High heat treatment of wood to impart specific characteristics has been around since the 1920s. Chief early protagonists for the research and experimentation with high heat treatment are Scandinavian and North American countries.
In all it takes up to four days to treat wood with high heat; up to two days are required to bring the wood up to temperature, between half an hour and five hours for the treatment itself, and as long as twenty four hours are required for cooling. Heat alone would cause burning, so plenty of water in the form of steam is part of the process. Half an hour to an hour at 150ºC (302ºF) or just above is used primarily to change wood colour and these ‘lower’ temperatures don’t cause undue brittleness; the treatments are used primarily on hardwoods, e.g., heat treated birch which I've worked with and was a rich brown and not noticeably brittle. Higher heat treatment at approximately 240ºC (464ºF) for five hours improves durability of the wood at the expense of a significant increase in brittleness. These latter treatments are used on pine or spruce to increase their durability when used for external non-ground contact structures.
Heating wood in this way causes the following effects:
- It alters the physical and chemical properties of the wood permanently. Degradation of the wood hemicellulose occurs, and hydroxyl[1] groups within the wood decrease. As the wood degrades it produces acetic and formic acids along with phenolic and other aromatic compounds.
- The wood darkens— a relatively light heat treatment causes this effect, but the new colour is rather fugitive when exposed to UV light.
- There is a reduction in changes in the wood’s volume in service with changes in moisture content; this is due to chemical changes in the wood, i.e., a decrease in the hydroxyl groups and the effect is to reduce the range of dimensional change, primarily across the grain, as the wood takes on or loses moisture.
[1] Hydroxyl: a chemical group in which oxygen and hydrogen are bonded and act as a single entity.
Slainte.