Heat-treated, or thermally modified wood, is an ecologically sound alternative to the more toxic methods used to strengthen wood and make it more decay-resistant. There are three related techniques currently favored for heat-treating wood, all developed in Europe. All increase the durability and hardness of the wood by one at least one class, rendering certain common woods a dark exotic color that makes an excellent substitute for tropical wood such as ipé from the Brazilian rainforest. Like ipé, heat-treated wood is dense and will split when nails are applied unless pre-drilled. Heat-treated lumber is typically used for siding, flooring, cladding, patios, outdoor furniture, and children’s playgrounds.
Retiwood was developed in France at the Mines Saint-Etienne engineering school and patented in the late 1990s. Retification dries wood at 160°C to 180°C and then exposes it to temperatures reaching 240°C in carefully designed ovens devoid of any oxygen so there is no risk of combustion. The company that holds the patent applies it to local species primarily ash, beech, popular and pine. Their products include cladding, floor boards and outdoor terrace boards.
The technique for ThermoWood was patented by VTT Technical Research Centre of Finland. It currently has two standard treatment processes, Thermo-D and Thermo-S. The process is based on the use of rapid high temperature (185°-225°C) and steam within a specially designed kiln. The peak temperature is dependent upon the desired end use and kept at a constant for 2-3 hours, then reduced using water spray. Average density is reduced by 10% with modulus of elasticity maintained or slightly improved. ThermoWood is not recommended for use in horizontal load bearing designs.
Platowood uses a multiple-stage treatment designed based on the research of H. Ruyter in the Koninklijke Shell Laboratorium in Amsterdam with commercial production since 2001. Both softwood and hardwood species are utilized, notably Norway spruce, Douglas fir, birch, popular and Scots pine. The wood is pre-dried in a conventional kiln to a moisture content of 14-18%. Afterwards, the wood is heated to 150°C-180°C within a water/steam environment at super-atmospheric pressure, then dried and cured before heated again to 150°C-190°C in a dry and lower pressure environment.
Other heat treatments are generally based on these three processes with alterations to avoid patent infringement. Heat-treated lumber is typically a dollar more per board foot than a non-treated counterpart of the same species. The lack of chemical treatments and rich deep colors make it appealing to many health and fashion-conscious home owners and a lucrative market for lumber manufacturers eager to promote themselves as “green” industries.
Peter Wendt is a freelance article writer residing in the Austin, TX area. A couple of his friends recently moved into new houses and he was intrigued by all the different wood designs that he had seen. He was very interested in learning more about how the lumber was treated in order to produce certain designs. Peter went to uslumberbrokers.com and learned everything he needed to know.