Discoloration of Norway spruce and Scots pine timber during drying

The effect of growth site, felling time (winter, spring and autumn) and wet storage on the discoloration of spruce (Picea abies (L.) Karst.) and pine (Pinus sylvestris L.) dried at different temperatures was studied. Two dominant trees were selected at each cutting season from a fertile and a poor site. Some of the logs felled in May were stored under sprinklers for 6 weeks. The butt logs were cant-sawn with a circular saw. The temperatures for drying were 50, 70, 90 and 110 °C. One group from autumn felling was dried in a vacuum kiln at 70 °C. Common drying schedules for 38-mm-thick pine boards were used. The surface colour (CIELAB L*, a*, b*) of boards was measured before and after drying with a spectrophotometer. Some of the boards were also measured after 0.5 mm and further 1.5 mm planing. Drying temperature was the most significant factor for discoloration of boards. The colour change of pine was quite similar to that of spruce except that there was a stronger darkening of pine heartwood compared to spruce at 90 °C. Discoloration in sapwood increased remarkably at temperatures above 70 °C. At 90 and 110 °C the lightness (L*) decreased significantly indi-cating darkening. The b* value increased with temperature. The a* value changed less. Timber felled in winter showed in-creased discoloration in drying compared to other felling times. There were no significant differences between fertile and poor sites. The extractive content of the wood surface was analysed after the drying. The amount of soluble carbohydrates was 5- to 10-fold larger on a 2-mm-thick surface layer after drying than in the fresh samples of sapwood. The highest sugar content was detected in the samples taken in winter. However, results did not show any clear correlation between extractive content and degree of discoloration.