Durability of heat-treated wood

Heat-treated wood from the French process was laboratory tested for decay using agar block test and a modified soil block test. Water absorption, bending strength, lignin content and acid number were also determined to evaluate the effect of heat treatment. Heat treated samples exhibit a higher lignin content and a lower acid number compared to the untreated controls indicating the degradation of some hemicellulose and extractives compounds. The significant amount of water absorbed during water soaking or exposure to different relative humidities suggest that the heat treatment helps in releasing the stress in wood after the removal of hemicellulose and degradation of lignin rather than the reported significant cross link reaction of organic acid and the benzene ring of lignin. Cubes extracted with water or acetone or chloroform, and exposed to pure fungus cultures show a considerable weight loss, which confirms the absence of any extractable compounds toxic for decay fungi during the heat treatment. Significant weight loss was observed after 12 weeks exposure for laboratory soil block or after 6 to 8 weeks for agar block test. For the soil block test, weight loss of 11% was obtained for heat-treated samples exposed to G. trabeum and 46% for P. placenta. About 56% and 54% weight loss were obtained for southern pine control exposed to G. trabeum and P. placenta, respectively. The weight loss of the water and acetone extracted heat-treated sample exposed to P. placenta was 49.7% and 53.9%, respectively. Only about 11% and 14.8% weight loss was obtained for water and acetone extracted samples exposed to G. trabeum. The moisture content of the tested samples was about 70+/- 10% for the unheated controls and 50+/- 10% for heat-treated samples. This treatment may modify the durability from non-resistant to moderate/resistant species depending on the fungus species as defined in the ASTM 2017 standard. The data from the bending test indicate that such treatment may create a 10 to 50% reduction or MOR and deflection, which will limit the use of such wood for structural purposes.