Drying stresses
March 3, 2011 | 6:00 pm CST

Q: We are fairly new at lumber drying and have a question. When running the kiln at the end of the cycle to relieve stresses, we call this process "conditioning," how can we tell when the conditioning is done? In the last load, the prong tests looked okay when we pulled the load, but now, two days later, we see a lot of stress. When we joint the lumber on one face, the lumber cups.

A: It is normal for drying stresses, also called casehardening, to develop during drying. I know of no commercial process that can avoid developing some stress; air-drying has the least amount of residual stress. To relieve the stresses, we traditionally add water back to the surface of the lumber quickly and at as hot a temperature as possible; usually we inject "live steam" into the drying chamber. Note that this steaming will leave a moisture gradient in the wood - the surface will be wetter than the core by a little.

To check for stress, or the lack of stress, we cut a prong that looks like the letter "U" from the interior of several pieces of lumber. This prong sample is initially cut so that it is the full width and thickness of the lumber and is 1 inch along the grain. Usually, for 4/4 and 5/4 lumber, the center half of the lumber (that is, the core) is removed, forming the letter U on the end grain of the lumber, where the legs of the letter are the width of the lumber. If the prongs pinch together, then the lumber is casehardened; if they are straight, then there is no stress.

The problem with the prong test is that moisture gradients can cause the prongs to move. So, if the prongs are evaluated in wood with a gradient, then the results will change as the gradient dissipates. So, when you read your prong test right out of the kiln, you really don't know what the stress situation is because of the moisture gradient. You can wait a day or two and then see what happens. But a better idea is to take the prongs, right after they are cut, and put them into a microwave oven for 15 seconds at high power. This energy will move the moisture around and eliminate the gradient. After microwaving the samples, let them sit for a few minutes and then "read them." The microwave results will be the same as the results your customer will find weeks or months later!

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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.