Thermally modified timber: AJ&B share their experience with TMT U.S. tulipwood

in Technical Information, Environment, Species focus, Architecture and Design, Comment

Last year AHEC collaborated with architect Asif Khan and carpenters Aldworth James and Bond (AJ&B) to create a space-enhancing playground structure for the pupils of Chisenhale Primary School. Not only was it an excellent chance to help in an important community project but it also gave AHEC the opportunity to trial the permanent, outdoor use of thermally modified (TMT) American tulipwood for the first time.

Untreated tulipwood has creamy white sapwood and is sometimes streaked, with the heartwood varying from pale yellowish brown to olive green. However, to survive outside it has to be thermally modified giving it a rich dark brown colour.

The thermal modification process involves heating the timber to between 180 - 215°C for three to four days, in a vacuum. During heating the part of the wood that bonds with water – hemicellulose decreases which reduces the timbers moisture content to less than 5% (very low). This means that it can no longer absorb moisture and as such is less prone to warping, cupping and other affectations from changes in humidity. Along with hemicellulose, heating also removes the carbohydrates found in wood, normally eaten by insects and rot-producing fungi making the timber more resistant to rot.

Whilst drastically increasing the timber’s outdoor performance, the drying process also makes it more prone to cracking and breaking. As AHEC has never had the opportunity to monitor its use, it was an exciting experiment to see how it would fair.

Dan Benson, Head of Operations at AJ&B was impressed with the timber from the moment it arrived from the timber yard; there were no surprises.  “There is little fluctuation in the thickness of the boards of timber, so there was little machining that we needed to do to manipulate any thicker pieces” which would be Dan’s normal concern when working with a new timber. There was also no difference in the amount of damaged stock than with any other timber.

Once the carpenters at AJ&B began to work with the thermally modified tulipwood and ash, using the planer/thicknesser, band and table saws as well as their CNC machine they were satisfied with its performance, “it behaved almost exactly the same as non-thermally modified tulipwood” the only exception being it’s slightly reduced flexibility. Del Bond, co-founder of AJ&B comments, “The material is very similar to non-thermally modified timber but slightly more brittle and tricky to glue. However it machines and takes screws and nails exactly the same.” He added, “if sanded the TMT turns an orange colour, but since it has a good looking natural finish, which fades from a rich brown to a silvery colour, we left it planed and unfinished.”

Read more about a Room on a Hill and see our short film about the project.

See also Paul’s Shed made from thermally modified American ash.