Robinia, acetylated or thermally modified wood – the comparison

Ferdinand Terlinden

1 day ago

7 min read

Robinia series · Comparing species

Natural durability, chemical modification or thermal modification: three ways to make timber last

Robinia (Robinia pseudoacacia) — also known as black locust, and often sold as acacia — lasts because its heartwood is durable by nature: class 1-2 under EN 350, use class 4 under EN 335, which means permanent ground contact. Acetylated wood and thermally modified timber change the material itself: one chemically, the other with heat.

Each family has its own ground. Acetylated wood, of which Accoya is the leading brand, claims durability class 1 (EN 350) and a manufacturer's warranty of 50 years above ground and 25 years in ground contact: an excellent timber for external joinery. Thermally modified timber reaches class 2 or 3. Robinia holds the ground and the structure.

× 1.6 to × 2

how much more impact-resistant a timber is before thermal modification than the same timber afterwards: the process takes away 37 to 51 % of its impact strength (Esteves and Pereira, BioResources, 2009). That is what rules thermally modified timber out for stakes and structures.

Wave-shaped public seating made from finger-jointed robinia boards on a seafront promenade — Wave-shaped seating in finger-jointed robinia boards on a seafront: scooters, sand and salt all year round. Robinia takes these knocks, whereas a timber destined for thermal modification was 1.6 to 2 times more impact-resistant before the kiln than after it.

Natural durability, acetylation, thermal modification: how does each one work?

Natural durability is a property of the tree. Robinia deposits extractives in its heartwood — flavonoids and tannins — that make the wood indigestible to wood-decay fungi. It leaves our sawmill in Teiu, Hunedoara County (Romania), debarked, with its sapwood (the pale outer ring) removed, and dry.

Acetylation is a chemical modification. The wood is reacted with acetic anhydride, a derivative of vinegar: the hydroxyl groups in the cell wall, which bind water, become acetyl groups, which do not. The manufacturer states that water uptake is reduced by about 80 %: without liquid water, fungi can no longer do their work.

Thermal modification works through heat alone. Heated to 180-215 °C in a low-oxygen atmosphere, the wood loses part of its hemicelluloses, the sugars that fungi feed on.

Which durability class and which use class does each one reach?

Two standards complement each other. EN 350 rates natural durability from 1 (very durable) to 5 (not durable). EN 335 defines use classes: class 3 above ground, use class 4 in permanent contact with the ground or fresh water. Robinia is rated 1-2 (EN 350) and meets use class 4 without treatment.

Acetylated wood is presented by its manufacturer as durability class 1 under EN 350, covering use classes 1 to 4. Its warranty certificate distinguishes 50 years above ground from 25 years "at or below ground level". This warranty covers fungal decay and excludes salt water.

Thermally modified timber is more modest. The International ThermoWood Association handbook distinguishes Thermo-S, at around 190 °C, aimed at stability, from Thermo-D, at around 212 °C, aimed at durability. Thermo-D pine reaches class 2 under EN 350, Thermo-S class 3: it is a use class 3 timber.

Why can't thermally modified timber be used for stakes, posts or structures?

The heat that destroys the fungi's food also attacks the wood's own framework. The review by Esteves and Pereira (BioResources, 2009) reports a loss of bending strength of 20 % from 180 °C and up to 50 % at 220 °C, and a drop in impact strength of 37 to 51 %. Put the other way round, which is what matters to a buyer: the same timber was 1.6 to 2 times more impact-resistant before it went into the kiln. It comes out brittle.

A point of honesty: this ratio compares a timber with itself, before and after treatment, not robinia with a thermally modified pine — they are two different species. But robinia starts from much higher up: a density of 740 kg/m³; around 9.5 in the French Monnin test (NF B 51-013) against 4.2 for oak, or about 7,560 N against 4,980 N on the Janka scale — about 50 % harder than oak; and no damage whatsoever to its fibres, since it has undergone no process at all. The real gap, on a stake driven in with a sledgehammer or a loaded joist, is therefore very much in its favour.

The ThermoWood handbook itself advises against using this timber in load-bearing structural applications. A stake driven in with a sledgehammer or a loaded joist demands exactly what the process takes away. Robinia does all of this: a density of around 740 kg/m³ and a bending strength of 88.4 N/mm² in the 2024 UCLouvain tests.

Stability and uniform appearance: where modified woods beat robinia

This is the strong point of modified woods. The manufacturer of acetylated wood states that tangential shrinkage is reduced from 7.9 % to 1.5 % in radiata pine. An external window or door needs timber that does not move: acetylated wood is made for that.

Thermally modified timber wins on uniformity of appearance. The treatment colours the wood an even brown right through to the core and evens out differences in shade; its equilibrium moisture content is about half that of untreated timber.

Robinia loses on both counts, and that needs to be said plainly. It is a lively timber that moves, and its colour varies from honey yellow to greenish brown from one piece to the next before it turns grey. Long straight lengths are rare, which makes finger-jointing necessary. For a window, the right choice is acetylated wood.

Finger-jointed robinia cladding, tongue-and-groove, machined to size in the workshop: dimensions and fit are perfectly consistent, the colour is not — from one board to the next, a hardwood always varies. This is where modified woods, uniform in colour right through, keep the advantage.

Price, origin and end of life: how does each one measure up?

The price ranking is stable, even if the amounts change: thermally modified timber — ash, pine or beech — sits above pressure-treated pine and below robinia; acetylated wood is the most expensive of the three. We do not publish a price per square metre, which depends too much on profile, lengths, quantities and sales channel to mean anything outside a quotation.

Acetylation is a patented industrial process, carried out at very few sites: one plant in Arnhem in the Netherlands, a second in Kingsport, Tennessee. The raw material is FSC-certified plantation radiata pine, mostly from New Zealand according to the manufacturer. Thermal modification is carried out in many European plants, but uses a great deal of energy.

At end of life, all three families are clean, unlike pressure-treated timber. The manufacturer of acetylated wood states that its timber can be incinerated safely, like untreated wood; thermally modified timber contains no additives and burns in the same way; robinia ends up as firewood, without ever having needed any process.

Pallets of robinia decking boards loaded onto a lorry at the sawmill, bound for Western Europe — Loading decking boards at Teiu, bound for Western Europe.

So which one should you choose, and for what?

The right choice follows from the job: ground contact, mechanical load, need for stability.

Use	Best choice	Why
Stake, pile, ground contact	Robinia	Use class 4 without treatment, and withstands driving
Structure, post, pergola	Robinia	Hard, dense wood; thermally modified too brittle, acetylated too expensive
Decking	Robinia or acetylated	Robinia for hardness and price, acetylated for stability
Cladding	Thermally modified or robinia	Thermally modified for even colour, robinia for longevity
Window, door, joinery	Acetylated wood	Dimensional stability and finish performance

Key points

· Three mechanisms: natural extractives for robinia, acetyl groups for acetylated wood, loss of hemicelluloses for thermally modified timber.

· Robinia reaches durability class 1-2 (EN 350) and use class 4 (EN 335) without treatment.

· Thermally modified timber reaches class 2 or 3, and the same timber was 1.6 to 2 times more impact-resistant before the kiln: not for stakes, not for structures.

· Acetylated wood claims class 1 (EN 350) and a manufacturer's warranty of 50 years above ground, 25 years in the ground.

· For an external window or door, acetylated wood is the right choice, not robinia.

Frequently asked questions

Can thermally modified timber be used for fence stakes?

No. Thermally modified timber is a use class 3 timber, so for use above ground, and thermal modification takes away 37 to 51 % of its impact strength — the same timber was 1.6 to 2 times more impact-resistant before the kiln. A stake has to be driven in with a sledgehammer, then withstand lateral loads for years. You therefore need a mechanically intact use class 4 timber: robinia, or use class 4 pressure-treated pine.

Does the acetylated wood warranty cover ground contact?

Yes, but not for the same length of time. The warranty certificate from the manufacturer of acetylated wood distinguishes 50 years when the product is used above ground from 25 years when it is used at or below ground level, fresh water included. The warranty covers fungal decay, excludes salt water and assumes correct installation. The 50-year figure applies only above ground.

Is thermally modified timber a natural, chemical-free wood?

Thermal modification adds no product: the wood is heated to 180-215 °C under steam or in a low-oxygen atmosphere, with no biocide or resin. Calling it natural wood is still misleading: the hemicelluloses are destroyed, the colour changes right through and the mechanical properties decline. Thermally modified timber is a wood modified without chemicals, not raw wood.

Robinia or acetylated wood for a deck around a swimming pool?

Both are suitable, with different priorities. Robinia brings hardness, and therefore good scratch resistance, at a lower price. Acetylated wood brings greater stability, useful around a pool where the boards constantly alternate between wetting and drying. For a saltwater pool, the manufacturer's warranty excludes salt water.