Date: 2009-09-30
By Gene Wengert
Q: We are in a discussion about the role that water and
moisture plays in bending wood. I know that how well a piece
bends is dependent on the moisture content of the wood but what
role does the moisture play in the plasticity of the sample? If
you were to take a piece of wood that is oven dry and we're able
to cool it in a chamber with 0 percent RH, would application of
heat alone be enough to bend the piece and maintain it's
shape?
A: Heat does indeed make the wood plastic, which is what we
need in order to bend wood and have it maintain its bent shape.
(If wood were only elastic, it would bend, but then when the
bending force was released, the wood would return to its
original shape.) Moisture extends the plastic range of wood. I
cannot say which factor, heat or moisture, is more critical;
both are important, but perhaps heat is a bit more important
than moisture. Note that at 0 percent MC, the wood is so
brittle that the plastic range is very small. Failure can occur
as soon as the wood bends a little bit even when using heat to
plasticize the wood.
Gene Wengert, "The Wood Doctor," has been training people
in efficient use of wood for the past 35 years. He conducts
many short courses and has written hundreds of practical
articles and books covering all aspects of converting logs into
lumber and lumber into finished products. He is presently an
extension specialist emeritus at the University of
Wisconsin-Madison. Fax your questions to him at +1.815.966.6416
or e-mail to wooddoc@uwalumni.com.
Updated: Sep 30, 2009