Q. What makes a 2 x 2 inch square (about 24 inches long) that initially has the sides at perfect 90 degrees so it has a square profile become slightly diamond shaped with the sides no longer at 90 degrees to each other?
A. This diamond in occurs for two reasons: 1) Because the moisture content is changing and 2) because the annual growth rings, as seen from the end of the piece, are not running parallel to two faces (and perpendicular to the other two faces), but are running at an angle to the faces.
The worst case is when the growth rings run from one corner to the opposite corner. The reason that the ring orientation on the end grain is important is that wood shrinks, when there is a moisture loss, twice as much along or parallel to the rings than across the rings. This difference in shrinkage results in a square becoming diamond shape. This "un-squareness" is easy to detect in many wood products, especially in a butcher block top made with end grain pieces.
This same shrinkage difference effect is why a perfectly round piece becomes oval shaped when it dries. There is considerably more shrinkage along the rings than across the rings when looking at the end grain. However, a slight oval shape is hard to detect visually in most wood products.
Note: A gain in moisture causes the same overall distortion, but in the opposite or reverse direction; there is more swelling parallel to the rings than across.
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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