Thermally modified wood - heat-treated wood or Thermowood, as it is also called - is wood that has been heat-treated to make it more resistant to certain conditions, especially outdoor, high humidity and aggressive weather conditions. Using it for wooden decking, outdoor decking, pergolas and gazebos or exterior cladding makes them more durable, more resistant over time. About the process by which wood becomes more resistant to various external factors, the transformations that take place in wood, how modified wood can be used and other such information, in the following.
Why thermally modified wood
Because as a result of these changes, the wood acquires qualities that make it very suitable for use in conditions of high humidity, wide temperature variations, scorching sun and bad weather. Here are some of these qualities:
Dimensional stability. Following treatment the wood becomes much more dimensionally stable, with dimensional changes due to moisture variation often being unnoticeable. Calculations show a decrease in changes of 50-90% compared to ordinary wood.
Resistant to outdoor conditions. Heat-treated wood can stand rain, damp, snow or scorching sun without significant changes. According to CE standard EN 350-2, wood types are classified according to their outdoor resistance into 5 classes. The most suitable for outdoor use are class 1 and 2, the least suitable are class 5. For example, tecul is in class 1, iroko in class 2 and birch and frasin out of 5. With this treatment, grade 5 wood can be upgraded to grade 1 or 2.
High insulation properties. As a result of the thermal change the heat transfer coefficient of wood decreases by 20-25%, making it a much better insulator. It is therefore widely used for cladding houses in Scandinavia and the UK.
Behaves the same no matter where it's used. The wood does not change regardless of the area of the house where it is used and regardless of whether it is used outdoors, in very hot or very cold areas, with high or very low humidity.
Wood is much more resistant to insect and fungal attack. The changes inside the wood lead to the destruction of the fungi's favorite substances. This makes the wood much more durable over time, with warranties for such products reaching up to 30 years and more.
Resinwood no longer contains resin. The treatment removes the resin from the wood so there is no risk of it coming out during very hot periods.
When did the idea for this type of wood come about
Studies on the possibility of wood gaining superior properties by modifying it with temperature have been around since the 1930s-1940s. The impossibility to better study the changes taking place in the wood cells and the rather high cost of the installations meant that the studies remained at the laboratory stage.
In the 1990s, VTT - the Technical Research Institute of Finland - found the idea interesting and developed it to the point where the patent for the Thermowood manufacturing process was granted in the late 1990s.
Thermowood is therefore a registered trademark and can only be used by companies that are members The International Thermowood Association (International Thermowood Association). To become a member of this association you must use patented technology and the finished product must meet certain quality standards. This is not to say that there are no producers of thermally modified wood outside this association, but they are not allowed to use the Thermowood trademark.
What is the process and what changes take place inside the wood
The wood thermal modification process goes deep into the wood, acting on the cell wall. It is no longer steaming to make the wood more elastic and bend it or about steaming to even out the color, as with beech.
This leads to decomposition of the hemicellulose in the wood and changes in lignin and cellulose. The breakdown of the hemicellulose is the reason why the wood becomes resistant to attack by fungi, which lose their food source.
The process consists of treating the wood with steam in autoclaves, at over 180ºC (up to almost 250ºC), under pressure and in the absence of oxygen. The process consists of 3 stages - drying, actual high temperature treatment and cooling. The treatment can take several days (2-7) because it depends on many variables: the essence, the thickness of the board, the desired results.
At the end, the equilibrium moisture content of the wood decreases by 5-6 percent to around 6-9%. The color of the wood darkens and becomes uniform. The color obtained depends on the process time and temperature. The wood is perfectly "clean" because no chemicals are used during the process.
Virtually any wood can be thermally modified. The process is more complicated for hardwoods (hardwoods) because of processes at the cell level.
Other treatments with similar effects
There are other ways of thermally modifying wood. But it is the most widely used worldwide.
Very important is that there is a risk that the wood will ignite at those high temperatures. That is why there must be no oxygen in the process. While Thermowood uses steam, there are processes that use nitrogen (RetiWood - France) or heated oil (OHT Process - Germany). Whatever process is used, the steps are the same.
Disadvantages
The main disadvantage of the process is the decrease in wood strength. The attack on the wood cell, and especially on the lignin, decreases the strength by 30%. Therefore wood treated in this way is not used as strength wood. In construction it will only be used for cladding, the strength being provided by other types of wood.
Another disadvantage is the color difference that can sometimes occur due to the different composition of the wood. Even if it belongs to the same species, the wood can be different depending on where it grows and this can be seen by slight changes in color.
How to finish
Heat treated wood is not used unfinished. Regular varnishes, oil or wax may be used for coating. If mounted outdoors and not coated with UV-absorbing products (special outdoor products), the chestnut-brown color obtained after treatment may fade.
The wood used in saunas is treated with paraffin oil, possibly with a bath to even out the color.
Where it can be used
As we have seen, wood was originally manufactured in Scandinavian countries and was mainly used for sauna construction. Its resistance to moisture and constant dimensional stability to variations in temperature and humidity quickly led to its use outdoors, for landscaping in the garden or by the pool, construction of terraces and pergolas or the cladding of buildings. In all these areas, exotic wood from Asia, Africa or South America was used, which was much more expensive than wood obtained by thermal modification.
But there are also more unusual uses. For example, using it to make guitars makes them sound much more like old instruments. Such guitars are called Baked Guitar or Roasted Guitar.
Find heat-treated wood here DIY stores, garden shops or directly from the manufacturers.
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