Effect of high-temperature drying and restraint on twist of Norway spruce

During drying, timber distortion is a major defect mainly due to shrinkage anisotropy, differences in longitudinal shrinkage, and spiral grain. The distortion can be reduced by external restraint and use of appropriate kiln schedules. The research presented here is part of a project on the improvement of shape stability of Norway spruce by high-temperature drying. The effects of drying temperatures between 80 and 170 degrees C and restraint on the extent of twist in Norway spruce were investigated on 30 cm lengths. Results show that significant twist reduction could be achieved in restrained specimens sawn from core wood. This effect was permanent even after exposure to subsequent moisture cycling. In addition to high-temperature drying, heat-treated material (Thermowood (R)) was also investigated. As for the high-temperature dried material, twist amplitude in moisture cycling was highest for pith-in specimens. Increased heat-treatment temperature reduced the twist amplitude, thus improving the shape stability. However, for the material dried in this study, shape stability was not influenced by the drying temperature.