Q. Why does our eastern 8/4 white pine lumber bow, both pieces, when we resaw it into 4/4? The bow is so the middle touches, but the ends do not.
A. It bows like this because it has lengthwise stress. This stress is caused by lengthwise shrinkage in the wood; such shrinkage and stress is not common in EWP, except in wood very close to the center of the log -- probably the first 10 years of growth. (In hardwoods especially, such stress can arise due to juvenile wood in the tree or growth stresses in tree.) I suspect that you likely have a moisture gradient, shell to core, with the shell being somewhat drier. Drier means the shell will be trying to shrink a little more than the core, which is wetter. In the 8/4 piece this shrinkage difference and the resulting stress is balanced on both faces, so the piece stays flat. But when you open the piece by resawing, the new lumber pieces have unbalanced stresses, face to face.
In drying, the standard procedure is to use a process called equalizing to get uniform moisture which is then followed by a steaming process called conditioning to remove any residual drying or shrinkage stress (also called casehardening stress). I suspect that these final two steps in drying have been omitted by your lumber supplier. You should be able to return the lumber, as it is not of the quality that is generally expected for lumber that will be processed further in furniture or cabinet manufacturing. On the other hand, such stress is okay for lumber used for construction lumber where remanufacturing is not required, other than cutting to length.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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