A local wood trader has shown me some thermally modifid ash planks (never seen that before!) with a stunning coloring that almost seduced me to immediately buy a table and two benches worth of it and build some new massive garden furniture.

Now, the wood is thermally modified. Which basically means it went into a kiln at a somewhat higher temperature and underwent a heat-induced chemical reaction. I had never seen such a wood (if you can still call it that) before. It's apparently a rather new thing (less than 20 years old) too.

From what I could gather from various (partially contradicting) sources, this means:

  • Greatly improved resistance to fungi and parasites, suitable for outdoor
  • Even woods like beech and oregon pine are resistance class 1-2 after treatment
  • Best invention since the invention of the wheel
  • Wood almost doesn't work any more (50-75% reduction in shrinkage/growth, nearly no water uptake)
  • No treatment needed
  • Treatment recommended (... OK?)
  • Varnish needed, oiling before varnish recommended to give the wood back its elasticity (how would that work???)
  • Insufficient data for close-to-earth placement, might not be good (WTF?)
  • Darker color (very obvious, and good looking, main reason to buy!)
  • Harder, but somewhat more brittle
  • Need to pre-drill big screws (Wow, I wouldn't have guessed that!)
  • Not recommended to have too sharp edges in the open (Again, wow, what an insight!)
  • Slightly less stable (slightly? how much?), not suitable for construction (why not, if it's just slightly weaker?)
  • Glue might take up to 10 times as long to dry (... but, will it hold?)

So, what does that mean in summary? And does one of you maybe have experience working with that and can confirm or deny any of the above?

For example, when someone tells me "hard and brittle", I come to think of mineral substances. As in, giving that ultra-fine dust that causes lung cancer when being worked. Not exactly what I'd like.
When someone tells me "controlled, partial combustion, chemical change", I come to think of "easily flammable" and "unknown evaporations".

Are there some obvious and maybe not-so-obvious reasons why one wouldn't want to use such a kind of wood?

  • I would list your sources for the above information so the rest of us can check out your findings.
    – grfrazee
    Commented Aug 5, 2015 at 18:31
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    Wikipedia has a short article on it. You list most of the advantages/disadvantages in your question. Minimal wood movement, reduced bending strength, and increased resistance to rot seem to be the main characteristics.
    – Doresoom
    Commented Aug 5, 2015 at 18:42
  • @grfrazee: Unluckily I didn't keep track of sources (but seeing how they're all in German, it's unlikely to be much useful anyway).
    – Damon
    Commented Aug 5, 2015 at 19:05
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    Just thought that this wood is probably harder on tool edges. Edited my answer as such.
    – grfrazee
    Commented Aug 5, 2015 at 20:12
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    @Dronehinge: I have only heard of "heat treated" with palette wood. In which case it's heated to kill bugs inside, not to modify the chemical structure of the wood (so both temperature and duration would be different).
    – Damon
    Commented Aug 6, 2015 at 8:51

3 Answers 3


Lee Valley has begun using what they call "torrefied wood" in their plane totes and knobs, which seems like the same thing as what you're explaining above. To quote:

All totes are made using domestic torrefied maple – wood treated with a heating process that changes the structure of the wood at the cellular level, stabilizing it against the swelling and shrinkage caused by humidity changes.

See this site and Wikipedia for an explanation of torrefaction.

If I had to guess, heating the wood to a higher temperature than normal might begin crystallizing the sugars leftover in the wood or making them react chemically with the rest of the wood fibers, essentially turning them into something more mineral-like than organic. I'm imagining something like petrification, but not to such an extreme extent.

This, in turn, would make the wood harder and more brittle since you are changing the fibrous nature of the wood. I imagine this is why it's recommended against for construction - brittle failure of structural elements is generally to be avoided since it's a sudden failure as opposed to a gradual (i.e., noticeable ahead of time) failure. I speak as a structural engineer here.

As far as your comment about partial combustion being more prone to future flammability, that's not necessarily true. Historically, wood was sometimes pre-flamed on the outside before use in building. This resulted in an outer skin that was initially more resistant to fire than raw wood. Also, since the torrefied wood is made harder and less fibrous, it would naturally be less prone to ignition. This is why it's easier to light a bundle of fluffy grass (loose, very fibrous) than a log (dense, not so fibrous).

One perhaps non-obvious disadvantage might be that since the wood is denser and having a slightly different chemical composition, it could be more abrasive or just harder on tool edges. Think of the difference between working pine and working a much harder wood like purpleheart or a siliceous wood like teak.

  • 2
    "Lee Valley has begun..." -- Huh, incidentially I own one of these, and didn't even know about the torrefaction! :-) Yes, this sounds exactly like it (only that it's maple, not ash).
    – Damon
    Commented Aug 5, 2015 at 18:59
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    I think a good way to look at it Is that the heat without oxygen is axactly the same process as making charcoal. The process just isn't taken so far. The basic constitituents, lignin, sugar and the complex organic long chain carbon fiber are probably all altered to lesser or greater extent. Fire hardening has been used for a long time. There is a high end Japanese product now being used for unfinished exterior wood now. I really don't think the process changes the abrasive edge dulling qualities. I have used Japanese hand planes and chisels extensively with this material and see no difference.
    – Eric
    Commented Nov 25, 2017 at 23:36
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    "Fire hardening has been used for a long time." -I'll say! The Clacton Spear is about 400,000 years old. Commented Nov 27, 2017 at 13:15

Now, the wood is thermally modified. Which basically means it went into a kiln at a somewhat higher temperature and underwent a heat-induced chemical reaction.

The thermal modification of wood into what is variously called roasted, toasted, caramelized or more technically torrefied wood takes place at significantly higher temperatures than are used in conventional kiln drying (more than double).

Although the process can turn a bland species into something that looks a bit special I think you were wise to hold out on buying some, for furniture applications at least.

While the modified material looks good and can feel great there are some significant issues working with it that may confront the woodworker trying to use it as you would normal lumber. Its brittleness is often cited as an issue (leading to flaking and splitting) and the hold of fasteners is/can be greatly compromised.

oiling before varnish recommended to give the wood back its elasticity (how would that work???)

I'd be sceptical about this claim too. Oil penetration, except in end grain, is usually very shallow in wood. Obviously that's with unmodified wood with a higher water content, but still I'd want to see some evidence before believing oiling would do anything other than provide a surface treatment.

Glue might take up to 10 times as long to dry (... but, will it hold?)

The glue taking longer to dry makes sense — most wood glues actually dry dry; that is, they set by dehydration and the roasted wood is inherently less absorbent.

There are some good reasons to suppose that glue joints, particularly with conventional wood glues, will not be as strong as with untreated wood. There is some technical data (I think from the US Forest Products Laboratory) showing this, and the practical experiences of some woodworkers bear this out.

I expect that epoxy will turn out to be the ideal glue for this material, after the joint faces are prepared suitably (roughened in some way, or undercuts created for a gross mechanical hold as some knifemakers use for attaching knife scales to the tang).

When someone tells me "controlled, partial combustion, chemical change", I come to think of "easily flammable" and "unknown evaporations".

I would guess that this material, just on general principles, would burn more easily than regular wood, even if merely because the moisture content is lower. And there is quite a bit of data online about torrefaction being used to increase the energy density of woody biomass to turn it into a better fuel for electricity generation.

The "evaporations" thing I would presume is a non-issue, the heating process actually removes some of the VOCs from wood so the modified material is lower in evaporative compounds (or extractives as they're called in the industry) than the starting wood.

when someone tells me "hard and brittle", I come to think of mineral substances. As in, giving that ultra-fine dust that causes lung cancer when being worked. Not exactly what I'd like.

Not relating to this material specifically but since this is a woodworking forum I feel the need to mention that there is no conclusive data to suggest that woodworking-associated dust inhalation increases risk of lung cancer.

There is however a causal link established with a (very rare) form of nasal cancer, and exclusively with occupational exposure (large amounts, over a long time) and to hardwood dusts specifically.
Source, Health and Safety Executive document (UK Government).
Source, SafeWork Manitoba document.
Source, OSHA regulations, section VI, Health Effects Discussion and Determination of Final PEL.

  • Interesting to see how different risks are assessed in different places. We have specific vacuum cleaners classified for type "L" dust (sand, wood, not oak/beech) and type "M" dust (oak and beech, and non-cancerogenous mineral dust) and then "H" for stuff like asbestos. Class M must guarantee 99.9% retention, and class H must guarantee 99.995%. Needless to mention that they have different price tags, too (no class = 60-90€, class L 300-400€, class M 600-800€, class H start at 1,200€). Beech and oak is definitively classified as cancerogenous here (also some tropical woods).
    – Damon
    Commented Aug 6, 2015 at 13:43
  • But with that lung cancer thing I was more thinking the way: If the material becomes more mineral-like, then it probably acquires mineral-like properties in that respect, too. For example silica dust is agreed to be a cause of lung cancer with little or no doubt (at least the IARC says so). It's for the most part not something inherently toxic or such, just something that is hard and brittle, with fine dust (that goes into the alveoles when inhaled). Sounds familiar?
    – Damon
    Commented Aug 6, 2015 at 13:48
  • @Damon, I didn't mean to imply that roasted-wood dust is not an inhalation risk; I'd fully expect that it would be at least as dangerous as the dust of the unmodified species, if not likely to be more so. Just wanted to make the point in relation to normal wood dust and lung cancer specifically, as the belief that there's a link between the two is quite widespread..
    – Graphus
    Commented Aug 6, 2015 at 16:44

I have worked with both poplar and maple cambia as it is called here in Portland. Carmalizing kills the wood in a way. I believe it breaks the cell walls. Gluing and fastening are OK figuring for a small amount of loss in strength. The memory quality is lost - much harder to swell a ding back out. I made a chair out of the maple cambia ( a very good test) and it is plenty strong. My biggest complaint comes with finishing tho. Because of the broken cell walls the surface of the wood is much more porous. I like polimerizing oil finish. These woods seem to have no bottom when it comes to absorbing solvents and oils. As a result way more finish is trapped near the surface and it takes a lot more finish to fill the surface. And drying time is much delayed. From a day in normal wood to a week or more till the smell of offgassing goes away. Milling is OK. I personally don't notice more tearout, splintering or brittle behavior. But I have only made a few pieces with it so far. I do like the deep burnt color a lot. It is quite consistent and much less expensive than walnut or dark tropicals. My current opinion is mixed.

  • I should add that next time I plan to try a good fast drying sealer to stop or reduce over absorption of finish.
    – Eric
    Commented Nov 25, 2017 at 18:34

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