Q. We have received some complaints about small cracks (I believe you call them checks) in the red oak stiles and rails of our kitchen cabinet doors. We replaced the doors a few months ago and have no more complaints, but what should we do to avoid this costly issue in the future?
A. Checks or cracks that open up in the finished product are the result of shrinkage of the wood after finishing. However, dry oak is very strong…so strong that it is virtually impossible to have enough shrinkage force that will initiate a new check or crack, even if the moisture loss that causes shrinkage is 3 or 4 percent MC.
Stated another way: It would be very unusual to have lumber that is so wet (always check the MC of incoming lumber) and air in a home or office that is so dry (measure the RH) that the moisture change would be large enough to crack the strong oak wood. Plus, the stiles and rails of a door are normally quite narrow, so the overall shrinkage is very small, so the force, even with a large MC change, is small.
So, what you are seeing are preexisting checks; the checks were created at very high MCs when the lumber was air drying or predrying. In the manufacturing process in your plant, it is often difficult to see these checks as they will be tightly closed (but not healed).
The only cure is to prevent the checks from developing in air drying. Perhaps you have noticed that the past few summers in the U.S. have been accompanied by unusual, extremely dry air for a week or so.
In Georgia, we have seen humidities under 30 percent RH in the warm afternoons…dry enough to check freshly stacked oak even with excellent drying facilities. When lumber is dried to 7 percent MC, but then is exposed to more humid air in shipment or storage (65 percent RH is a common average RH in much of the U.S.), this will swell the surface slightly and close the checks so tightly that they cannot be seen. Exposure to the dry air in a heated plant, home or office will reopen them as the surface shrinks.
A varnish or other finish will not stop moisture movement enough to prevent this from happening. The best that you can do, in addition to working to improve air drying, is to cut surface check samples from wide, flatsawn pieces. See page 102 in Drying Hardwood Lumber (https://www.fs.usda.gov/treesearch/pubs/5710) for instructions on how to cut these test pieces.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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