Effects of High-Temperature and Low-Humidity Pretreatment on the Drying Properties of Sugi Boxed-Heart Timber with Black-Colored Heartwood

In this study, the optimal conditions of high-temperature and low-humidity (HT-LH) pretreatment to prevent surface checks of sugi boxed-heart timber with black-colored heartwood were investigated. The focus of this study was on the effects of log diameter, temperature, and pretreatment time on the drying time and checking of the timber dried at a constant temperature and relative humidity. The results obtained were as follows: The drying time of specimens obtained from 22-cm-diameter logs was longer than that obtained from 20-cm-diameter logs. The drying time of specimens obtained from 22-cm-diameter logs was influenced by both pretreatment temperature and time. Higher pretreatment temperature and/or longer pretreatment time resulted in longer drying times. In contrast, in the case of specimens obtained from 20-cm-diameter logs, a longer pretreatment time resulted in a shorter drying time. The average surface check area of specimens tended to decrease and the average total number and length of internal check tended to increase as pretreatment temperature and time increased. Pretreatment at a temperature of 135°C for 10 h or 150°C for 7 h was effective in preventing surface checks of the specimens.