Thermally Modified Wood
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
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
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% MC (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 (for more information,
Their 2×6 lumber costs about $2.50
per lineal foot. 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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