the timber on tramcars, which 

 are run into a large steel cylinder, bolting the cylinder door, and pressure 

 applying the preservative until the required absorption has been obtained. 

 Two principal types of pressure treatment, the full-cell and empty-cell, 

 are in common use (U.S. Department of Agriculture, 1952). 



(1) Full-Cell Processes . In pressure treatments with the so-called 

 "full-cell" or Bethell process, a preliminary vacuum is first applied to 

 remove as much air as practicable from the wood cells. The preservative is 

 then admitted into the treating cylinder without admitting air. After the 

 cylinder is filled with preservative, pressure is applied until the required 

 absorption is obtained. A final vacuum is commonly applied immediately 

 after the cylinder has been emptied of preservative to free the timber (or 

 charge) of dripping preservative. When the timber is given a preliminary 

 steaming-and-vacuum treatment, the preservative is admitted at the end of 

 the vacuum period following steaming. 



It is impossible to remove all the air from the wood cells regardless of 

 the method of treatment employed. For this reason, even under the most 

 favorable conditions, there is some unfilled airspace in the cell cavities 

 of the treated wood after impregnation by the full-cell process. 



(2) Empty-Cell Processes. Two empty-cell treatments, the Lowry and 

 the Rueping, are commonly used, both of which depend on compressed air in 

 the wood to force part of the absorbed preservative out of the cell cavities 

 after preservative pressure has been released. In the Lowry process, which 

 is also designated as the "empty-cell process without initial air," the 

 preservative is admitted to the treating cylinder at atmospheric pressure. 

 When the cylinder is filled, pressure is applied and the preservative is 

 forced into the wood against the air originally in the cell cavities. After 

 the required absorption has been obtained, pressure is released, a vacuum is 

 drawn, and the air under pressure in the wood forces out part of the pre- 

 servative absorbed during the pressure period. This makes it possible, with 

 a limited net retention, to inject a greater amount of preservative into the 

 wood and to obtain deeper penetration than when the same net retention is 

 obtained with the full -cell process. The Lowry process is convenient to use 

 in any pressure-treating plant, since no additional equipment is required. 



The Rueping process is called "empty-cell process with initial air"; this 

 process differs from the Lowry empty-cell process in that air is forced into 

 the treating cylinder before the preservative 'is admitted. The air pressure 

 is then maintained while the cylinder is filled with preservative; thus, the 

 wood cells are left more or less impregnated with air pressure. 



c. Classification of Wood Preservatives. Wood preservatives may be 

 grouped into two broad classes; preservative oils and waterborne preserva- 

 tives. Each of these classes may be further subdivided in various ways. 

 For example, preservative oils include petroleum refining byproduct oils 



252 



