Drying Technique and Wood Hardness

Those in the know describe the effects of case-hardening (a drying defect) and how to diagnose it, as opposed to hardness that would affect machinability of wood. July 30, 2007

Question
We have some red oak that we just milled into tongue and groove flooring. It milled extremely hard. I suspect that it has some defects related to drying. If it is case hardened, and we install this flooring, will this defect show up later in some ugly form?

Forum Responses
(Solid Wood Machining Forum)
From contributor F:
The terms "casehardening" and "reverse casehardening" refer to kiln drying defects that cause the boards to become bowed and crooked when being parted into narrower rippings.

I may be wrong, but I don't think it has anything to do with a wood being harder than normal and dulling saws and tooling faster than what is normal for that species of wood.
If your flooring stayed as straight as other runs you have done in the past and you know the moisture content in the wood is correct for your application, you shouldn't have any further problems.

There may have been something on the wood that caused premature dulling of your tooling? Red oak has typically been normal for hardwoods as far as machining for me. Oregon white oak, I have noticed, really dulls my tooling fast. Any explanation for Oregon white oak being so abrasive, Gene?

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From contributor D:
Contributor R is correct about the term casehardening and how it should not have any effect on the tooling. You can tell if your material is casehardened by making a crosscut in the middle of a rough board. Then another crosscut so you have a section about 1-1/2" long. Turn this 90 degrees so the end grain is down on the band saw table, and cut in from the edge so you remove the center, leaving a C shaped "shell" of the material, with about 1/4" thick walls. The center can be wasted. What you examine is the C shaped piece, to see if the legs converge toward each other. If they converge, the wood is casehardened. If they stay the same, it is properly dried. I have seen the legs come in so hard on badly casehardened 8/4 W oak that they touch once the center is removed.

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From Professor Gene Wengert, forum technical advisor:
Contributors F and D have the correct information. Casehardening only causes immediate warp when machining.

The hardness you are experiencing can be due to high density of the wood or to over-drying (under 6% MC). I suspect you have low MC wood.

Some trees will also bring silica from the soil up into the wood structure. I am not aware that this happens with white oak, but maybe. If so, then an ash test would show increased mineral content.