Fred:

Interesting.... but several thoughts come to mind...

The heat in the box around the vacuum chamber may heat the walls of the
chamber. Heat can move via convection or conduction. The problem is,
that with no (or very little) air in the chamber, convection cannot take
place. So, conduction is the only way of transferring the heat to the
wood. Since the wood is minimally in contact with the chamber, I doubt
that the heat can have any effect at all.

Vacuum kilns are a proven technology. But, they rely on heating
blankets in contact with each and every piece of wood. Also, if the
wood is thick, the surface is going to dry much faster than the core
leading to all sorts of problems.

When the pressure is lowered, the boiling point of water is lowered --
that 212F is at a standard pressure only. Vacuum kilns work by raising
the temperature and lowering the pressure so as to change the water in
the wood to vapor which passes through the cell walls more easily than
water.

I tend to think that Fine Woodworking is right.

Bill

Fred Holder wrote:
The various discussions on ways of drying wood prompted me to post this message.

The following article was published in the March 1998 issue of More Woodturning.
I authored the article with technical help from Gilbert Brown of Omak,
Washington. Gilbert had done considerable experimenting with his process and was
trying to market the system with little success. I wrote the article and
submitted it to Fine Woodworking. They kept it for a year and then returned it
saying that their experts said it would not work. I leave it to the reader to
determine whether this might be useful.

I first met Gilbert Brown at the 1993
“Art in the Park” craft fair at Omak,
Washington. Several of the bowls on my
table were turned from wet wood and
had warped appropriately. Gilbert said,
he was working on a project to dry fresh
cut wood fast! With his planned system,
he said, we wouldn’t have to turn green
wood and guess at what its final shape
might be. Being a writer, I told him it
might make a good article. Since then,
we’ve crossed trails several times. Each
time we met, he hadn’t perfected his
dryer yet. He was still working on it.

Then “out of the blue” he called me.
His project had finally been worked out.
His system can dry a kiln full of wet
(newly cut) fruit wood to 6 percent moisture
content in three to seven days depending
upon the thickness of the timber,
about the best I’ve heard of is almost a
month. This new system uses a low temperature
(100 to 150 degrees) and a
vacuum pump. Gilbert says, he isn’t going
to mass produce these units but will
make one up for someone on special order.
If interested, you can contact him
at: Gilbert Brown, Route 3, Box 336-B,
Omak, WA 98841. You can call him at:
509-826-4058. (Note: Gilbert wasn't in extremely
good health at the time I wrote this article
and he may no longer be able to produce a unit
on order.)

The unit that Gilbert has devised uses
a two-foot diameter steel tank that is 10
feet long as his vacuum chamber. One
end of the tank has a door that can be
opened for loading and unloading the
chamber. The door is equipped with a
small Plexiglass window so that the wood
can be viewed during the drying operation.
A light bulb is also installed in the
tank to assist in this.

The vacuum tank is mounted inside
a wooden box with an air chamber between
the tank and the sides of the box.
The walls of the box are one to two feet
thick with sawdust used as a filler and
for insulation. A tube type thermometer
is used to measure temperature within
the vacuum chamber. Finally, a vacuum
is pulled; a reading of 15 on the gauge is
adequate.

When loaded with wet wood that has
been brought up to temperature, beads
of water will appear on the ends of the
boards about six minutes after the
vacuum is established. Apparently, the
moisture is removed from the wood along
the channels that the moisture moved in
when the tree was alive. You don’t get
to view what’s going on for very long before
the window is fully fogged with moisture.

Gilbert has been making lumber from
fruit wood stumps. He says, a cherry
wood stump that is three feet long and
about 16 inches in diameter weighs about
300 pounds. Such a stump will produce
about three gallons of water in three days.
This water collects in the bottom of the
tank. He has installed a drain valve in
the bottom of the vacuum tank so that
the water can be removed and saved in
another smaller tank without losing the
vacuum.

This dryer was developed to quickly
dry fruit wood from the orchards around
Omak, Washington to produce cabinet
grade lumber at the lowest possible cost.
Gilbert uses the stumps that regular mills
leave behind. Here, much of the figure
in the tree exists so that really fine wood
can be obtained from a part of the tree
that would otherwise be burned. Of
course, this process doesn’t yield long
pieces of timber, but most cabinets don’t
need very long pieces. Gilbert also makes
up fancy flooring from this wood by laminating
the pieces to form larger boards
with an appropriate pattern.

Fruit woods will twist a good deal
during this fast drying process, unless they
are clamped down while drying. Other
woods seem to be alright, he says. Gilbert
described a typical load of fruit wood
to be boards that are about one inch thick,
either trimmed up or with the bark left
on. The boards are stacked on a 1/4 inch
metal plate. They are then clamped down
to the plate to hold them in position while
they dry. Once dry, Gilbert says, the
boards are OK. Wax sealer on the ends
of the boards will help to keep them from
cracking.

Currently, I have quite a bit of wood
on the shelf that was cut six months to
two years ago that still isn’t ready to turn.
I’m waiting for it to air dry at about one
inch per year. I can see how a small kiln
of this type that would hold a few green
wood bowl blanks or several small pieces
for other turning projects would be very
useful. I turn small pieces such as wine
bottle stoppers and spinner tops. These
must be dry before you turn them. A
small kiln with a vacuum tank about the
size of a five gallon bucket, which
wouldn’t take up too much space in my
shop, would work well for me. I could
dry more wood with such a small unit
than I have time to turn.

Editor’s Note: This article was
written for Fine Woodworking in cooperation
with Gilbert. After almost a
year, Fine Woodworking returned the
article, saying that the “experts” they
had contacted said the system wouldn’t
work. I don’t make any claims about
Gilberts system, but I believe he is honest
and that he has worked out a system
that will dry wood quickly. I have
held off publishing this in More
Woodturning in hopes that I could find
a publication with larger circulation
to spread the word about this system.
Anyone wishing to investigate this further
should contact Gilbert Brown directly
at the address given above. The
way he talked, he would be willing to
make up systems of virtually any size.

Fred Holder
http://www.fholder.com