Colour changes of Scots pine and Norway spruce - A comparison between three different drying treatments

Surface of boards, dried with wet bulb temperature above 60 degrees C are often called discoloured caused by severe resinflow. Discolourisation was evaluated using a tristimulus colour analyser, for measuring the colour changes during drying at three different drying schedules; 50 degrees C, 60 degrees C and 70 degrees C wet bulb temperature. The colour properties were pictured by lightness (L*), chroma (C*) and hue (h degrees). Results showed that lightness decreased and chroma increased during drying. When comparing the three differnt drying schedules the lightness decreased and chroma increased with higher drying temperature. No significant change of hue during drying was noticed, but when comparing the three schedules, hue decreased with higher drying temperature. Drying time and drying temperature level effect the value of lightness more than any other colour properties. Values of lightness and chroma returned to its original value after 3 mm planing, while the value of hue for sapwood of Scots pine decreased after 1 mm planing, but increased again after 3 mm, even more for series C (70 degrees C) than for series A (50 degrees C). For sapwood of Scots pine, lightness decresed and chroma increased more than for sapwood of Norway spruce, both during drying and with higher drying temperature. A hypothesis is that red-making (or blue) substance such as nutrients and extractives are transported toward the surface during drying and making the surface look more red. That leaves the yellow-making substance in the inner parts of the board and when planing the board, the surface will be more yellow than before.