Thermally modified timber is manufactured from pine, spruce or deciduous trees using the thermal modification process. The process is based on the use of high temperature (about 200 C) and steam, so no chemicals are used in the process. Thermally modified timber has better biological durability and lower moisture content than sawn timber that has not been thermally modified. Furthermore, thermal modification can change the colour of the timber into a shade more reminiscent of a hardwood, with the colour changing throughout the entire piece of timber.
In Finland, thermally modified timber is manufactured using the ThermoWood®process, which produces it in two product classes, Thermo-S and Thermo-D. These classes determine the properties and uses of the end-product. In the ThermoWood® process, the sawn timber to be thermally modified is sorted by quality prior to thermal modification based on its own quality criteria. Because of this, the quality of thermally midified timber is not inspected according to the quality classification of untreated sawn timber.