Moisture gradient stress

Q: We have some 5/4 hard maple, and when we resaw the lumber, the two 1/2-inch-thick pieces cup right off the saw. When they sit around for a day or so in the plant, then the cup goes away. Our foreman calls this casehardening and blames the kiln operator; the kiln operator says that the lumber is okay and that we have a storage problem and that we should not process cold lumber. We want you to resolve this please.

A: It is indeed correct that when lumber is machined - ripped, resawn, planed, moulded, etc. - and it has stress, it will warp immediately. However, if the lumber also has a moisture gradient, then the lumber will warp over the next few hours or days as the moisture gradient dissipates. If it has casehardening stress due to improper stress relief in the kiln, then the warp will not disappear.

In your case, the lumber leaving the kiln was apparently perfect. It then sat around in storage and the surface gained a little moisture, trying to swell. However, as both top and bottom faces of the lumber are swelling equally, nothing much happens - no warp in the lumber. But there is a little stress in the lumber from this restrained swelling at the surface. In fact, if you cut a stress sample now, you will see that the prongs pinch inward and indicate stress. But this is moisture gradient stress, and not casehardening stress.

Now, when you resaw this lumber, the outer surface is able to immediately swell, creating cup. However, when the surface moisture evaporates in the warm room, the surface shrinks and the lumber becomes flat again. Temperature is not the problem; it is a moisture problem. The heat allows the surface to dry, but the humidity in the plant is much lower than the humidity in cold storage. The solution to your problem is to heat the storage area slightly in order to achieve about 35 percent relative humidity.


Have something to say? Share your thoughts with us in the comments below.

Profile picture for user genewengert
About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.