France is a now a leading producer and exporter of heat-treated hardwoods, which complement its range of naturally durable timber species, such as oak and black locust (false acacia).

While naturally durable hardwoods can be used in damp or humid conditions, without the need for any chemical preservatives, other species require pressure treatment if they are to be used for exterior applications such as decking, cladding and garden furniture.

The alternative is heat-treated, or thermally modified timber, the production of which has become an industrial process using highly developed modern technology. This is particularly the case in France, where thermally modified wood products are actively developed to meet a growing market demand.

Gilles Négrié from the FCBA, France’s largest timber research institute, says heat treatment results in a new material.

"Some constituent elements like hemicelluloses decompose, lignin molecules are fused together and the crystalline structure of the cellulose is altered," he said.

The decay of the hemicelluloses reduces the hydrophilic properties of the wood, making it less likely to change shape – and, as a result, be more durable. The heat treatment also makes the timber more resistant to attack from fungi and less attractive to insects.

Researchers have also pointed out that the formation of chemical bonds through the cross-linking of lignins leads to a hardening of the material. And other benefits include reinforced impermeability, even colouring, improved thermal insulation and fixed tannins. On top of that, heat treatment results in an environmentally-friendly material, which can be recycled.

Demand in France and across Europe has been increasing, according to Mathieu Blanc, sales director for Sivalbp. "We have been developing thermo-stabilised products for four years and in the past two years there has been a real surge in the market and sales are increasing every year," he said.

French producers now manufacture 25,000m3 of heat-treated wood per year and the product range is "large and varied", according to Louis Naudot, manager of Dumoulin Bois.

His company has been heat treating species such as oak, beech, ash, poplar and chestnut for applications such as timber frame, decking and cladding for a number of years.

"Each species has its own characteristics and the aim is to offer a very wide variety to suit many requirements," he said.

Ducerf is one of France’s leading manufacturers and distributors of hardwoods and also a major user of heattreated timber through its Profiled Wood unit. It too has experienced an expanding range of applications.

"Heat-treated woods are being used in an increasingly diverse range of situations, including in interiors, with jointed gluedlaminated panels and solid laminated panels," said Julien Guénard, the company’s quality manager.

Heat-treated timber is proving popular with end users, particularly furniture and interior designers and those manufacturers who are looking to use different species. For example, Réné Bruger, who runs fencing and gate specialist Jardimat, was looking to expand his range of wooden doors with a new hardwood that would be both easy to work with and affordable. The answer was heat-treated poplar.

"This little-known wood needs to be rediscovered," said Mr Bruger.