Effect of a microwave drying process as a pretreatment for preservative impregnation on sugi [Cryptomeria japonica] round timbers

A microwave drying (MD) process was applied for sugi round timbers, as a pretreatment for effective impregnation of wood preservatives. In most MD process, the center layers of round timbers were maintained at higher temperature than the outer layers. As a consequence, the moisture content (MC) of the inner layer dropped down significantly in comparison with that of the outer layer in the round timbers. At the same time, the MD-dried timbers could reach a low level of MC more quickly than the kiln-dried timbers. In addition, the timbers dried by MD showed less surface checks than those dried by kiln drying (KD), even if without kerf. However, internal checks occurred frequently in most of the MD-dried timbers, which was related to the low final MC. The timbers dried by MD, but not by KD, showed an elevated ratio of impregnated area of the preservative in the cross-section. Among timbers with 0, 1 or 4 kerfs, there was no significant difference in the penetration area of the preservative.