Born in the Nordic countries to upgrade poor, soft species, heat treatment turns timber into a hyper-stable exterior material, roasted-coffee in colour, ready to face the weather without chemicals.
The process involves heating the wood to temperatures between 160 degrees C and 215 degrees C in a steam or nitrogen atmosphere (absence of oxygen). This thermal shock melts the resins and irreversibly destroys hemicellulose, the sugary component on which fungi and wood-boring insects feed. Deprived of its nourishment, the wood becomes immune to rot.
By destroying the hydroxyl groups, the cell loses its ability to bind water: the typical shrinkage and swelling movements are reduced by up to 50%. The price to pay is partial fibre breakage: the material becomes more brittle and prone to splintering. Its bending modulus of rupture collapses, making it unsuitable for primary structural loads and limiting its use to cladding and secondary finishes.
Standards
European and international references applicable.
Physical properties
Usage environment
Despite the heat treatment, under UV rays the timber greys exactly like untreated wood. To maintain the original dark colour, pigmented oils must be applied. Always pre-drill before screwing to prevent instant splitting.