Effect of drying force on birch wood colour change during high temperature drying

Sawn birch timber is traditionally dried very carefully at low temperatures in order to avoid discolouration of wood. To be able to increase the capacity of old kilns and enable the drying of birch in high temperature drying kilns, experiments of high temperature drying of birch were made. The main aim of the study was to investigate the intensity of discolouration of birch during high temperature drying and test the novel combination drying (hot air drying/high temperature drying). The results have shown that sawn birch timber darkens in normal high temperature drying when drying force is kept low above ESP. Especially, reddish colour will appear in the planks. The results also have shown that this kind of discolouration can be decreased considerably by increasing the drying force during the first steps of the drying process. The results indicate that hot air/high-temperature-dried birch timber could be used particularly in gluelams and I-beams for the lower price categories. Further studies are needed on other mechanical and physical properties of birch wood dried by the hot air/high temperature method.