Chris Schwarz's Blog

How to Make a Quick Kiln

When I released my Compwood arm bows
from the clamps this morning, I saw a little bit of springback. It was
less than I’d get with other bending techniques, but it was more than I
expected.

I had expected zero springback.

When Chicago furniture
maker Jeff Miller had introduced me to the material, he said that he was
impressed because the material kept its shape perfectly after it was
dry. So I started to dig a bit.

I called Miller this morning and
chatted about the species he was using, the type of bend and how he
dried the wood. Miller said he was bending pieces of ash that were 8/4
thick and 5″ wide – and bending them on the 8/4 dimension. The radius of
the bend was about 20″ – this was a shape useful for a chair back.

After
bending the wood, which took so much effort that it eventually busted
the form, Miller dried the wood in a shop-made kiln. He made the “kiln”
using loosely assembled 2″-thick pink foam insulating boards from a home
center. And he heated the kiln using a ceramic heater that had a fan.

After leaving it in the kiln for a week, Miller removed the Compwood from the form.

“It was pretty much flawless as far as springback is concerned,” Miller said.

So
I decided to talk to Chris Mroz at Fluted Beams LLC, the company that
makes and sells Compwood here in the Americas. Mroz purchased the burly
press that compresses the wood in length, which makes the wood flexible
when it is cold.

Mroz said that working that press is equal
parts art and science – so much so that it took him a couple years to
master the process and that he still compresses every piece personally.

As
to drying Compwood, Mroz recommends a kiln made from insulating foam
that can maintain the temperature at 100° to 110° Fahrenheit. He has
used ceramic heaters with fans, but says they tend to burn out quickly.
Now he uses radiant dish heaters and places a fan directly at the work.

As
to timing, Mroz said he would dry a piece that was 1-1/8″ thick for
about six days. If the piece were just sitting out in the form – no kiln
– he would let it sit for two to three weeks.

But what about my moisture meter readings? Our pinless moisture meter said the wood was at 7 percent.

Mroz
said that pinless meters tend to measure surface moisture for the most
part. His guess is that the wood was still wet at the core but dry on
the edges.

So it sounds like my arm bows might still be a bit
wet. I clamped them both back in the form and will head off to the home
center to get some foam.

— Christopher Schwarz

9 thoughts on “How to Make a Quick Kiln

  1. Ross

    Chris,

    I think I get it now. this is all about bending this bendy wood into a shape, drying it out with warmth and time, and hopefully it retains its shape. This isn’t a multi-ply bent lamination but a completely different kind of material and process to acheive the same goal, and you are doing two at once. Got it.

    Ross

  2. Wagner Electronics

    Mroz is correct – many studies have concluded that most pinless moisture meters are highly affected by surface moisture (pin meters are much more prone to false readings when there are wet pockets in the wood).

    However, Wagner moisture meters are able to see the true moisture content in the wood, bypassing the surface moisture, because of their IntelliSense technology. You can verify this by taking two pinless moisture meters, one a Wagner with IntelliSense, and one not, and wipe a damp cloth across the surface of a board. The Wagner meter will be barely affected, but the other will likely give a highly inaccurate reading.

    Try the damp cloth test yourself or watch Wagner’s IntelliSense video on youtube.

  3. Christopher Schwarz

    Ross,

    You are seeing two separate laminations in the form. Nothing is glued to anything. Hence the horizontal seam and the small gap between. When I release the clamps I will have two armbows.

    Hope this clears it up.

    Chris

  4. Steve

    "Mroz said that pinless meters tend to measure surface moisture for the most part."

    That would certainly be consistent with what you measured. But it’s completely contradictory to what the moisture meter manufacturers say. For example, from Wagner’s web site: "Wagner Moisture Meters, featuring IntelliSense™ Technology, go beyond surface conditions for accurate measurements of moisture conditions IN the wood."

  5. Steve

    @Michael,

    The type of kiln being described here is, in effect, a low-temperature dehumidification kiln. (There was an article on how to build a dehumidification kiln, using a household dehumidifier, some years ago in FWW.) A dehumidification kiln uses a relatively small amount of heat, usually just enough to raise the temperature to "lukewarm" or thereabouts. In a dehumidification kiln, the purpose of the heat is not so much to dry the wood directly as it is to dry the air, so that water is more effectively transferred out of the wood.

    The commercial kilns that are used to produce kiln-dried wood work on a different principle: Water migrates through the wood via a process known as diffusion, and the rate of diffusion is temperature-dependent. So the higher the temperature, the faster the water leaves (within reason, of course–heat it too high and the wood starts to decompose…).

  6. Ross

    I’ve never used this compwood product but I have made hundreds of various bent laminations over the years. I’ve always pressed a curve like the one you’re attempting with the glue seams going vertical, not horizontal. I just don’t see how that would ever work. Is this a special property of this product, that it’s supposed to be able to do this? I’d try it again with an inner piece and an outer piece, not a top piece and a bottom piece, if Im seeing this picture correctly…No snark here, I probably just don’t get it.

  7. Mark

    The one kiln I made so far consisted of some foil wrapped around the setup, a lightbulb, and a probe thermometer to monitor the temperature. It worked pretty well, but the piece was smaller (about 1/4" thick) so I wasn’t trying to heat a lot of air.

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