Q.  We have begun to use wood with more knots showing in the final product.  We finish these knotty pieces and they are flat and smooth.  However, after finishing and shipping, we sometimes hear about a customer that says the knots have grown and are protruding up a small bit higher than the surrounding wood.  Obviously, the wood is dead so the knots are not growing.  Can you explain what is going on?

A. To understand what is going on, you need to appreciate that wood does not normally shrink lengthwise, or more precisely, does not shrink along the grain when the moisture content drops.  However, wood does shrink across the grain with lowered MC.  So, when we look at a flat surface, such as a table top, if the moisture content drops slightly (such as in the wintertime when the interior humidity drops), there will be a small decrease in thickness of the top.  I suspect you already know this.

There is an exception. The knots in the top have their grain running at 90 degrees to the grain in the rest of the top.  (That is, when looking at a knotty top, we are looking mainly at side grain except where we have a knot: the knot is end grain.)  So, when the top losses moisture, all the wood will decrease in thickness except for the knots, as the wood in the knots does not shrink along the grain, which is the thickness of the top.  The end result is that the knots protrude slightly, or seem to grow.

The reverse is true if the top grains moisture- -the knots seem to shrink.

As this is normal behavior for wood, the only way to control this is to minimize any moisture changes in the top.  Also, we know that slow changes will have less impact than fast changes in 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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