Discolouration during kiln drying of South African Pinus elliottii

Abstract The effect of different parameters on the surface discolouration (yellow stain and kiln brown stain) and thermal discolouration of Pinus elliottii during kiln drying was investigated. Boards were dried with different kiln schedules and discolouration was assessed at different depth levels from the surface of each board. The discolouration data were analysed using notched boxplots, multivariate analysis of variance and canonical variate analysis biplots. Thermal and surface discolouration were distinguishable by the distribution of colour data point plots in the CIE-L*a*b* colour system. Thermal discolouration was less intense and less varied than surface discolouration. The greatest degree of thermal discolouration occurred in one of the higher temperature (Tdb≥90°C during first drying phase) schedules that was run for an excessively long period due to low air velocity. Discolouration progressively decreased as the boards were planed to a greater depth. Higher temperature schedules yielded 77.5-82.5% discoloured replicates at the 2 mm depth level. Lower temperature (Tdb≤71°C during first drying phase) schedules yielded no discoloured wood at the surface or at the centre of the boards.