Problem definition and objectives: Among the various methods of thermal modification of wood, the modification process in the steam environment under the trade name "Thermowood" is the most common method. Dry wood or wet wood can be used as a raw material to produce Thermowood, but due to economic and technical reasons such as the occurrence of surface cracks during drying at high temperature, dried wood is usually used as consumable wood.  In the Thermowood production process, the method adopted to dry the used wood is often not considered and only the moisture content of the dried wood is controlled before loading the wood into the Thermowood kiln, while the quality characteristics of the Thermowood may be affected by the dring method.
Methodology: In this research, Scots pine wood (Pinus sylvestris) dried by different methods was used to produce Thermowood. Scots pine is the most commonly used wood for Thermowood production in Iran. The drying operation was carried out in three ways: 1- Radio frequency/vacuum kiln with a capacity of 14 cubic meters and a frequency of 50 MHz, a power of 100 kW and a maximum temperature of 65 °C; 2-Conventional kiln at low temperature (50°C) and relative humidity of about 50% and 3- Conventional kiln according to a drying schedule in 20 steps with a maximum temperature of 71°C. Thermal modification was performed at 212°C for 3 hours and Thermowood type D was produced. The wood samples were subjected to various tests after thermal modification and conditioning in a climate room (temperature 20°C and relative humidity 65%).  The physical properties (color, equilibrium moisture content and dimensional swelling), mechanical and weathering resistance of Thermowood samples were measured. Colorimetry of samples was done based on CIE-Lab system. The equilibrium moisture content and volumetric swelling of the samples were measured in two ranges of relative humidity 60% and 90% and temperature 20°C. Mechanical tests including compression strength parallel to the grain, impact resistance and modulus of rupture of the samples were measured. Accelerated weathering test was performed using Gardner weathering machine for 750 hours and after weathering, the intensity of color change of the samples and surface cracking were measured.
Results: The results showed that the impact resistance of Thermowood produced from wood dried in a conventional kiln at low temperature compared to the other two methods showed an average increase of 60%, but its modulus of elasticity was 22% lower. Usually, incorrect Thermowood production process leads to a reduction in impact resistance beyond the permissible limit; therefore, measuring this type of resistance is an important parameter for quality control of the thermally modified wood. No significant difference was observed in the physical characteristics, resistance to weathering, bending strength and compression strength parallel to the grain, among Thermowood samples.
Conclusion: In general, it can be concluded that the equilibrium moisture content and dimensional stability of Scots pine Thermowood type D are not affected by the method of drying the wood before loading it in the Thermowood kiln, but some its mechanical properties such as impact resistance and modulus of elasticity depend on the applied drying method. Considering the different drying behavior of wood species, it is suggested to investigate the effect of the drying method of each wood species on the quality characteristics of thermally modified wood, separately.