Warp on resawn basswood

Q. Is it possible for a dull sawblade to cause stress?

A: Basswood, at times, has stress in the tree that will show up in the lumber. This stress will mainly appear as lengthwise warp (commonly called sidebend or crook). For this reason, the stress relief procedure in the kiln has to be done more carefully and differently than for other hardwoods. In general, the initial high humidity during stress relief must be obtained within two hours. This high humidity is often 5 percent RH higher than normal as well. It seems that the best and most uniform stress relief is obtained at temperatures over 170F.

Further, appreciate that if lumber leaves the kiln in perfect shape, but then is allowed to lose or pick up a bit of moisture, so there is now a moisture gradient from shell to core, we will have problems. When such lumber is resawn, it will cup immediately. Also, as the moisture adjusts to its new environment and the gradient disappears, the pieces will cup over the next few days. Your description seems to fit this picture quite well.

In the kiln, if lumber in the kiln is not at a uniform moisture throughout, the stress relief will be spotty. Also, stresses after drying must not be measured on hot lumber, as the results will not agree with the stress tests in cooler lumber. Further, the stress tests that are used today (most often cutting a “U”-shaped test piece, as shown in all drying texts) all require the lumber to be uniform in moisture content (a point that is often not mentioned in the textbooks). A moisture gradient will result in erroneous results. For this reason, I am not always confident when someone says that they think the lumber was dried properly. As a Missourian would say “Show me the data!”


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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.