Drying of sugi logs with microwave heating

Sugi logs were dried with the microwave (MW) heating. The temperature and moisture content distributions in the logs were measured during MW heating. And the surface stresses were measured at the same time as released strains on the surfaces. As a result, the temperature in the center layer was higher than that in the outside layer during drying because of rapid temperature decrease for irradiation stop in intermittent heating system of MW. The moisture content in the center layer was relatively low at the end of drying. The surface stress of dried log was compression for the tangential direction related to surface checks. During seasoning after MW drying, the moisture was removed mainly in the surface layer, and the surface stress changed to tensile stress. As a result, the surface checks occurred for all logs after seasoning in about two or three months. Some dried logs had been sawn into the boxed-heart square timbers. Many surface checks appeared on these timbers because of the existence of internal checks of sugi logs, which occurred by the MW drying. These check widths did not change largely for the efficiently dried timbers as the core part of sugi logs after MW drying.