Thermally modified timber (TMT) was first developed to improve the performance and durability of softwoods, but it has more recently been extended to boost the performance of hardwoods, allowing certain timbers to be used externally with no additional protection. Following thermal modification, ash, soft maple, tulip wood and red oak (with the best results from quarter-sawn timber), can achieve Class 1 durability, the highest possible rating, and equivalent to teak.
The treatment process consists of gradually heating the timber to a temperature of between 180 - 215°C for three to four days (the length of time depends on the thickness and the species of timber). This has to be done in an inert atmosphere (that is, one that contains no oxygen) to prevent the timber igniting. This is normally done either in steam or in a vacuum. This process permanently alters the wood’s chemical and physical properties. This is very different to kiln drying, which only reduces the moisture content of the timber.
The thermal modification process reduces the timber moisture content down to 4-6% (very low). This has the effect of drastically reducing the equilibrium moisture content. Put simply, the physical structure of the wood is changed, which limits the ability of the wood to absorb moisture, so products are more dimensionally stable and less prone to cup, warp and twist with changes in humidity.