Modifying Behaviours

Modifying Behaviours

ARTICLE  · By Camilla Hair · 26 March 2020 

Architects across the globe last summer declared a ‘climate emergency’.  The RIBA followed suit, laying down a ‘2030 Challenge’ to its members to reduce the embodied carbon in the materials they use.  The time is thus right for more widespread specification of timber and wood products. 

Demand can only increase for the skills of IoC members in using wood to its full advantage, aesthetically and environmentally.

Architects, though, are also looking at the whole life impacts of a building, and thus demanding materials that will last as long as possible. All IoC members, from the longest-standing to the increasing number of student members, could benefit from making themselves aware of the numerous modification options now available for extending the longevity of timbers.

Although much progress has been made with sustainability licensing schemes such as FLEGT for tropical hardwoods, as yet their supply through to the UK is still limited.  Modified woods, produced using a variety of processes, can represent a more easily available alternative to some tropical species.

Modified woods include Accoya®, Lignia®, ThermoWood® and Kebony® are gradually making their way into the carpentry and joinery sectors, with Accoya® perhaps being the most well-known to date. Each has its own process for extending durability.


Accoya® wood combines chemistry with wood science to create a durability said to be better than that of tropical hardwoods, starting from a base of sustainably-grown redwood (Pine).

Accoya’s patented acetylation production process changes the free hydroxyls within the wood structure into acetyl groups using a product derived from acetic acid (better know as vinegar in its dilute state).

This change in the wood reduces its ability to absorb water. Two effects stem from this modification: Accoya wood becomes more dimensionally stable and extremely durable.  The reduced ability to absorb water also decreases Accoya’s attraction to wood’s normal agents of destruction, fungi and rot.

Sustainably-grown northern European softwood is further kiln dried to ensure the acetylation process is complete.

There is thus no need for further preservative treatment of cut ends when Accoya timber is used – a time-saving for carpenters and joiners alike. As the Accoya process adds nothing to the wood that does not already occur within it, Accoya wood is not likely to add any toxins to the environment during or at the end of its life.

It also has lower maintenance requirements to normal timbers as it has an in-built resistance to UV degradation.  Coatings will therefore last longer, giving greater cost-effectiveness over its extended lifespan.

For clients looking for longevity, Accoya carries a 50-year warranty used above ground and a 25-year in-ground or freshwater warranty. It is also one of the few wood products to be ‘Cradle2Cradle’ certified, a form of measurement of its usefulness in terms of the Circular Economy.

The use of Accoya will also contribute, both directly and indirectly in terms of scoring, to client schemes looking to achieve BREEAM New Construction certification. For interested IoC student members, a worthwhile ‘watch’ is the Accoya wood video created by Westgate Joinery on YouTube, which gives an excellent overview.

Accoya’s process has also fed into other developments, such as the innovation involved in creating Medite® Extreme Tricoya fully external MDF, which is based on acetylated wood fibre.  This machines like a normal MDF product, but can be used for applications from front doors and flower planters to external cladding, and in areas where previously only plastics and metals could withstand the elements.

An example of its use is to replace one of the mountain rescue equipment boxes stationed across the Lake District, which have to withstand all that the great British weather can throw at them all year-round.


Kebony® is the trademark name for another method of enhancing the durability of sustainably-grown softwood.

The ‘Kebonization’ process modifies the wood by forming stable, locked-in furan polymers in the wood cell walls. This is achieved by the impregnation of furfuryl alcohol, which is produced from agricultural crop waste. The wood is then heated and cured, sealing the resulting stable polymer into the wood cells.

This permanently alters the cell walls by thickening them by around 50%. Similarly, to Accoya, this produces greater dimensional stability and gives Kebony durability ratings akin to tropical timbers.

Again, an environmentally benign process, Kebonization reduces swelling and shrinkage by 50% from that of the original timber, e.g. Pine. Lifespan is said to be increased between 7-9-fold compared to the original timber.

From the woodworking perspective, Kebony creates almost no splintering when used. The process also increases stiffness and hardness of the resulting timber, essential for durable construction projects.

From the wood-worker’s perspective, Kebony contains no toxins or checmicals and the wood does not get hot in summer.

Used above ground, Kebony wood is resistant to rots, fungi and wood-destroying micro-organisms.

Cladding facades, decking, piers and promenades, roofs and floors have so far been made from Kebony. For furniture-makers there is also a Clear Grade version of Kebony.

Lignia® Wood

Lignia® Wood is the first modified wood to be manufactured here in the UK, in Wales.

A different type of resin impregnation and heat modification process, it also improves the dimensional stability, durability and resistance to rot and fungal decay of its Radiata Pine base material.

A 50-year warranty is offered for certain applications. The resin used in made locally in Wales.  A low-movement timber, Lignia machines in the same fashion as a medium density hardwood and any dust created in the workshop can be treated as per normal wood dust. It is also non-corrosive to standard steel fixings.

The resin content of the modified Lignia wood gives additional resistance to weathering. Lignia retains its post-manufacture darker colouring, weathering better than many other timbers, and any apparent variation in colour can be standardised by planning.


An entirely different form of wood modification is represented by ThermoWood®, a process of heat and steam modification originating in Finland. Metsä Wood, one of the key proponents of ThermoWood technology, uses Finnish Pine and Spruce as its base, treating it with heat and steam to temperatures reaching in excess of 200 degrees Centigrade.

During this treatment, chemical and structural changes occur within the timber, altering some of its basic characteristics and removing resin from the wood.

The process reduces the timber’s ability to absorb or lose water, again giving today’s much-desired dimensional stability, limiting softwood’s natural tendency to distortion, shrinkage and swelling. Used as cladding, for example, Metsä Wood’s ThermoWood-D offers a service life of 30 years without the need for chemical preservatives.

Modified wood products manufactured by Timber Trade Federation members also represent ‘Timber you can Trust’.

Each is made from a sustainably-sourced, certified material, with the producers submitting their sourcing practice to independent audit under the TTF’s Responsible Purchasing Policy.

Modified wood products manufactured by Timber Trade Federation members also represent ‘Timber you can Trust’.

Each is made from a sustainably-sourced, certified material, with the producers submitting their sourcing practice to independent audit under the TTF’s Responsible Purchasing Policy.

First published in Cutting Edge, Institute of Carpenters, March 2020