The effect of the ThermoWood Process on sapwood of the Japanese cedar, Cryptomeria japonica, was investigated. Seven matched specimens were cut from a sapwood board of Japanese cedar and subjected to thermal treatments according to the ThermoWood Process. The decay resistance and chemical and physical properties of the treated specimens and untreated specimen were investigated. The decay test was carried out according to the Japan Industrial Standard K1571-2004. The specimens were cut to the size of 2 (R) ? 2 (T) ? 1 (L) cm or 2 (R) ? 2 (T) ? 10 (L) cm and subjected to the laboratory decay test and the fungus cellar test, respectively. The results of both tests clearly indicated that the ThermoWood Process at temperatures above 210 °C effectively increased the decay resistance of Japanese cedar sapwood. Mass loss was less than 13% in the sapwood treated at 220 °C for 2.5 hours, but was about 50% in the untreated specimens. Thermal treatments at higher temperature and for a longer period improved the decay resistance. No mass loss was found in the specimens treated at 237.5 °C for 5 hours. A similar tendency was observed in the fungus cellar test. The average decay rate of the untreated sample reached 4.6% after exposure for 0.6 year in the fungus cellar while that of the samples treated at 237.5 °C for 5 hours was still 0%. Determination of holocellulose contents suggested that the percentage of holocellulose was a good index for assessing the degree of thermal treatment and the level of decay resistance.

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