Thermally modified wood
Thermally modified wood is wood that has been modified by a controlled pyrolysis process of wood being heated (> 180 °C) in absence of oxygen inducing some chemical changes to the chemical structures of cell wall components (lignin, cellulose and hemicellulose) in the wood in order to increase its durability. Low oxygen content prevents the wood from burning at these high temperatures. Several different technologies are introduced using different media including nitrogen gas, steam and hot oil.
The first technological approach of thermal modification processes of wood is the paper of Burmester. There are 5 different thermal modification processes : Thermowood (or Premium wood) in Finland, Retification process (Retiwood, New Option Wood) and Les Bois Perdure in France, Plato process in Netherlands, Oil-heat treatment in Germany (OHT-Process)
Description of the process
Three of the processes are performed in one step, using oil (oil-heat treatment), nitrogen (Reti wood) and steam (Le-Bois Perdure). The Thermo wood process consists of drying, heat treatment and finally cooling/conditioning, and takes up to 72 hours. The Plato process consists of hydrothermolysis, dry curing and conditioning, and can take up to 7 days. The required time depends on wood species, width and initial moisture content.
A two step process which first subjects wood saturated with water to hydrothermal treatment at 160 °C–190 °C at a high pressure. After the moisture content in the wood is reduced to 10% then a curing step is performed at 170 °C–190 °C under 1 atm of pressure.
Retification refers to the French word rétification, which is the abbreviation of réticulation (creation of chemical bonds between polymeric chains) and torréfaction (roasting). Wood in this process must have a moisture content at 12% or lower which can be attained through simple drying processes. The wood is then placed in a high nitrogen atmosphere with no more than 2% oxygen content.
Les Bois Perdure
Freshly cut timber must first be dried and then will be heated in a steam environment using the leftover moisture in the wood.
Oil Heat Treatment
The wood is placed in an oil bath between 180 °C and 220 °C. This process can also be used with chemical additives to alter the wood in other ways.
This process is similar to the Les Bois Perdure treatment in that it uses a steam environment at atmospheric pressure to treat the wood. However, this process can also be used on "green" wood and was the most widely used commercial process as of 2004.
Characteristics of thermally modified wood
The main advantage is that softwood can be used for applications that require high durability, which might be seen as environmentally friendly. Durability class 1–3, according to European Standard EN 350-2, can be obtained out of non durable (class 5) softwood species. 
The main disadvantage is that the strength is decreased as a result of the high temperatures. In general the bending strength is reduced up to 30% with more reduction at higher temperatures. 
The biological resistance against some (not all) micro-organisms and insects is improved. Shrinking and swelling is reduced up to 50–90%. The treated wood is somewhat darkened in colour. As a principle the heat treatment process can be done on all wood species.
Matureness of the technology
Thermally modified wood is not just an academic topic these days. Thermal wood treatment has already been penetrating the market for a number of years. There is also some industrial input in the research and development. The Natural Resources Research Institute at the University of Minnesota Duluth has a pilot-scale kiln to research the effectiveness of thermal modification on underutilized Minnesota tree species to expand the wood markets.
In the guitar-making industry, this process is called "Thermo Curing", "Baked" and "Roasted", among other names. Some guitar manufacturers have begun using acoustic sound boards and electric guitar fretboards that are thermally cured in order to help prevent the typical warping and cracking that often occurs from seasonal humidity swings. As a secondary benefit, acoustic guitars tend to sound like well-broken-in aged instruments much sooner than do non-thermally modified guitars.
There is ongoing research to optimize the industrial processes and the various parameters to produce it and to develop the applications for this wood. There is also research in order to find new wood heat treatment processes, even trying to combine different wood modification processes. Finally, research is ongoing in an effort to clarify the chemical changes undergone by wood during the thermal treatment process.
- Burmester, A. (1973): Einfluß einer Wärme-Druck Behandlung halbtrockenen Holzes auf seine Formbeständigkeit. Holz als Roh- und Werkstoff 31, pp. 237–243
- "Principles & applications of wood retification" (PDF). Thermotreatedwood.com (Georgia, United States). Retrieved April 4, 2016.
- "Strength and Colour Response of Solid Wood to Heat Treatment" (PDF). Luleå University of Technology. Retrieved April 4, 2016.
- European Standard EN 350-2 (1994); Durability of Wood and Wood-based Products – Natural Durability of Solid Wood: Guide to natural durability and treatability of selected wood species of importance in Europe
- "Recent Development of Thermal Wood Treatments" (PDF). International Association of Wood Products Societies (IAWPS). Retrieved January 24, 2016.
- "Wood modification developments" (PDF). HERON. Retrieved January 24, 2016.
- "8th European Conference on Wood Modification October 2015 Helsinki". ECWM 2015. Retrieved January 24, 2016.