Timber can be modified by a variety of methods to enhance properties and change appearance. Modification using temperature is relatively simple and extensive trials using Welsh timber have been undertaken, but so far the process has not been developed commercially in Wales.

Dr Morwenna Spear of the BioComposites Centre, Bangor University.

Why can’t I get thermally-treated Welsh timber?

This was a question raised by one of the delegates at WoodBUILD 2019 where we were discussing how Wales could become a high-value forest nation. Ceri Loxton decided to find out more and talked to Dr Morwenna Spear, a wood scientist at the BioComposites Centre at Bangor University. Morwenna has been involved with a number of research projects over the years looking at thermal modification systems for Welsh timber.

Thermal treatment: what it is?

Thermal modification, or thermal treatment, of timber is a form of timber modification for the purposes of changing one or more properties of the timber compared to the untreated starting product. It should not be confused with the term heat treatment, that refers to a sterilisation process for pallets and packaging timbers (done at 60°C). Thermal modification refers to timber that has been heated to high temperatures so that a specified core temperature has been reached for a minimum period of time, allowing changes to occur within the chemical components of the wood. It also differs from kiln drying where heat has been used to dry the timber to a specified moisture content, usually approximately 18% for external use and 10 to 12% for internal uses.

Inspecting the quality of treated planks during unloading.

During thermal modification the timber is exposed to elevated temperatures (typically 180 to 240°C) for a sufficient period of time to alter the chemical composition of the hemicellulose within the wood cell wall. This results in a colour change, a reduction in density, an increase in dimensional stability and may also result in elevated durability or reduced strength depending on conditions (Esteves and Pereira 2009). The extent of decay resistance is largely governed by the duration and the temperature achieved during this treatment stage. Some systems treat at higher temperatures, to increase durability, for example.

Thermal treatment is an industrial process which improves some wood properties, such as dimensional stability and durability. There has been a steady increase in heat-treatment companies and process across Europe, particularly in the last 15- 20 years. This has been driven by a number of factors, including wanting to add value to European timbers, reduced availability of some tropical species, and development of alternative methods to chemical modification and preservative treatments. Brands include Thermowood, Lunawood

Dylan Jones, who works for the Cwm a Mynydd RDP project through Caerphilly Council, and who previously worked on thermal treatment as part of a wider value adding project for Coed Cymru, explained more because “the process adds no additional chemicals to the timber end of life options for reuse, recycling or disposal of thermally treated timber could become a key advantage over other treatment methods in the future.” This is because Governments are starting to realise that we must look seriously at how products can be reused, recycled or disposed of at the end of their life.

Heat treated timber in the UK

Both softwoods and hardwoods can be thermally modified and a number of different, patented systems are available. Most thermally modified timber is imported into the UK and primarily is used to supply the timber cladding and decking market.

In the UK, Vastern Timber have a system for thermally modifying British hardwoods and the resulting product is known as Brimstone timber. While some of the timber they treat may have originated from Welsh forests, the treatment takes place outside the UK. At the moment no Welsh or UK sawmills or processors are producing thermally treated timber, although potential has been demonstrated by Coed Cymru, Coed Mon and Bangor University.

Uses of heat treated timber

Heat treated wood can be used outdoors for: cladding, decks, garden furniture, doors and windows; indoors it can be used for kitchen furniture, parquet, decorative panels and the interior of saunas.

Research into heat treatment of Welsh timber

Over a number of years Coed Cymru and the BioComposites Centre (in association with a number of different project partners) have undertaken research on the heat treatment of Welsh species including larch and western hemlock, as well as various hardwoods.

Conventional, commercial heat treatment processes such as Thermowood and Brimstone require relatively large set ups and high operating temperatures for extended periods of time for the primary purpose of improving dimensional stability. But a CIRP (Collaborative Industrial Research Programme) project involving Coed Cymru, the BioComposites Centre and a number of other partners focused upon small scale treatment units. One output was improving the joinery properties of Welsh larch through a mild treatment.

Heat treatment of Larch timber. Schematic showing a typical three day treatment, where each day is kept within working hours for a small enterprise.

The system was devised primarily to heat the timber to achieve a mild or moderate thermal modification, therefore the timber experiences temperatures that are lower than for the commercial thermal treatments. The main characteristic of this system is its applicability for small companies with limits on the operating hours, either due to number of employees, or to insurance or practical time limits on operation at the premises. The conventional continuous multi-day processes for heat treatment are not suited to smaller Welsh sawmills and timber processors. So a system using separate working days to achieve a drying stage, treatment stage and a conditioning stage was developed.

Planed surface of thermally modified larch for cladding

Mild to moderate heat treatment (maximum 160°C for 5 hours) rather than high temperature conventional heat treatment was explored because (1) high temperature heat treatment had already been explored and commercial ‘off the shelf’ solutions are available, (2) as a possible treatment method for larch to improve workability and machinability, (3) as a practical method for small (single shift) operations.

The mild thermal modification treatments were carried out in three stages (1) drying (2) treatment and (3) reconditioning. Stages were chosen so that they could be carried out in a single working day to allow small businesses to operate kilns on a single shift work pattern.  A definition of the treatment phase (set temperature of the kiln) is provided in “Physical properties of UK grown larch subjected to mild and moderate thermal modification processes” here.

The trials successfully showed that mild and moderately heat-treated larch has improved machining properties, the timber planed well, with relatively few chip bruising marks and post-machining raised grain was considerably reduced, as was fuzzy grain where grain deviated around knots etc.

Warnings and advice

Thermally modified Welsh larch cladding installed at Halen Mon on Anglesey in 2014/2015

Mild to moderate heat treatment of larch (made by Coed Mon and Menter Mon) proved very successful in this project. Cladding produced during the trial and installed at Halen Mon on Anglesey is still performing well after four years of service. “However as a method for a small enterprise it would require careful consideration, particularly if you are thinking about it as a sole operator” says Dr Morwenna Spear principle scientist/project manager on the project, “Loading and unloading the kiln is hard manual work if being done single-handed, so a small team may be needed. You also need to think about supervision of the kiln during its operation. It could work if co-located with a number of smaller companies working together, to share responsibilities and develop a business model.”

Is mild to moderate heat treatment commercially viable?

“If it can get more of our homegrown timber into high value cladding and joinery applications (which then act as carbon stores) then yes the sale price may justify investment of money and energy.” It’s also a relatively environmentally benign process, so an added benefit may occur when that carbon store is acknowledged. It is a chemical free process, so the environmental impacts are lower than some other treatment processes.

Why haven’t we seen more Welsh larch thermally treated in this way since the project ended?

“Lack of momentum. If there had been a follow-on project after completing the Halen Mon cladding, then the consortium may have been able to keep the ball rolling and produce more product.” Explained Dr Spear. “It’s all been developed and demonstrated, the next step is getting the timber on the market. We had timber merchants showing an interest due to the distinctive colour and local story. Scale up of production needs a secure market, and a market needs a ready supply. It can be a chicken and egg thing, where someone needs to take the next step.”  “Certainly, more demonstration projects would help and a clear understanding of where demand would come from and how large it could be”.

Should it be pushed further?

Morwenna believes that the process has potential and the feedback from joinery companies was good. The potential for use as cladding was clearly demonstrated during the project. Cladding could be a substantial market, even though it goes in and out of fashion and particularly design constraints (particularly for social housing), related to Secured by Design etc., cladding brings benefits in a climate emergency.

Dylan Jones, RDP project officer at Caerphilly County Council, who worked on the same project with Dr Spear agrees, “I believe the time is right to get people back into a room together and discuss what could be done”. Mr Jones went on to say that a dedicated project with staff, recourses and funding would have the potential to take the previously good work done on thermal treatment on Welsh timber forward and make products available for commercial use. The potential is there to add considerable value to what is often underutilised timber stock and add a boost to Welsh forestry and manufacturing industries.

Recommendations for next steps

“If a business case could be made” says Morwenna “then the expertise gained in the project is still available and could still be accessed.” So, if there is a reader out there who is thinking about heat treating Welsh timbers please get in touch with Dr Morwenna Spear at The BioComposites Centre, Bangor University to find out more.

Further sources of Information

Building success in modified wood. Link to summary of BioComposites Centre & Coed Cymru project with photos of modified larch cladding on a building. Link to document here.

Esteves, Bruno & Pereira, Helena. (2009). Wood modification by heat treatment: A review.  BioResources. 4. 370-404. Link here.

Determination of verifiable programmes for heat treatment of Welsh larch . Link to report here.