Reversible and irreversible dimensional changes of heat-treated wood during alternate wetting and drying

Spruce wood specimens were heat-treated in saturated water vapor (steaming) and in the absence of moisture (dry heating) at 120–180 °C, and their wet volumes were measured during alternate wetting and drying cycles. After the first wetting and drying cycle, the wet volume of steamed wood decreased and then remained unchanged during the following alternate wetting and drying cycles. The wet volume regained its initial level when soaked in dimethyl sulfoxide (DMSO), followed by washing in water, but decreased again after subsequent drying and rewetting. Such reversible changes were not observed in unheated and dry-heated specimens. This suggested a hydrophobic structure formed during drying or plastic expansion of cell lumen due to steaming and soaking in DMSO. Although heat treatments reduced the hygroscopicity of the wood, steamed wood showed more swelling, i.e., lower dimensional stability than unheated and dry-heated wood. This is probably because steaming caused serious degradation of cell wall components, therefore loosening its fiber-reinforced structure restricting the swelling of the cell wall. When wood is exposed to alternate wetting and drying, steaming is not advisable because it may reduce the dimensional stability of the wood.