Kiln drying is a key factor for the utilisation of our hardwood forest resources, providing our industry with high quality raw material. The increased use of transparent surface treatment in the furniture and carpentry industries during the last decades has put an increased focus on wood colour. Fashion trends in furniture and interior design call for blond colours on our Scandinavian woods species. While commonly recognised as a major colour-controlling process in wood utilisation a fundamental understanding of how the drying procedures affect the final colour of the wood is still lacking.

This thesis is an attempt to increase knowledge of how different process parameters during kiln drying affect the final colour of the common hardwoods beech and birch. The investigation is based on experimental dryings in climate chamber and laboratory kilns using designed factorial experiments.

During the capillary drying phase, time is more important than temperature level for the final colour. Speedy drying even at moderately increased temperature levels results in brighter more yellowish colours. Diffusive drying from approximately the fibre saturation point to 20 % moisture content seems to be the most important colouring phase in conventional drying. Temperature and the time a local section of wood spends in the moisture content region are decisive for the final colour. To create blond wood the temperature should be kept low and the drying fast, which calls for low relative humidity levels and high air speeds.

The common problem of grey-stain in non-steamed natural beech is caused by to slow air- or kiln drying using high relative humidity levels in the initial stages of the drying process. The problem may be avoided by increasing the speed of drying. Log storage at a low temperature during wintertime on the other hand seems to have little effect on the final colour of natural beech only affecting the hue. Winter storage of logs is thereby not a high priority issue regarding final colour of beech wood.