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Thread: Thermally Modified Wood

  1. #1

    Thermally Modified Wood

    I know it's a step outside the woodshop and onto the porch, but has anyone worked with thermally modified wood (Westwood, Thermory, etc.)? I'll be building a porch and a breezeway which will be open but covered with roofs. I want something low-maintenance (stain is OK, but I don't want to have to scrape and paint), I'd rather not use tropical hardwoods, and I don't like the look or price of the plastics. I would consider just doing good job of selecting clean PT lumber and finishing it nicely, but my wife doesn't want gaps in the deck boards. Thermally modified wood sounds like a good solution: durable, rot resistant, minimal shrinkage, and affordable. Does anyone have any experience with these products? Thanks.

  2. #2
    Join Date
    Mar 2003
    San Francisco, CA
    Deck boards have gaps partly due to shrinkage, but mostly to assure water drains off the deck. Water pooling on wood for long times is going drastically shorten the service life, no matter how weather resistant the wood is.

  3. #3
    No experience, but I first saw thermally modified wood at IWF in 2008. Display had some "canal boards" from Holland that had been in place for over ten years, with not signs of rot. Best I can remember, thermally modified is hard and more prone to splitting and splinters. Farmers used to char the bottom of fence posts to keep them from rotting. Wish they did that to today's treated posts.

  4. #4
    They make it near me, I left several pieces outside on different sides of the house. It wasn't very stable.

    Install t&g ipe porch flooring if is covered like a porch and you have air flow underneath and its sloped a little for runoff.

    It's not horribly expensive, but will outlast anything else and you can just let it slowly silver and it will last forever basically.

  5. #5
    I saw a Utube by Matthew Cremona where he used thermally modified elm? i think to build a garden bench. Sounds ideal to work with, like the properties of the wood have changed & reduce internal stresses. I'm looking for something like this to build a couple English garden benches.

  6. #6
    I have experimented with roasted poplar for alternate uses (in musical instruments). It is interesting stuff. Roasted maple is sometimes used for guitar necks and so on. Both can be beautiful stuff, especially the maple. I found that the torrefied wood tended to be pretty stable, but was a little brittle.

    I have not used it for it's normal intended purpose of outdoor use other than one small piece that has held up a bird feeder for a number of years. That piece has weathered well in the Tallahassee sun and rain, but it is not in a very demanding function.

  7. #7
    An alternative to consider is Accoya, a trade name for wood modified chemically by acetification. As far as I know only treated Radiata Pine is available in the US. Rex Lumber carries it here in the Northeast.

  8. #8
    I've worked a lot with Accoya. Great for cladding. For decking, I prefer this...

    Felder USA Territory Representative: Central & South Texas

  9. #9
    Erik, that looks interesting, thanks for the links. How would you compare Kebony to Accoya and thermally modified wood in appearance, workability, performance, finishing, price or any other characteristics?

  10. #10
    Quote Originally Posted by Kevin Jenness View Post
    Erik, that looks interesting, thanks for the links. How would you compare Kebony to Accoya and thermally modified wood in appearance, workability, performance, finishing, price or any other characteristics?
    Kevin, both Accoya and Kebony were heavily used products back in my Delta Millworks days. Here is how I would characterize them:

    -Accoya: The acetylation process closes the wood cells in a low-temp kiln/pressure cooker. I think they might acetylate SYP but Radiata is the species you want. Acetylated Radiata is the most dimensionally stable modified wood out there. Also very lightweight. The Dutch dyke someone was referring to earlier is made from Accoya. It is as waterproof as you could ever need. Absolutely will not rot and bugs won't eat it. The down side is that since the cell walls are closed, it does not accept stain very well and has sort of a sickly green-tan color when raw (though it ages nicely to silver in the sun). Because Accoya is so dimensionally stable and lightweight, it's great for cladding/siding and joinery like windows and doors.

    -Kebony: Different process, called furfurylation, where sugar alcohol is pressurized into the wood. No heat, only pressure. This has the effect of substantially thickening the woods cell walls and also gives it a natural walnut-type look. I've worked with both SYP and Radiata Kebony and the result is so dense and heavy, you could probably drive nails with a chunk of it. Radiata Kebony has pretty good dimensional stability (since it's Radiata) but not as good as Accoya. At least as rot-resistant as Ipe or other tropical hardwoods. Personally I like Kebony best for decking since it doesn't require any staining and looks natural from the get-go.

    As far as the thermally modified woods, we had samples but I never actually did a project with them. My understanding is that they are created by super-heating the wood in an anoxic environment. I think the concern is that the superheating somewhat deteriorates the wood and you might see long-term stability issues but again, that's just my suspicion. Since there is no heat used in the production of Accoya or Kebony, that's not a concern. The thermally modified woods are probably a lot cheaper to manufacture, which is why they are better know.

    Felder USA Territory Representative: Central & South Texas

  11. #11
    Thanks for the info. Just one quibble about Accoya being "waterproof"- according to a self-identified Accoya rep in this Wooden Boat forum thread "
    Accoya is permeable, thus it still soaks up water, the only difference is that Accoya will have a very much reduced swell character" Just so no-one gets the wrong idea.

    I have seen Accoya used in exterior millwork projects and it is very stable- comes flat and stays that way when ripped and milled. It is extremely thirsty for glue, both waterbased and epoxy, but it does glue well. The appearance calls for paint or serious staining or weathering. The greater hardness of Kebony and initial dark color does seem to make it a better candidate for decking and clear finishes.
    Last edited by Kevin Jenness; 04-03-2020 at 2:05 PM.

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