Heat-treated wood is being developed by the forest industry into a new successful product for the export market. Thermo wood is ideal for interior fittings such as parquet and floorboards, wall and ceiling panelling, sauna fixtures and kitchen furniture, etc.

The protective effects of treating wood thermally have been known for hundreds of years, but it is only in the last decade that science has been used to research the phenomenon.

Several years ago, the Finnish forest industry became interested in the heat treatment of wood and began to fund research. The quantities produced are still modest - an estimated 30,000 cubic metres of thermo wood this year, which is less than the annual production of a medium-sized saw mill. But when the new production facilities of the major forest companies, Stora Enso Timber and Finnforest (part of the Metsäliitto Group), are finished at the end of this year, production capacity will be large enough to merit serious attention.

Improved rot resistance without chemicals

When wood is heat-treated the fungi that cause decay are unable to survive, and the wood does not 'live ' in the same way as untreated wood. As a result, it hardly swells or shrinks, which makes it a very useful material for doors, windows and the like.

The properties of wood change when it is treated at high temperatures. When wood is 'baked' at temperatures of over 200ºC, its sugar content breaks down and becomes unusable by fungi. At the same time, natural protective agents present in the wood spread and protect the wood. In other words, the wood's rot resistance is greatly improved without any need to use chemical substances.

Thermo wood is ideal for interior fittings such as parquet and floorboards, wall and ceiling panelling, sauna fixtures and kitchen furniture, etc. It can be used in making garden furniture, doors and windows and is well suited as external panelling for buildings. It can even be used for building piers. Thermo wood cannot, however, endure direct contact with the ground.

Thermo wood has also been used experimentally in musical instruments, and has proved to be an extremely useful material. Heat-treated wood is like aged wood, which means that it has better acoustic qualities.

Heat treatment turns the wood a soft brown. When used in interior fittings it bears comparison with teak.

Production capacity to be multiplied at the end of year

Approximately ten companies in Finland currently produce thermo wood. About half of production was exported last year, and thermo wood is expected to become a major export product within a few years. Thermo wood can be produced from pine, spruce, birch, aspen and alder. The raw material must be very high quality for it to produce good thermo wood. For example, knots in the wood are prone to scorching during thermal treatment, which spoils the board.

In addition to the currently operating production plants, two more major facilities will be completed this year. These will multiply production capacity.

Heat treatment lasts from 24 to 72 hours with cooling and final stabilising. The temperature of a batch of wood is raised gradually in a drying kiln and then lowered slowly during the cooling phase, to keep the wood from splitting. The temperature in the drying kiln is raised to 180-230ºC using steam. As the wood contains no chemicals, it can be destroyed by burning.

'Baking' reduces the splitting strength of the wood somewhat. Predrilling and the use of screws with widely spaced threads reduces the risk of splitting, however. A protective face-mask should be worn when sanding, as thermal wood dust is extremely fine.

Source: VTT (Technical Research Centre of Finland)

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