Q: We just ripped our lumber and everything was OK, but then we resawed it and the two strips bowed with a gap of almost a foot between them. Moisture seems OK. We are dealing with 6/4 basswood. We cut the standard prong test and found no stress. What causes this?
A: The phenomena can be caused by a severe moisture gradient, growth stress in the tree or longitudinal (lengthwise) casehardening. We can eliminate a severe gradient, as you indicated that you checked the MC with a pin meter using insulated needles. We can eliminate growth stresses because the strips were straight when you ripped them. So, it is longitudinal casehardening (also called drying stress).
With basswood and a few other species, there are wood cells that shrink lengthwise or try to shrink longitudinally during drying. They are restrained by the rest of the wood fibers in the piece, so this drying stress develops. Normally, we would relieve such stresses by using a conditioning period in the kiln -- rapid rewetting of the wood at 180F. I would have to look at the kiln temperature records to determine if conditioning were used, if the temperature was high enough and if the RH was high enough within two hours. Have the kiln operator check the appropriate material in the Drying Hardwood Lumber textbook.
Incidentally, the prong test you refer to only measures across the grain stress, not longitudinal casehardening. For longitudinal casehardening, take an 8-inch-wide piece of lumber, cut a 30-inch-long sample, rip the piece into two 4-inch-wide pieces, and put the two pieces back together. If there is a gap at the ends or in the center more than the thickness of a dollar bill, there is longitudinal casehardening that may cause problems for some manufacturing schemes.
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