Heat Treatment of Wood
For centuries, wood is a reliable and proven material which is used in construction because its characteristics have not changed over time. It is, however, changed in time, but always in the expected boundaries. Top experts and scientists have revealed all the secrets and also have tried to promote it. Have they managed to do that?
During the 1920s, the Finnish people followed the heritage and found an idea of dried wood, heat-treated, or thermowood. Thermo-wood is processed on high temperatures, from 160°C to 260°C, depending on the species, chemical structure, but with no traces of any chemical agencies.
The process of heat treatment takes place in special chambers without oxygen, in order to avoid fire of wood. During the first phase, wood is heated up to 100°C and then the temperature is raised to 130°C and it is constant until the moisture index becomes 0. The second phase of heat treatment takes place at the temperature of 215°C (it can be lower or higher, depending on the wood species). The temperature is constant for a preset period of time which depends on the species and on the visual effect which is to obtain. The third phase covers the period of cooling and adaptation of wood to the new structure. The temperature of the wood is lowered to 80°C to 90°C, the index moisture is raised to the new balanced moisture which now becomes much lower in comparison to the untreated wood.
What is the purpose of heat treated wood?
First of all, heat treated wood has bigger dimensional stability, in other words, every “movement” of wood, including warping, swelling and strangulation, is lowered for about 50%. Lignin, which is one of the key components of wood, regulates water, changes its structure under high temperatures and thermowood is not any more hygroscopic – it does not absorb water. Hemicellulose, the basic food for different types of fungi, mildew or insects, decomposes itself and lengthens the life of wood because the possibility of rotting is smaller. This means that thermowood can be also used in the conditions including high relative moisture of air and when the wood is directly open to atmospheric conditions. During the heat treatment, the sugar in the wood is caramelized and the wood changes its colour. Depending on the temperature and length of the treatment, the colour goes from light beige to completely dark and almost black nuances.
With heat treatments, the insulatory properties of wood are also better. Although, it must be said that thermowood is less elastic that untreated wood, in other words, some species become brittle after the treatment and are easily damaged.
What is the application of heat treated wood?
Thermowood is used in flooring industry for making high quality parquets. It is especially useful for larger dimensions of parquets which in their appearance remind of deck flooring. It can also be installed in rooms with high moisture, such as saunas, bathrooms or indoor pools, respecting certain conditions.
If the final finish is done using natural oil and wax, these parquets can be laid not only in residential, but also in the rooms intended for commercial use, which suffer a greater load or the ones which have strict non flammable regulations, since the thermowood is harder to be on fire. A wide range of colours which are obtained by heat treatment offers a vast number of solutions for interior design without the use of poisonous chemicals.
Thermowood can also be used for production of wooden flooring for outdoor use, for terraces, porches, gardens, around pools, but also on the bridges, piers, docks, etc. Since the atmospheric influences cannot do a lot of damage, thermowood can also be applied to facades. Thermowood is also used for carpentry and furniture, so that the interior and exterior designs can be encompassed into a unique solution which is very important for architects and designers. To put it in another way: heat treated wood is stabile with less possibility of dimensional deformations, the insulatory properties are better, resistance to rotting and impacts of different insects is higher which makes thermowood a long-lasting material.
The previous sentence leads to the conclusion that, except for the fact that the process of heat treatment is ecological because it does not use poisonous substances, thermowood with its properties enables regeneration of forests. At the same time, warm and exotic colours which are obtained during the heat treatment, reduce the demand for rare and endangered species which are uncontrollably used and cut. Luckily, the Scandinavians have started, and the whole world accepted, a new trend of heat treatment which promotes the characteristics of thermowood.