Anatomical explanations for the changes in properties of western red cedar (Thuja plicata) wood during heat treatment

Thermal treatment is an alternative to the chemical treatment in wood preservation, which has been used to some extent in improving timber quality. Despite the enormous works done so far on the effects of heat treatment on wood properties, very little is known about the anatomical changes in the various wood species during the process. Wood samples from western red cedar (Thuja plicata) were heat-treated at a temperature of 220°C for 1 and 2 h. The anatomical structures were examined before and after the heat treatment process by using scanning electron microscope (SEM) and related to density, water uptake, thickness swelling and modulus of rupture of wood samples obtained from the same board. Heat treatment of red cedar wood resulted in the destruction of tracheid walls, ray tissues and pit deaspiration. The destroyed tracheid walls and ray tissues appeared to blow up, thus increasing the size of the specimen. The process of pit deaspiration also resulted in increasing size of the pits, thus creating more openings in the wood. These changes in wood anatomy indicate that the well-established chemical degradation is not the only reason for changes in wood properties during heat treatment. However, it is believed that the effects of the chemical changes still outweigh those of the anatomical changes based on the modification observed during the process of heat treatment.