Colour responses to heat-treatment of extractives and sap from pine and spruce

Abstract

Scandinavian sawmills have over the last 20 years adopted ever-higher temperatures in the dry kilns. The reason for this has been improvements in drying quality and kiln capacity. However, some problems relate to this strategy, mainly discolorations and extensive resin flow from knots and heartwood in pine. In this study, underlying factors for changing wood colour were investigated for spruce and pine. From the sapwood portions of wood, sap was extruded by mechanical compression, put in glass bottles and heat-treated at different temperature levels ranging from 60 °C to 95 °C during 1 to 5 days. Then the colour of the sap was measured using a standard colorimeter. Extractives from pine heartwood were also heat-treated and analysed in the same way. From these experiments it was concluded that much of the colour change in solid wood emanated from colour changes in constituents of the sap and extractives. Pine sap was more susceptible to colour change than sap from spruce. Also, an accelerated colour change was noticed above 70 °C for pine sap. Finally, it was observed that extracted wood that was soaked in water and then heat-treated still underwent some colour change, probably due to secondary hydrolysis of hemicelluloses.