Improvement of shape stability by high-temperature treatment of Norway spruce - Effects of drying at 120 degrees C with and without restraint on twist

During drying, timber changes its shape mainly due to shrinkage anisotropy, radial differences in longitudinal shrinkage and spiral grain. The warp, causing severe downgrading of the timber, can be reduced by restraint and appropriate climate treatments of different types. The research presented here is part of a larger project on the improvement of shape stability of Norway Spruce (Picea abies) by high-temperature treatment. In this part, a method for determining twist as well as results for a drying temperature of 120 degrees C are presented. The effects of presteaming, drying and steaming with and without restraint on the size of twist in Norway Spruce were investigated in laboratory scale. Short-term twist-reducing effects as well as the permanency of the reduced distortions in subsequent moisture cycling were investigated. Results show a clear dependency of twist on the distance from the pith. Furthermore, the twist is reduced in restrained specimens sawn close to the pith. This effect is permanent even after exposure to subsequent moisture cycling