Hand Tools

Subject:
Re: wood drying/checking
Response To:
wood drying/checking ()

Larry Barrett
I have tried putting a round in the kiln to speed up drying and the whole piece checks and splits.
What I need to understand is what is the process that causes checking? What happens to the cells to make the wood crack?

A living tree contains both free and bound water. When you buy a log and split it into billets, each billet will still contain free and bound water. Free water is not connected to the wood fibers in any way, either physically or chemically. It will evaporate/drip out of the billet as quickly as possible. As it leaves, there is no change in the dimension of the wood. Bound water molecules are different. They are chemically bound to the cellulose polymers in the log/billet. If they are removed from the cellulose fibers, the fibers shrink.
The bound is a weak one (hydrogen bond, I think - Bill Tindall helped me out with this a couple of years ago). But even though they are bound to the cellulose, the moisture in the billet still seeks equilibrium with moisture in the surrounding air. (Exactly why this is the case is covered in third year chemistry class according to Bill; I did not get this far). When you put your 2" round in your kiln, the warm air in the kiln can hold much more moisture than the moisture in your shop; the water molecules near the surface of the round leave their hydrogen bond with the surface cellulose molecules. The water molecules in the interior of the round will migrate toward the surface, but not fast enough to replace those that left. The cellulose fibers near the surface shrink and eventually pull apart.
I do essentially the same thing when making post and rung chairs. But my rungs are much smaller, about 3/4" octagons when I place them in my kiln. I run mine at about 120 degrees F. The rungs go in at around 20% MC and I take them out when they are bone dry. Because they are much smaller, I do not see any checking. You did not say what the temperature is in your kiln. If you lower the temp in your kiln I suspect you will not have a problem, or as much of one. I do not have any suggestions for drying your rounds any faster.

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