Thermally modified hardwoods offer great potential for outdoor applications

Commercial thermal modification of timber was developed and introduced in the 1990s, and is becoming increasingly popular with architects and designers globally as a substitute for tropical timbers and traditionally preserved timber.

The American Hardwood Export Council recognises thermal modification as a developing market with great potential for American hardwood producers.  There are various species that can be successfully treated such as – ash, soft maple, tulipwood, red oak, yellow birch and hickory. 
Thermally modified timber (TMT) was first developed to improve the performance and durability of softwoods, but it has more recently been extended to boost the performance of hardwoods, allowing certain timbers to be used externally with no additional protection. Following thermal modification, ash, soft maple, tulipwood and red oak (with the best results from quarter-sawn timber), can achieve Class 1 durability, the highest possible rating, equivalent to Teak. The treatment process consists of gradually heating the timber to a temperature of between 180 - 215°C for three to four days (the length of time depends on the thickness and the species of timber). This has to be done in an inert atmosphere (that is, one that contains no oxygen) to prevent the timber igniting. This is normally done either in steam or in a vacuum. This process permanently alters the wood’s chemical and physical properties.  This is very different to kiln drying, which only reduces the moisture content of the timber.
The thermal modification process reduces the timber moisture content down to 4-6% (very low). This has the effect of drastically reducing the equilibrium moisture content. Put simply, the physical structure of the wood is changed which limits the ability of the wood to absorb moisture, so products are more dimensionally stable and less prone to cup, warp and twist with changes in humidity. The thermal modification process also destroys the hemicelluloses and carbohydrates in wood that provide the main food sources for insects and rot-producing fungi. The timber is therefore more resistant to rot, and does not necessarily need any surface treatment, painting or pressure treatment. This increase in dimensional stability and decay resistance significantly extends the service life, and has the potential to reduce the maintenance needs of products made from thermally modified hardwoods.   
In addition, the timber has an attractive dark colour running through the whole cross section of the timber.  The timber will still turn grey over time due to the degrading action of ultraviolet light. For a timber such as ash, which was a popular choice for furniture several decades ago, this opens new possibilities in durable and attractive external applications. Although the thermal modification process does use energy to heat the timber, this increased carbon footprint has to be set against the environmental impacts of the various protective treatments that are no longer needed.

During the summer of 2014, AHEC collaborated with fashion icon Sir Paul Smith and Nathalie de Leval as part of ‘The Wish List’ – an extraordinary collaborative project created by AHEC, Benchmark Furniture and Sir Terence Conran who invited nine other famous commissioners to work with talented up and coming designers to create “the object they had always wanted”. Paul’s request was for a garden shed, “Somewhere you can go to switch off, somewhere to relax,” he says. As Smith wanted the shed to be all wood, thermally modified hardwoods, with grade-one durability, seemed a sensible choice. Together, Smith and De Leval chose thermally modified ash for its dimensional stability, rich colour and grain. De Leval consulted with Arup’s timber specialist, Andrew Lawrence, on the structure, “Paul’s Shed is the first structural use of thermally modified American ash,” says Lawrence. “The thermal modification makes the ash resistant to decay and imparts a wonderful dark tone, while at the same time preserving enough strength for modest structural use. The next step will be to do some formal strength testing.”  The thermally modified ash was kindly donated by UK importer Morgan Timber and US exporter Northland Forest Products, who produce the TMT under the commercial name “Cambia”.

AHEC European Director David Venables says, “Paul wanting a shed gave us the opportunity to really test the performance of thermally modified American ash. We believe this technology is a key part of the future for using wood externally. The market is growing and designers and architects want to use wood but it has to perform well, look good, and last, or it just won’t compete. TMT uses no chemicals, improves stability and has a very low environmental impact relative to other material solutions. So it was amazing to have the shed in the courtyard of the V&A for six weeks, in all weathers. The public reaction was huge and to top it all Paul came to the launch, he loved it and was especially delighted we had taken up the challenge he set us to make it rotate!”

Notes to Editors:

American Hardwood Export Council
The American Hardwood Export Council (AHEC) is the leading international trade association for the U.S. hardwood industry, representing the committed exporters among U.S. hardwood companies and all the major U.S. hardwood product trade associations. AHEC concentrates its efforts on providing architects, specifiers, designers and end-users with technical information on the range of species, products and sources of supply.
AHEC produces a full range of technical publications which are available free of charge by visiting

Thermally modified ash was supplied by:
Morgan Timber  
Northland Forest Products

Paul Smith

Paul Smith is a quintessentially English designer who has created a hugely successful business that is known around the world.  The Paul Smith empire now spans, menswear, womenswear, accessories and items for the home.

Nathalie de Leval

Nathalie de Leval is an independent furniture designer and maker, focusing on bespoke commissions and products predominantly in wood. Originally a sculptor, she studied cabinet-making in order to improve her making skills, but the furniture quickly took over, taking her to an MA at the Royal College of Art.