Date: 2007-10-22
By Gene Wengert, wooddoc@uwalumni.com
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.
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.
Updated: Oct 13, 2012
This article appeared in FDM, April 2000. ©Copyright 2013, All Rights Reserved.