Variation of colour and selected physical and mechanical properties related to artificial drying of sawn silver birch (Betula pendula Roth) timber from plantations

2 Variation of colour and selected physical and mechanical properties related to artificial drying of sawn silver birch (Betula pendula Roth) timber from plantations Veikko Möttönen Dissertationes Forestales 13 3 Möttönen, V. 2005. Variation of colour and selected physical and mechanical properties related to artificial drying of sawn silver birch (Betula pendula Roth) timber from plantations. Dissertationes Forestales 13. Birch is the most important domestic hardwood species for the wood products industries in Finland. In the near future, the wood from silver birch (Betula pendula Roth) plantations will broadly increase as a source of raw material of saw and veneer logs. Due to the differences in growth rate and silviculture, the raw material from plantations is likely to have characteristics different from those of the wood harvested from naturally regenerated forests. From the point of view of the wood products industries, however, little is known of the quality of the wood of plantation-grown birch trees. In addition, environmental factors such as growing site, felling season and storage of timber are also known to have an influence on the wood properties, but there is little knowledge of the mechanism of those factors and the possibilities to control them. In the further processing of sawn birch timber, the drying of wood is one of the most difficult phases: discolouration, deformation, moisture content gradient, being typical defects for birch timber produced during drying. The objective of this thesis was to study the drying behaviour and related wood and timber properties of plantation-grown silver birch for the raw material of wood products industries. The wood procurement season and storage of logs before further processing were studied as a source of variation of wood properties and drying behaviour. To study the variation in wood properties and drying behaviour caused by the drying method, the conventional kiln drying (heat and vent drying) and vacuum drying were used, which are the most common drying methods for sawn birch timber. On the subject of drying method, wood colour, shrinkage, weight density, final moisture content, Brinell hardness and equilibrium moisture content were studied. The role of proanthocyanidins (condensed tannins) and their polymerisation as a chemical background for the discolouration was studied. Owing to the variation in the wood properties, the drying behaviour was found to differ between the wood procurement seasons. In addition, due to the low temperature used during the first steps of the conventional drying process, the …