Thermal modification of American hardwoods

Thermally-modified timber (TMT) is not a new concept. The ancient Vikings knew that when building defensive fencing for fortifications, poles made from timber with a burnt surface lasted longer than those without. Modern studies on the possible benefits of TMT began in the 1930s and 40s in Europe and the United States, but no product reached commercialisation. It wasn’t until the late 1990s that the first commercial method of thermal modification that we recognise today was first introduced, rather fittingly, in Scandinavia, as a way of improving the durability and stability of native softwoods. More recent experimental and development work has shown that the thermal modification process works equally well with some American hardwood species and the first commercial kilns were introduced into North America in the mid 2000s. The TMT process is essentially a high intensity kiln schedule that lasts between 3 and 4 days, depending on the species and dimension of the timber being treated. The temperature reaches 180 to 215 Celsius, subject to the level of durability required. The process requires an inert atmosphere, devoid of oxygen, to prevent combustion and this is usually either steam or a vacuum. During the process the chemical and physical properties of the timber undergo permanent change. The two most beneficial effects of the TMT process are dramatically improved stability and durability of the timber. The latter is particularly relevant to species that have an inherently low natural durability, such as American ash and tulipwood. After the process the moisture content of the timber is 4 to 6 percent. The equilibrium moisture content is permanently reduced, which means that the timber does not react to changes in humidity as drastically as in its natural state - in effect reducing the ability of the timber to absorb moisture and so greatly improving its stability. The durability is improved by removing the hemicelluloses and carbohydrates from the wood, which are the main food sources for wood destroying fungi. Recent independent durability testing conducted at the Italian timber research laboratory, CATAS, has shown that the durability of four American hardwood species - ash, tulipwood, soft maple and red oak (quarter sawn) - can be improved to durability class 1 (very durable), which is the highest rating possible and equal to that of tropical timber species such as ipe. This now means that these species can be used for exterior applications such as cladding, decking, shade structures and high quality outdoor furniture. Other benefits of thermal modification include a reduction in thermal conductivity by around 20 to 25 percent, which is a significant benefit for window manufacturers. Surface hardness is also improved, although this is yet to be quantified. The other major characteristic of TMT is a change in appearance; the colour of the timber will darken and this is influenced by temperature and species. TMT also machines beautifully, particularly American soft maple. Thermally-modified U.S. hardwoods offer a real alternative to traditional preservative treated timber and tropical timber for most external applications. There are no special handling precautions required with TMT and there are no environmental issues associated with wood disposal, or issues of leaching of chemicals whilst the timber is in service. Also, there are no potential sustainability or legality issues that may be associated with some tropical timber species. There are disadvantages, but these are few. Bending strength is reduced, so major structural applications are not recommended. However, engineering by finger jointing and glue lamination is still possible. The process is not suitable for thicknesses over 50mm. There are now around 120 TMT operations around the world, with nearly 100 in Europe and 10 in the United States producing commercial quantities of thermally-modified timber.

Neil Summers

Opening photo: Wish list, Paul Smith's Shed. Photography by Petr Krejci Photography