PEER-REVIEWED ARTICLE bioresources.com Čabalová et al. (2018). “Thermal treatment of oak,” BioResources 13(1), 157-170. 157 Effect of Thermal Treatment on the Chemical, Physical, and Mechanical Properties of Pedunculate Oak (Quercus robur L.) Wood Iveta Čabalová, a, * František Kačík, a,d Rastislav Lagaňa, b Eva Výbohová, a Tatiana Bubeníková, a Ingrid Čaňová, c and Jaroslav Ďurkovič c Changes in the chemical composition and selected physico-mechanical properties of pedunculate oak (Quercus robur L.) wood samples were assessed after thermal treatment. Heat treatment was performed at 160, 180, and 200 °C in an oxidizing atmosphere. The contents of the extractives, lignin, cellulose, holocellulose, and saccharides, and the structural changes in the functional groups were determined. Changes in the colour traits, wood density, compression strength parallel to the grain, and compression modulus of elasticity were also determined. The decrease in the holocellulose content caused by the degradation of non- glucosic saccharides was observed during thermal treatment. The contents of both the extractives and lignin increased. The syringyl to guaiacyl (S/G) ratio in the lignin increased because of the preferential condensation of guaiacyl units. The physical and mechanical properties of pedunculate oak wood, such as density, equilibrium moisture content, colour lightness, and yellowness, decreased as the temperature increased. The compression strength and redness varied during thermal treatment, and reached maximum values during the treatment at 180 °C. The modulus of elasticity showed non-significant differences. Three groups of heat treatment clusters were distinguished in the multivariate wood trait analysis and were clearly segregated from each other. Keywords: Pedunculate oak wood (Quercus robur L.); High temperature treatment; Wood chemical components; Infrared spectroscopy; Colour of wood; Physical properties; Mechanical properties Contact information: a: Department of Chemistry and Chemical Technologies; b: Department of Wood Science; c: Department of Phytology, Technical University in Zvolen, T.G. Masaryka 24, 960 53 Zvolen, Slovak Republic; d: Department of Wood Processing, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6- Suchdol, 16521 Czech Republic; *Corresponding author: cabalova@tuzvo.sk INTRODUCTION Wood properties are influenced by the modification of its main constituents by heat treatment, moisture, UV radiation, and chemical agents. Heat treatment generally improves the dimensional stability and resistance to wood-deteriorating fungi, and it can enhance some wood properties. This process was first developed in 1920 as a high temperature wood drying process and has become a wood modification process that occurs at various conditions. Heat treatment is the most advanced wood modification process that is used commercially. It causes extractives and main wood component changes due to heat transfer, which can be influenced by nanoparticles. Further development of heat treatment started in 1990 in Finland and continued across Europe for processing softwood and hardwood species, such as spruce, pine, aspen, and birch (Hill 2006; Laoutid et al. 2009; Sun et al. 2012). The American Hardwood Export council suggested thermally treating oak