stain, JIT panel
Q:I heard you talk about the possibility of sap soft maple with a
gray-stained center surrounded
by normal white color after KD.
The lumber grader cannot see the gray so
does not reject the lumber. We have recently
experienced this phenomenon for the first
time. In your opinion, is it due to sick logs
or bad kiln procedure?
These stains are
caused by slow
drying at warm
A:First, the NHLA hardwood
lumber grading rules specify
that the lumber is graded
as it appears. If the interior
graying cannot be seen during inspection of the outside of the lumber, the
interior graying cannot be used to
lower the grade unless there is a special clause in the purchase contract.
Many suppliers of lumber will, however, when informed of the interior
graying, exchange the lumber, as they
recognize the wood is not useful to you
and has a man-made defect they are
Second, the interior gray stain we
see in various species, but especially
the maples, is an enzymatic oxidation
stain where the sugars in the sapwood are oxidized during the drying
process. This oxidation results in a
dark color that can be pink, brown
or gray. In fact, sticker stain is one of
these stains. Basically, these stains are
caused by slow drying at warm temperatures. As such, the staining can
actually begin in the log (but it is not
noticed at this point), making it hard
to prevent the eventual discoloration
seen at the end of drying.
Third, if log storage was poorly done
or air drying was poor, the stain can
develop before the lumber reaches the
kiln. The best that can be done in the
kiln in any case is to use a very low RH (a
15 degrees F depression) and stay under
110 degrees F for as long as possible.
Fourth, sometimes lumber is dipped
in a fungal stain preventative chemical.
Some of these chemicals have a bleaching effect on the surface, effectively
hiding the interior chemical stain.
Note the interior graying, browning or
pinking are not fungal stains; hence the
fungal dip does not prevent them.
Q:I have read every Wood Doctor
column you have written and
have gained a lot of practical information. But here is a
question I have not seen addressed before.
As background, our fairly large company
has really gone into JIT, just-in-time,
manufacturing, which means in-process
materials cannot sit around very long at
all. Well, this has recently translated into
machining our glued up panels (edge-glued
on a clamp carrier) within 24 hours after
they are glued, or sometimes less. Of course,
you know what the problem is: sunken glue
joints that are obvious after finishing. My
suggestion of waiting three days after gluing, as we have always done, has not been
well-received. I am hoping that you have
some help for us.