by Gene Wengert
email@example.com WOOD DOCTOR’S Rx
Symptom. In dry lumber, the pitch or
resin exudes from the lumber, especially in warm conditions. When the resin
moves in dry lumber, it can actually migrate through paint and plastic films.
The resin, when heated in sanding, can
clog (technical term is load) sandpaper
and sanding belts.
Cause. The pitch or resin in pine
consists of a myriad of chemicals. Some
of these chemicals evaporate at room
temperature and give us the wonderful fresh wood aroma. Some of the
chemicals are fairly liquid at room
Cures. A heat treatment given to the
lumber at the end of drying will evaporate the components of the resin that
would be liquid at room temperature.
After the heat treatment, the resin left
Fine Hairline Checking
will be hard and will not cause prob-
lems unless the wood is heated (such as
in sanding or when used in a hot place
such as a fireplace mantle). The hotter
the temperature during this treatment,
the more that is evaporated. From a
practical point of view, 160F is fairly
good and 180F in the kiln is even bet-
ter. Of course, such high temperatures
may create color problems and may
also make the wood more brittle.
Symptom. From time to time, a few hairline cracks (usually called a check) will
show up in the finishing room or even
after finishing. They were not seen
prior to finishing. They are fractures
within the wood.
Causes. All such fractures are the
result of a surface check or end check
(formed at very high moisture levels
because drying was too rapid) that
went deeper into the wood because fast
drying was continued throughout the
drying process (Fast drying is usually
associated with high temperatures,
high air flow and low humidities.) or
a rapid increase in surface moisture
content (This increase causes the
surface to swell rapidly and in the
interior is still quite wet, this swelling
stress will pull the interior apart. Fully
dried lumber is usually strong enough
to resist this selling force, so the risk
is with partially dried wood or wetter
wood in the kiln.)
Cure. Avoid surface checking and end
checking, as if they are avoided, the interior will not develop checking. Avoid
rewetting partially dry lumber.
End Checks and End Splits
Symptoms. End checks develop in all
species of lumber, but are rare in
spruce...that is, they are short. They
are worse in thicker stock. End checks
can become internal checks and no
longer visible from the surface. Splits
are cracks that go through the lumber,
from face to face.
Causes. Checks result because the ends
dry faster (due to the end grain which
dries perhaps 10 times faster than face
grain) than the result of the piece. As
a result, the ends want to shrink but
are restrained by the rest of the piece.
Stress develops and often a check
See more at the Wood
Dr. Knowledge Center
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Rx for Better Drying, Part II
More on symptoms, causes and cures for a variety of problems.
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Wood Doctor’s Rx question and answers, go to
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.