Changing shape in a square profile
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 shape 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.

Is it normal for catalyzed finishes to crack over joints?
Q: We recently made a cabinet with stile and rail joints that were mortised and tenoned, then glued and finished with a catalyzed varnish. Now the joints are opening about 0.03 inches maximum, but the cracks are unsightly.  Is there something we can do to avoid such problems?

A: Whenever two pieces of wood are joined with the grain of one piece running at 90 degrees to the other, we have the potential for some serious misbehaving. What happens is that the one piece doesn’t move with moisture changes, while the other does. More precisely, along the grain (or lengthwise), wood is stable, while across the grain (thickness or width), wood moves.

However, wood only moves if the moisture content changes. So, did you provide wood that was too wet or too dry? Or is the customer’s environment too wet or too dry? (Or possibly, the piece was stored or transported at a high humidity and then dried out when unloaded.)

The only way we know for sure is if you measured the moisture content of the wood items when you manufactured them. I am not talking about the moisture content that the kiln operator indicates or even the moisture content measured on a few pieces at the rough planer. You need to make some measurements – about 30 – on your finished pieces each day so you can guarantee that you are not shipping anything wet. (Wet means over 7.5 percent moisture content in most cases.)

Then, you will know for certain that any moisture problems with your finished goods will not be your fault. Measuring the moisture content of finished goods is a very cheap insurance program, a lot cheaper than liability insurance and certainly much more beneficial. In fact, if you do indeed measure the moisture content, maybe you will be the first one in our industry to guarantee that your furniture was made at the correct moisture content, or you’ll replace the piece within 48 hours!

Sounds risky, but there is no risk if you indeed measured the moisture content before shipping.

Source: Gene Wengert, “The Wood Doctor.” Read more of Gene’s Wood Doctor and Wood Explorer columns at WoodworkingNetwork.com/genewengert.