Chemical Seasoning of Lumber

This is a general article with no experimental research results. It describes the causes of checking in lumber and how chemical seasoning works. If a proper balance exists between the relative humidity of the air and the vapor pressure of the chemical solution in the wood, moisture content of the surface fiber will remain high enough to prevent shrinkage and thus surface checking. Combining the different chemicals can help to improve wood drying and at the same time impart certain beneficial properties to the wood. Combining a crystalline agent such as salt, urea, or mono-ammonium phosphate with a non-crystalline chemical such as glycerine, diethylene glycol molasses, invert sugar, or corn syrup gives the maximum anti-shrink effect. Resin-forming chemicals increase the hardness of wood. The three methods of chemical seasoning are 1) steeping, 2) spraying and dipping, and 3) dry spreading. Because of the enormous tank capacity required, steeping is impractical. In general, dipping or spraying is restricted to specialty items or small scale operations. The dry spread method has proven most successful. Chemicals retained in lumber may modify properties of the wood by 1) corrosion, 2) condensation, 3) electrical conductivity, and 4) color. A satisfactory chemical seasoning agent must be one which is low priced and quite soluble in water. It should be in equilibrium with about 75 percent relative humidity and noncorrosive to metal fastenings and kiln parts. Only buffered sodium chloride, buffered urea aldehyde and urea meet these requirements. There is no effect on drying time when chemical agents are added assuming treated and untreated lumber are dried with the same schedule; on the other hand, chemically treated stock can be subjected to more severe drying schedules and thus dry faster. If the previous conditions for proper chemical seasoning are met, then drying degrade can be cut up to 95 percent.

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