Low temperature predrying appears to be a promising method of evaporating free moisture from tanoak lumber. The most important drying defects, surface checking and collapse, have been reduced below the level encountered in kiln drying by low temperature drying. Tanoak showed considerable variation in drying characteristics particularly between bright colored and dark colored lumber. Separate drying of these two categories should improve drying time and grade recovery. The early period of drying is the most critical and it is necessary to keep the relative humidity high and temperature low. As soon as stresses in the boards become reversed, the drying rate can be increased by stepping down the humidity. In addition to the normal shrinkage of cell walls, a study of shrinkage characteristics showed the occurrence of collapse also determined the dimensional changes of tanoak specimens dried to various moisture content levels. The change in dimensions became apparent when the wood was still above the fiber saturation point. A distinct difference in the shrinkage of bright colored sapwood and dark colored heartwood occurs. Shrinkage values measured on boards in these tests must not be considered standard information because method of sawing, amount of heartwood present, method of drying, and other variables affect the shrinkage of tanoak lumber. However, these tests offer a good indication of dimensional changes to be expected from such drying of gang sawn or slash sawn boards.

This study was made possible through the assistance of the Union Lumber Company, Fort Bragg, with the particular cooperation of N. V. Poletika and L. Andreani.