What Are Some New Techniques for Making Wood Last Longer?
The Wood Guide · Treatment
TreatmentWood preservation
There are four techniques that are genuinely on the market, and a handful that exist mainly in press releases. This is the difference.
- Thermal · acetylation
- Furfurylation · pressure treatment
- The rest is still lab
Why wood is treated at all
Rot is not a chemical process but a biological one: fungi eat the wood. Every preservation technique tries to make that meal unappetising.
Fungi need three things: moisture, oxygen and food. There is little you can do about oxygen outdoors, so all the techniques target the other two. Either you make the wood absorb less moisture, or you change the sugars in the cell wall so there is nothing left to eat, or you introduce an agent that kills the fungus.
Those three routes explain the whole landscape. Thermal modification heats the wood until the sugars break down. Acetylation and furfurylation change the cell wall chemically so that it barely absorbs water any more. Pressure treatment introduces a biocide into the wood. Every other technique you come across is a variant of or an addition to these.
Important to know: none of these techniques makes wood indestructible. They shift the durability class — from class 4 or 5 to 1 or 2 — and that is exactly what you need to make spruce last decades outdoors instead of years.
Preservation is about rot and insects and works from within. An oil or stain is about UV radiation, surface moisture and colour. You need both if you want lifespan as well as looks — one does not replace the other.
- Thermally modified
- heated to 180–230 °C in a low-oxygen environment
- Acetylated
- cell wall chemically converted; virtually no moisture uptake
- Furfurylated
- impregnated with furfuryl alcohol from plant residues
- Pressure-treated
- biocide forced into the sapwood layer
- Gain
- from durability class 4–5 to 1–3, depending on the technique
- What no technique does
- stop greying
- Nano-coatings and plasma
- exist, but barely outside the laboratory
To be fair: the good techniques make wood more expensive than the alternative. Acetylated wood costs many times as much as pressure-treated spruce, and thermally modified wood is more brittle: it splits more easily when screwed and is not suitable for load-bearing structures. If you look only at lifespan, you pay for it in another way.
In pictures · What is really on the market
Schematic drawing.
What is really on the market
Thermal modification
The wood is heated in a low-oxygen kiln to 180–230 °C. The sugars in the cell wall break down, so fungi no longer find food. The wood turns dark brown, much more stable and considerably lighter — but also more brittle.
Acetylation
Acetic anhydride is used to convert the moisture-attracting groups in the cell wall. The result absorbs virtually no water and therefore barely moves. The best-known brand is Accoya; the price is to match.
Furfurylation
The wood is impregnated with furfuryl alcohol, obtained from agricultural residues, and then cured. The cell wall becomes hard and dark. Sold under the name Kebony.
Pressure treatment
The classic: in an autoclave, a preservative is forced into the sapwood layer. The cheapest, the most widespread, and the reason pressure-treated spruce is affordable outdoor timber.
The four compared
| Technique | Strong point | Weak point | Price |
|---|---|---|---|
| Thermal | very stable, no chemicals, a lovely dark colour | more brittle, not for load-bearing work, colour fades to grey | medium |
| Acetylation | virtually no moisture uptake, very long lifespan | expensive, few suppliers, vinegar smell when sawing | high |
| Furfurylation | hard surface, bio-based raw material, dark colour | expensive, the colour is not to everyone's taste | high |
| Pressure treatment | cheap, widely available, also for ground contact | greenish colour, biocide in the wood, end grain needs re-treating | low |
| Naturally durable hardwood | no treatment needed | heavy, expensive, and its origin deserves attention | medium to high |
Nano-coatings, plasma treatment and 'self-healing' surface layers crop up in articles on wood innovation, but you rarely find them at a real timber merchant. That doesn't make them any less interesting — you just can't lay a deck with them today.
Which technique for which application
| Use | What is usual | Why |
|---|---|---|
| Posts in the ground | pressure-treated | the only affordable route that can handle permanent ground contact |
| Decking boards | thermally modified or hardwood | stability counts for more than load-bearing capacity |
| Cladding | thermal, acetylated or furfurylated | dimensional stability prevents warping in the cladding |
| Window and door timber | acetylated or durable hardwood | a window must not move; otherwise it loses its seal |
| Garden furniture | pressure-treated spruce or hardwood | price and weight count most here |
| Load-bearing structure | pressure-treated or untreated structural timber | thermally modified wood is too brittle to bear loads |
The environmental trade-off
Thermal modification uses only heat, no chemicals — but a lot of heat, and that has to come from somewhere. Acetylation and furfurylation use substances that remain in the wood but do not leach out; the waste is therefore ordinary wood waste. Pressure-treated wood does contain a biocide and does not belong in the fireplace or on the compost heap.
The fairest comparison is not technique against technique but lifespan against replacement. A treatment that makes a board last three times as long almost always beats an untreated board that you replace three times — even when that treatment costs energy or chemicals.
Frequently asked questions about wood preservation
What is the difference between preserving and pressure-treating?
Pressure treatment is one of the ways to preserve wood. Preservation is the umbrella term for everything that extends lifespan: thermal modification, acetylation, furfurylation and pressure treatment.
Is thermally modified wood suitable for a structure?
No. The heating process makes the wood more stable but also more brittle, so it can take less load. Use it for cladding, decking and furniture, not for load-bearing work.
What is Accoya and what is Kebony?
Brand names. Accoya is acetylated wood, Kebony is furfurylated wood. Both turn ordinary softwood into a material with the durability of tropical hardwood.
Does preservation stop greying?
No, no technique does. Greying comes from UV radiation on the surface; only an oil, stain or lacquer helps against that — and you have to maintain it.
Can I burn pressure-treated wood?
No. It contains preservatives that are released when burned. Pressure-treated wood waste belongs at the recycling centre, not in the fireplace or the fire pit.
Do nano-coatings for wood really exist?
They exist and there is plenty of research into them, but they are hardly available for building and garden timber. For a real project today, you end up with one of the four established techniques.
Which species needs nothing?
Some wood species are naturally durable enough for outdoors. The Wood Guide gives the class for each species.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Untreated wood on the left, the same wood after thermal modification on the right — Anubis100, CC BY-SA 3.0 · Pressure-treated boards with the typical greenish tint — TS Eriksson, CC BY 3.0 · Top of a wooden post, badly affected by wood rot — Lamiot, CC BY-SA 3.0