Changes of Wood Quality in Sugi (Cryptomeria japonica D. Don) and Karamatsu (Larix kaempferi Carriere) Woods by Smoke Heating

摘要

In the present study, sugi (Cryptomeria japonica D. Don) and karamatsu (Larix kaempferi Carriere) logs were smoke-heated with different temperatures and times using a modified food smoker. After smoke heating, several wood qualities were examined, and then the effects of treatment temperature and time of smoke heating on wood quality were discussed. Moisture contents (MC) were decreased with increase of temperature and time in each treatment. The difference in MC between heartwood and sapwood became small due to great decrease of MC in sapwood with increase in time, resulted in uniform distribution of MC around fiber saturation point. However, prolonged treatment caused tension stress at surface layer due to drying, which led to the increase in frequent occurrence of surface checks. In the treatment at 60 deg C, relative degree of crystallinity (RDC) increased by smoke heating over 40 hours in karamatsu wood, but not changed in sugi wood. However, RDC of both species increased with increase in time at 80 and 100 deg C treatments. Equilibrium moisture content (EMC) did not change in the treatment at 60 deg C. In the treatments at 80 and 100 deg C, however, EMC decreased with increase in treating time. This trend corresponds to increase of RDC. In both species, almost no change of sapwood color occurred in the treatment at 60 deg C. However, total color difference increased with increase in treating time at 80 and 100 deg C; the values showed more than 5, which means appreciable color change. Bending properties showed almost no change in the treatments at 60 and 80 deg C in both species. In karamatsu wood, however, specific Young's modulus increased with increase of time in the treatment at 100 deg C. These results suggest that no thermal degradation of wood occurred by smoke heating within 100 hours at a temperature inside the log below 100 deg C.