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Heat-treated wood

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The properties of wood can be permanently altered through a special process of heat treatment. The essential changes are:
- increased physical stability (the wood is less susceptible to expansion and shrinkage)
- increased durability (increased fungal and weather resistance)
- reduced equilibrium moisture content
- change in colour (red-brown to almost black colouring)

In principle all wood types can be heat-treated. Most common is the thermal treatment of native coniferous (pine, spruce) and deciduous woods (maple, beech, birch, oak and ash) for use outdoors without the need for ecologically unacceptable chemical treatment. Heat-treated woods are also used as an alternative to tropical woods e.g. for outdoor terraces and garden furniture. The darker colouring is also suited for use in interior fittings as an alternative to exotic woods transported from far-off locations.

The heat treatment process involves the precisely controlled heating of the cut timber in a vacuum chamber often with the addition of water or steam. Temperatures of between 190-240 degrees and steam extract certain substances (i.e. sap) from the wood and effect a change in the cell-wall structure of the wood.

The ability of the cells to take up water is significantly reduced. Moisture absorption is therefore greatly reduced and much slower than untreated wood. Wood expansion or shrinkage is reduced by 50% and also exhibits a greater resistance to fungal and bacterial infection.

In general the surface hardness improves whilst the elasticity and bending strength decreases. Thermally-treated wood is therefore less suited for structural use. Because of this increased brittleness, sections and panels are typically cut and profiled before treatment. Predrilling is also recommended when using heat-treated wood.

The heat-treatment process can take between 30 and 60 hours depending upon the wood being treated and the process is carried out in special chambers. The desired wood properties can be controlled by adjusting the treatment duration and temperature. The process differs slightly from producer to producer and as a result there are at present no uniform standards.

The process is regarded as ecologically preferable in comparison to treatment with chemical additives, coatings or impregnations as the wood can be recycled, composted or burnt after its useful lifetime.

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