Dehumidification drying of thin stock

Simple drying methods for small, thin stock. (Sawing and Drying Forum) March 23, 2003

Question
I am about to try DH drying some thin stock I have for small boxes. It is from 3/16" to 5/16" thick. I made a small coffin shaped box and have the wood stickered in it. I was planning to buy a small residential type dehumidifier and put it in the box with a small fan. The wood is air dried to 20% or so and I need to get it down to 9% or so.

Any suggestions on how I should approach this? I have a lot of thin stock I would like to use in boxes but have had problems with warping. I'm hoping that thoroughly dried stock might help.

Forum Responses
(From WOODWEB's Sawing and Drying Forum)
From contributor M:
Simply sandwich your boards between thick newspaper or fibre board and put a piece of plywood on the top and bottom of the sandwich and clamp together tightly with bolts through each corner. Put in a hot, dry place like the shelf above the woodstove in the shop. Make sure the fibre board sticks out beyond your wood by about 1" because this is where the moisture will evaporate as it is drawn off the wood. The wood will come out dead flat and very dry. How fast depends on how much heat you give it - maybe a week or two.


What happens if you do this to thin, green, right-off-the-saw stuff?

From contributor M:
With green stuff I usually make a change of paper after a day and again after a few days. Lots of dry heat because it is still very humid where the wood contacts the blotter paper.

Are you talking cardboard, as in box?

Shoe boxes and the stiff piece on the back of a pad of paper is cardboard. The stuff that many cartons are made of is called corrugated fiberboard.

You can dry the thin wood easily in a DH kiln, but because it warps more easily than thicker wood, the key is to use enough pressure and stickers about 12" apart.

The sandwich idea was patented by NC State University and called lumber drying pallets.

The problem with drying any wood is getting the correct final MC. If you try to do that in your shop, it can take a long time. Final drying is usually done at a humidity a little lower than the shop and at a warmer temperature. This is how billions of board feet are dried every year.

Gene Wengert, forum technical advisor


From the original questioner:
I have the wood on 1/2" stickers spaced about every 4". The pieces are less than a foot long themselves.

What I'm hearing is that pressure on well-stickered wood is as important as the drying schedule? The wood in question has been living in 80% relative humidity here on the wet side of the Big Island of Hawaii. I'm embarrassed to say I don't even have a moisture meter to see what the actual content is but have been told that wood air dried here won't get any lower than 20%.

What if I put the DH in the box with a small fan, put them both on a timer and then close it up? It doesn't seem to me that there can be much moisture in the wood really.

I have a lot of 3/16" thick seconds from making guitar sets and want to make boxes with this material.


From contributor J:
The process that was patented by Dr. Hart at NC State involves stickers with plywood or similar material attached to the stickers. This looks like pallets. The wood is layered between the pallets. This allows air to flow through the pallets as in regular stacked wood. The difference is that the inner surface of the pallets is exposed to the dry air instead of the lumber. This reduces the moisture gradient in the lumber, thereby reducing the stress gradients in the lumber.

Without the stickers, the water in the wood/paper/fiberboard in the middle of the stack will take a long time to move to the outside. This would increase the likeliness of mold in the middle.


The initial question and discussion was about 20% MC lumber, so mold is not an issue in that case.

Gene Wengert, forum technical advisor


From contributor M:
Contributor J, that's why I change the paper or fibre board several times in the first week of drying. I also keep it hot - too hot for fungus to grow, if possible. The moisture will move quickly if it is hot.

I don't use this method very often, as I seldom use thin wood for anything that I make.


From contributor J:
This particular instance the MC may be 20%, but people not posting read these posts. If they don't see the full process they may use it incorrectly and have problems. Also, what was patented was a different process then was described.
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