Mortised and tenoned joints opening
November 19, 2015 | 6:00 pm CST

Q: We recently made a wooden item that has some moulding pieces on the outside. The stile and rail joints were mortised and tenoned, and then glued. We then finished them with a catalyzed varnish. Now the joints are opening about 0.03 inches maximum, usually less. Yet, the cracks are unsightly. Are small cracks like this "normal?" Is there something we can do in the future 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 MC of the wood items when you manufactured them. I am not talking about the MC that the kiln operator indicates or even the MC 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 MC in most cases.) Then, you will know for certain that any moisture problems with your finished goods will not be your fault. Measuring the MC 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 MC, maybe you will be the first one in our industry to guarantee that your furniture was made at the correct MC or you'll replace the piece within 48 hours! Sounds risky, but there is no risk if you indeed measured the MC before shipping!

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Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.