Q: What causes checks in white oak lumber? I am referring specifically to checks that are open at the end of drying. Can these be closed?
A: Checks (or small cracks) on the surface, which may penetrate inward 1/2 inch or more, are usually caused because the lumber was dried too quickly when the moisture content was over 45 percent.
Checks can also be caused because the kiln temperature was too hot too early in the cycle when the moisture content was more than 25 percent (rarely the cause), or because the wood is abnormally weak, probably due to bacterial infection. We are seeing more of this. If the checks are a result of drying too fast during air drying, most of the time there will be dirt inside the check. You can crack the checks open and look inside for dirt.
Typically, if the lumber is not subjected to rewetting during the drying cycle, checks will close at about 25 percent MC and will remain closed on the surface after drying (but possibly open below the surface, so the cracks will show up after planing). If, however, the slightly checked lumber is exposed to wetting during or after drying, then the checks will close as the surface fibers swell shut, but at the same time the checks will deepen and will be open at the end of drying.
Can you ever close these checks that are open at the end of drying? No! At least not permanently. Some people say that the lumber can be steamed at the end of the drying cycle to close the checks. This may work because exposure to moisture will swell the surface fibers and will close many of the checks. But when the surface moisture leaves after steaming is completed, the surface fibers will shrink and the checking will be worse (deeper and wider) than ever.
If the checks are open at the end of drying, then it is extremely unlikely that any lumber grader would overlook the checks. They would be considered defects and unacceptable. That is, open checks would not be considered "ordinary" checks. There are probably many more checks that the grader cannot see, but will show up in processing; I suggest that a load with some open checks be carefully examined for hidden damage in the "check-free" pieces.
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