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A remarkable drying process gives wood
a new character.

By Chad Stanton

Some day, you’ll be able to build an outdoor project with a new kind of wood, grown right here in America, which resists decay, stays absolutely flat and is totally free of chemicals. Sound too good to be true? Well, that day isn’t way off in the future— this wood is here, right now.

It’s called thermally modified wood, or TMW for short. I’ll go into the details later, but basically TMW is wood that’s been dried at a really high temperature. This turns it brown all the way through—like a chocolate cookie. But it’s a cookie that mold and fungus can’t digest. TMW won’t rot.

Any species of wood can be turned into TMW—hardwood or softwood.

Origins of TMW

Credit goes to Finland for figuring out how to make TMW. Actually, TMW’s rot resistance was an accidental discovery. Back in the early ’90s, Finnish scientists were experimenting with a drying process that would make wood more dimensionally stable—that is, free from cupping, bowing and twisting. Good luck with that, you might think. But they hit the jackpot. Not only did they achieve their goal, but they found that the process made the wood rot-resistant, too.

Of course, baking the wood in a super-hot kiln changes it in other ways, too, not all of which are  desirable. More on that below.

For a few years, TMW was an exclusively Scandinavian product. Today, a few American companies
have licensed the process and are busy converting domestic woods into TMW.

How TMW is made

Making TMW is a complicated, four-step process. To start off, the untreated lumber is dimensioned at the sawmill. Then it’s brought to the kiln and the first step begins: slowly heating the wood to 212 degrees. In the second step, the wood is preconditioned by drying it to nearly 0% moisture content.
This wood resists decay, but it’s totally free of chemicals.

Now it’s ready for the crucial third step, where the temperature of the wood is raised to 374-482 degrees for several hours. At this high temperature the natural sugars in the wood are converted into substances that all the agents of rot—insects, mold and fungus—cannot eat. In the final step, the wood is cooled and some moisture is restored, bringing it up to around 6% to 7% MC.

Properties of TMW

I first heard about TMW from a friend who’s in the deck-building business. He buys thermally  modified Southern yellow pine from PureWood, a company based in North Carolina.  I used some of that wood to build a large picnic table.
Here’s what I’ve learned.

Color and smell. The TMW process darkens the wood all the way through to a cocoa-brown color.
If left unfinished outdoors and exposed to sunlight, it will turn gray. TMW has a pleasant smell when you cut it—like toasted marshmallows.

Stability. TMW planks are exceptionally straight and flat. I resawed some wood into thinner pieces and they didn’t warp one bit. That’s a rare experience with any wood—and a welcome one.

• Strength. The drying process seems to make the wood more brittle. It splits and splinters more easily than wood of the same species that’s been kiln-dried. TMW is not recommended for use as joists and posts.

• Dust. Sawing and routing TMW creates very fine dust, like working MDF. It’s a good idea to wear a mask.

• Planing and jointing. No problem. Freshly machined surfaces take glue well, too. Old surfaces should be sanded or milled before gluing.

• Dimensions. The TMW I used was slightly thinner and narrower than standard dimensional lumber. Check before you buy.

The bottom line

Like any wood, TMW has its pros and cons. But it’s amazing stuff, and I hope it catches on.

This story originally appeared in American Woodworker June/July 2011, issue #154.

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