Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood

In this study the effect of high temperature on mechanical properties, dimensional stability and color of spruce was investigated. Wood specimens conditioned at different relative humidities (50, 65, 80 and 95 %) were subjected to heat treatment at 200 degreesC for 2, 4, 8, 10 and 24 h and at 100, 150 and 200degreesC for 24 h. Color changes were measured in the Minolta Croma-Meter CR-300 color system. Bending strength and modulus of elasticity were determined according to DIN 52186. The results show that heat treatment mainly resulted in a darkening of wood tissues, improvement of the dimensional stability of wood and reduction of its mechanical properties. The darkening accelerated generally when treatment temperature exceeded approximately 200degreesC. Most of the darkening occurred within the first 4 h of exposure. For the specimens heated to high temperatures, the average decrease in bending strength was about 44-50 %, while modulus of elasticity was reduced by only 4-9 %. We found that treatment time and temperature were more important than relative humidity regarding the color responses. Strong correlations between total color difference and both modulus of elasticity and bending strength were found. Thus, the color parameters can be estimated quantitatively and used as a prediction of wood strength.