Ash is converted into a thermally modified timber (TMT), by heating to 180-230 degC in a controlled atmosphere. Known by trade names – such as Cambia, Thermowood, Thermory, Thermo Ash – the process reduces the moisture content to around 5% and in doing so permanently alters the wood’s chemistry by cooking off the sugars (hemicellulose). This results in a much more durable timber, resistant to decay and insect attack, and lower thermal transfer. The colour is darker (from caramel to dark brown, depending on the cooking temperature) to begin with, but will gradually lighten if exposed to the elements without surface protection. It is used for decking, flooring (and is suitable for high traffic areas), cladding, window frames and furniture.

Thermal modification alters the mechanical properties. However, the affects vary depending on the density. Generally, there will be an increase in brittleness, and reduction in strength, stiffness and density. However, the cooking process requires very high quality timer free from significant defects. Any imperfections in the wood could lead to emphasised weaknesses after thermal modification. In other words, specific strength may look good, but this is compromised by potentially significant structure flaws. Therefore, it is not recommended for load-bearing applications.