Date: 2009-03-18
By Gene Wengert
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?
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 responsible for.
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.
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 847.390.7100 or
e-mail to wooddoc@uwalumni.com.
Updated: Mar 18, 2009
This article appeared in FDM, March 2009. ©Copyright 2012, All Rights Reserved.