The water absorption of sapwood and heartwood of Scots pine and Norway spruce heat-treated at 170°C, 190°C, 210°C and 230°C

Heat-treatment changes the chemical and physical properties of wood. Wood polymers are degraded, dimensional stability is enhanced, equilibrium moisture content is lowered, colour darkens and biological durability is increased. The properties of heat-treated wood have been researched considerably, but the differences between sapwood and heartwood have not been reported separately. In this research, water absorption differences between sapwood and heartwood of Scots pine and Norway spruce heat-treated at temperatures 170 degrees C, 190 degrees C, 210 degrees C and 230 degrees C were investigated. The results were compared to industrially kiln-dried reference samples. Water absorption was determined with a floating test based on the EN 927-5 standard. The heartwood of both wood species absorbed less water than sapwood. Heat-treatment evidently decreased the water absorption of spruce and pine heartwood. The higher the heat-treating temperature, the lower the amount of absorbed moisture. However, a very interesting exception was pine sapwood, whose water absorption actually increased with heat-treatment after the three lowest heat-treatment temperatures compared to the reference material. Water absorption did not decrease until the heat-treatment temperature was 230 degrees C.