Corner miters opening

Q: We receive mouldings from Europe in a variety of species. We use the mouldings to make picture and mirror frames and find that because the wood is 6 to 12 percent MC (and even 14 percent MC at times), the corner miters are opening up. So, what can we do about this problem?

A: Reading between the lines, I think you already know that the problem is high MCs. Shrinkage after making a miter joint results in opening of the joint. I suspect you also are aware that the moisture content (MC) inside most offices and homes in North America is typically 7 percent MC, with winter values being closer to 6 percent MC.

If the wood you purchase comes in at 6 to 12 percent MC, you will experience some shrinkage and poor joint performance (if you do not like gaps).

From the information you presented in your memo, it seems that the only option you have is to take the raw material, when you receive it, and put it in a controlled heat and humidity room (100F and 30 percent RH), in order to dry the wetter pieces to an acceptable level. This will of course cause some width and thickness shrinkage. But if the pieces are dry when you cut the miters, you will have acceptable joints in use.

I worked with a small item manufacturer in Texas that had similar concerns and this "heat room storage" approach was perfect. Initially, they used some extra cargo containers for their heated and controlled RH storage facilities. Rejects went from about 24 to 4 percent. Basically, you have a very small fan (4 to 6 inches diameter) to exhaust a small amount of the air from the chamber at all times. You also supply heat (up to about 120 F) to lower the humidity to the desired level (30 percent RH); heating air lowers its RH. You would put the heater on a humidistat; when the RH is too high, the heat comes on. You do not need a dehumidifier; lower the RH with heat.


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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.