afford to work with this lower quality
A third concern is that if the trees
are not promptly harvested and the
logs promptly sawn, it is likely that
the logs will start to dry and develop
large cracks, both in the ends and on
the faces. Sawing logs with cracks is
often considered a safety risk. Further,
lumber with cracks is not worth much.
Some sawmills do have water sprinklers and use end coating on the log
ends to reduce this cracking during
log storage, but such actions must be
initiated promptly after harvest.
QDue to a recent severe storm with lots of rain, some of our kiln-dried
lumber in storage has
increased its moisture above the ac-
ceptable moisture content. How can
we dry this out?
ARedrying lumber that has increased its MC due to a severe storm or from be- ing kept in uncontrolled
storage requires great care to achieve
the required low MC. Redrying is best
done in a standard kiln. The first step
is to re-sticker the lumber using normal stickering procedures.
Two temperature steps are suggested for the redrying operation.
The first step should be about 18
to 24 hours long at 125 degrees F to
140 degrees F. The lower temperature
settings are for thicker and hard-to-dry
The humidity, which is set after the
correct temperature is achieved, is at
an EMC that is numerically halfway
between the current MC of the lumber
and the MC desired. (Every kiln operation has a chart of EMC values, relative
humidity values and temperatures;
use this chart to find out the correct
settings for your equipment controls.)
Do not use steam spray or water spray
during kiln warm-up.
The second step should be at the
final temperature of the normal
schedule, usually 150 to 160 degrees F.
This temperature can be achieved by
using several smaller steps (each about
an hour long), rather than making a
single 20 degree F or greater jump in
temperature. Set the humidity controls
to give an EMC 2 percent below the
desired final MC. Then, when the
wettest kiln sample reaches the desired
MC, stop the drying. There is no need
for stress relief or conditioning if the
lumber was dried correctly before it
Redrying is very risky if the species
being dried is prone to surface checking.
The risk is that surface checks, that
were small and tightly closed, may
become permanently opened, or may
grow into internal hairline checks
(sometimes called honeycomb). This
is a problem with oak and a few other
hardwoods, but not with easy-to-dry
hardwoods and not with softwoods,
With these risky species, go a bit
cooler and make changes in conditions
more slowly than suggested above.
Avoid rewetting at all costs; that is, do
not use higher than suggested humidities and do not add moisture before
the correct temperature is achieved.
See us at IWF Booth #1841