Casehardening explained

The Wood Doctor describes casehardening, its causes, and cure. July 18, 2000

Gene, I saw a reply to a question about casehardening and you said you thought the guy used a dehumidification kiln. My questions: First, what is casehardening, and is it a problem that dehumidification kilns suffer from? If so, what can be done to prevent it?

Casehardening is the result of the outside fibers trying to shrink and the inside, still very wet fibers not letting the outside shrink. As a result, the outside fibers dry in an enlarged shape, and after drying, when you cut the wood, these outside fibers try to get longer.

The amount of Casehardening is the same no matter what kiln is used, but most DH kilns do not have the equipment to relieve stresses.

So, at the end of drying, you need to rapidly add moisture back to the surface fibers at warm temperatures, causing the fibers to try and swell; meanwhile, but the core resists. The amount of restricted swelling at this time will offset the restricted shrinking, producing stress-free wood.

This stress-relief process is called conditioning.
Gene Wengert