Splits, if open more than ¼ inch,
are due to stress in the tree.
Cures. Splits cannot be controlled in
Checks are fully controllable if action is taken before drying begins. The
first step is to document the extent of
the end checking. Once a monetary
value for end checking can be established, then it is easy to determine
what one can afford to do. Control is
achieved by slowing down the drying
rates of the ends of the lumber. This
slowing is achieved by using commercial coatings (close to 100 percent
control), using burlap or plastic mesh
on the ends of the packs, or by loading
the kiln or predryer so that any exposed ends are shielded by the lumber
in the next row that has the ends offset
or in a different location.
Symptoms. Small cracks on the surface or face of the lumber are called
surface checks. They are especially a
problem with oak lumber. Thicker is a
Cause. At very high moisture, the
surface fibers dry and begin to shrink
while the core is still soaking wet and is
not drying much and is not shrinking.
This sets up a stress condition between
the shell and the core of the lumber.
If the stress at the surface exceeds the
strength of the wood, then a small
crack, called a check, results. The
surface stress is larger if the RHs in the
early stages of drying are lower; low
RHs cause more attempted shrinkage.
A dull circle saw increases checking in drying, as the dull saw tears the
wood fibers and creates tiny checks
that will worsen easily in drying.
High temperatures weaken the
wood, so 105 F is often the highest
temperature used at high MCs…some-times cooler than this is even better.
Some species, like southern
swamp white oak, are inherently weak
and prone to checking. Bacterially
infected wood is also very weak and
Cure. Avoid drying too fast in the early
stages of drying. This means that we
need high humidities and low air flow.
Note that high humidities can increase
Incorrect Final MC
the risk of staining. Cool temperatures
also help to reduce checking.
Symptom. The final MC can be considered incorrect because the average
MC is not what is desired or because
some pieces are too wet or two dry. In
past years, the final MC was an average
of the MC of the entire pile and we
did not worry much about individual
pieces of lumber. Today, a few wet
pieces in an otherwise dry load can
cause significant manufacturing problems. Softwood construction lumber
often requires under 19 percent MC;
softwood boards, under 12 percent
MC; hardwood lumber is often 6.0 percent to 8.0 percent MC. The customer
determines the desired value in most
Better tolerances of final MC
require time and effort; that is, are
Causes. Incorrect moisture measurement, especially not choosing sample
pieces that represent the entire load,
during the kiln run is a common
cause. Improper or no equalization is
also a cause.