Q. I have read that you suggest a target MC for oak and other hardwoods for cabinets and furniture of 6.8 percent MC to 7.0 percent MC. Can you explain why please? Can we get every piece between these two values? Presently our kiln operator shoots for 5 percent MC.
A. Appreciate that the range of 6.8 percent to 7.0 percent MC is a target to aim for. We might miss once in a while by a fraction of a percent. Second, this is the average MC of a load or batch of lumber. We do expect, in a good kiln drying operation, that about two-thirds of the MC values will be within +0.3 to -0.3 percent MC around the average.
So, if the average of a load is 6.9 percent MC, then two-thirds of the pieces will likely be between 6.6 percent to 7.2 percent MC. Further, 99 percent of the pieces will be between 5.9 percent MC to 7.8 percent MC. Technically speaking, this is a standard deviation of MC of 0.3, which good kiln operations can achieve and which mean that you will likely not experience MC problems.
Why do we choose this target? Most homes and offices in North American range between 30 percent RH and 50 percent RH weekly average. This is equivalent to 6.0 percent to 9.0 percent MC in the wood, although any moisture changes in a finished product do take time before we see any issues, a week or two at the least, and maybe longer. We also know that moisture losses cause shrinkage and shrinkage results in cracks or splits. So, we want to make sure that our wood is quite close to the driest expected condition of 30 percent RH, or 6.0 percent MC.
On the other hand, small amounts of swelling often do not create severe issues. Plus, when we switch from drying to moisture gain, there is about a 1 percent MC delay before the wood begins to swell. So, targeting a little bit on the dry side is ok, as with the delay, even though the humidity is 50 percent RH, the wood actually behaves as though it was only 43 percent RH. The bottom line is that with small moisture changes in use with the average 6.8 percent to 7.0 percent MC target, will not create defects in most products.
Note that if you are shipping to a humid climate like Florida or a dry climate like Colorado, you might indeed want to adjust your target. If that is not reasonable, then consider storing the products in your shop in a small room with walls sealed with plastic sheets and with a high RH for Florida or a low RH for Colorado.
Then any defects that develop can be fixed before shipping rather than in the field. Further, after your week-long test in the controlled humidity room, if you wrap the products in plastic wrap or bags, you know that the MC will not change one bit, because moisture cannot get in or out of the wrapping.
Finally, your kiln operation needs some attention, as it is virtually impossible in standard kiln to achieve and average of 5 percent MC. Such lumber would be extremely brittle when machining and would not glue well. So, you are being told incorrectly or the operator is measuring MC incorrectly. To measure these low MCs, the operator would have to use an oven test, or use a pinless meter. The pin meter is not very accurate under 6.5 percent MC.
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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