Effects of environmental and internal factors of trees and timber treatment on colour of dried birch (Betula pendula) wood

Abstract

Wood of silver birch (Betula pendula) has good properties that lead to its extensive use in mechanical wood industries in Finland. However, a problem with sawn birch timber is the darkening of the wood during kiln drying. The purpose of this study was to investigate the reasons for darkening of silver birch wood in order to suggest a way to avoid it. In these experiments, birch boards were sawn into the dimensions needed for parquet billet boards and dried by conventional and vacuum processes in laboratory kilns. The changes in wood colour and in proanthocyanidin concentration during conventional drying and after all drying processes were measured and compared to factors connected with timber handling, environmental factors and internal factors of the trunks, and selected anatomical characteristics of the wood were measured or observed visually. When different schedules for conventional drying were compared, the higher the temperature, the darker was the colour of the wood at the end of drying. Two critical points in wood colour changes were observed. After the conventional drying processes were started, the colour of wood became lighter until moisture content of about 30%; then darkening started and continued until the moisture content of the wood was ca. 15-20%. After that, during further drying the colour of the wood lightened again. A decrease in concentration of soluble proanthocyanidins, i.e. their polymerisation and/or oxidation to coloured compounds, occurred simultaneously with darkening of the wood. Differences between drying methods in terms of the measured anatomical characteristics that correlated with the colour coordinates were observed; in particular, the parenchyma cells of the wood and the width of the latewood layer were important in darkening during vacuum and conventional drying, respectively. The effect of anatomical differences, which develop during the whole life of a tree, on the colour reaction of wood suggests a new approach for controlling the darkening of birch wood that occurs during drying. With this approach, birch could be tried to breed for lighter wood or development of cells that affect wood darkening could be tried to minimize with silvicultural practices.