It is well known that Sugi (Cryptomeria japonica D. Don) boxed-heart square timber, which is usually used as columns for wooden residential houses in Japan, is one of the hardest species to be kiln-dried. Then we tried to apply the high-temperature setting method, which is one of the high-temperature kiln-drying that green timber is thermalsoftened and rapidly kiln-dried under high temperature and low relative humidity condition before the subsequent various usual drying. It was expected by the method that drying tension sets at the shell of timber should reduce occurrences of surface checks. Sugi green square timbers (132mm×132mm) without back-splitting were kiln-dried on various schedules to examine drying time, surface checks, and honeycombs (inner checks). The results were shown below.
For each schedule, variations of initial moisture content (MC) were very large, and then final MC also varied widely. When the focus was limited on the timber whose initial MC was from 60% to 120%, average of final MC was 18.5% after 4.25-day drying time in high temperature kiln drying at dry bulb temperature (DBT) of 110°C after the high-temperature setting process. In medium temperature kiln drying at DBT of 90°C after the pre-process, drying time was 8 days and mean final MC was 19.8%. In conventional kiln drying at 70-80°C, drying time was 14 days and mean MC was 25.6%.
By the high-temperature setting method, reduction of surface checks was particularly obvious, and timbers with lower final MC tended to cause fewer surface checks. When MC dropped, the widths of surface checks decreased or closed. On the other hand, honeycombs increased when MC decreased. The subsequent medium temperature kilndrying after the pre-process obviously reduced occurrences of honeycombs, though differences between DBT and wet bulb temperature was 30°C, that meant very low humidity condition. The honeycombs were also few in the conventional kiln-drying.
