| Abstract: | Abstract Change in the colour of silver birch wood is a serious problem in the mechanical wood industry. Here, colour was correlated with microscopic characteristics of wood, such as cell types and dimensions, by drying processes. In conventional drying, with lower temperature than in vacuum drying used here, the most important factor causing darkened wood was wide latewood. In vacuum drying, thickness of the vessel walls affected wood darkening, as did broad rays and large amount of axial parenchyma. Axial and terminal parenchyma cells contained very small amounts of phenolics, but after drying at elevated temperature, a thin dark layer could be observed on the innerside of their walls. Phenolics were abundant in ray parenchyma; these compounds darkened at elevated temperatures, less in conventional drying than in vacuum drying. Phenolics were observed only inside cells, mainly in the parenchyma, but in vacuum-dried wood also in fibres and vessels. Anatomical characteristics are known to be affected by both environmental and genetic factors. Thus it might be possible to influence the colour reaction of birch wood during the drying process by choosing wood according to growing-site conditions, or by choosing the seed source for birch plantations according to given anatomical characteristics. |
