Stages of Thermowood Process
Drying; Using heat, the oven temperature is raised to 100⁰C quickly. Then, the temperature is constantly raised to 130⁰C, during this time the wood is dried and the moisture is reduced to zero.
Heat treatment; The temperature in the wood is increased to 180⁰C (Thermo S) and 212⁰C (Thermo D) with water vapor. When the target temperature is reached, this heating process is applied for 2-3 hours.
Cooling and humidification; In this last part, the temperature of the wood is lowered to 80⁰C – 90⁰C by the use of the water spray and it is continued until the humidity of the wood reaches 4–6%.
Wood is Durable For a Reason
Heat treatment increases the stability and durability of wood.
Wood is composed of 50% cellulose, 23% hemicellulose, 20% lignin, and 7% other organic compounds, called extractives. Heat treatment removes resin, other extractives and their attached OH (Hydroxil) groups from the wood.
This process reduces water absorption of wood, consequently increasing decay resistance while decreasing swelling and shrinking. Another factor that contributes to the high durability of wood is the crystallization of cellulose. The change in hemicellulose increases the durability. Hemicellulose is separated into furfural and carboxylic acid. Caramelization of lignin, caused by heat, results in a darker color.