Interior gray stain
October 30, 2012 | 7:00 pm CDT

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.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.