Warpage in bookmatched curly maple?
by Professor Gene Wengert
Should I expect to receive warped wood and if so, how badly warped? What can I do if the pieces are warped? I will need to plane one side of the maple (3/4" thickness final) in order to achieve a proper gluing surface.
Warping is caused by a moisture change in the wood, so if the moisture is wrong, there is a chance of warping in any piece, regardless of the "style." I suspect that the information you received refers to taking a 2 inch piece of wood and resawing it into two 1-inch pieces (approximately). In this case, if there are any residual drying stresses (also called casehardening), then the two pieces will cup (warp across the width of the pieces) immediately when sawing.
But the presence of drying stresses is a defect; there should not be any stress. If there is, return the wood promptly for a full refund.
Often, the more you plane casehardened wood, the more it warps. Oftentimes we see casehardening in lumber dried by a very small operation or a hobbyist who does not take the time to relieve the normal drying stresses.
In fact, some kiln manufacturers even state that they dry the wood stress-free naturally, but it isn't true. Stresses are a natural event in drying; they can be easily relieved at the end of drying through rapid addition of moisture, usually by briefly steaming the wood. (There is a water spray system available too, but I have not heard how well this is working.)
Note: You can test the wood for stresses by resawing it. However, a quicker test is to cut a prong-shaped sample. The exact procedure is covered in most drying texts (or I could send you by FAX a picture).
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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