by Professor Gene Wengert
Q.
I have a problem with cracks in 10/4 hard maple. Each order of lumber received from our suppliers is oven tested for MC content. We refuse the load if the lumber has an MC higher then 7%. At each operation through the machining all the parts are inspected for quality, because parts that were perfect in one area of the factory develop cracks when stored in another part of the factory. We have contacted Moore Dry Kiln Co. & Coe Manufacturing Co. and we are still trying to solve the problem.
A.
Regarding cracks in the maple, it is not uncommon for maple to have fine hair-line cracks, especially in the thicker pieces. I don't mean that these cracks are in 5% or even 1% of the pieces; I mean that I do see these cracks from time to time. I find that most of the time, these cracks are pre-existing cracks that developed during the mid to late stages of drying (often when the wood was rewetted).
If you can imagine that a piece of lumber is drying and the core has finally begun to shrink. But the outside fibers are dry and very strong, so they do not give in or accomodate the shrinkage. Hence, there is quite a bit of tension stress in the inside of the piece of wood. Now, let the outside fibers be exposed to water or high humidity (which shouldn't happen, but sometimes does due to operator error [or rarely, equipment malfunction]). The outer fibers swell, pulling on the inside, which adds to the internal tension stress. The result in some cases is a crack. Now when all the moisture leaves as drying continues, the stress is reduced and the crack closes tightly so that it is invisible (but it isn't healed). As you begin to process the wood, you will remove the outer fibers which are the driest part of the wood. The core, which is slightly wetter is exposed. Further, the RH in the plant, especially if the heat is on, is quite dry. As a result, the core shrinks a little, creating stress which results in the pre-existing crack re-opening.
The cure is to dry the maple properly.
If the wood isn't dried properly, you can sample the load for defects, cutting check samples. Or you can cut a few pieces in half and expose them to the dry air in the plant and see what happens. You could also raise the RH in your plant, which would cause the cracks to stay closed in your plant, BUT they would likely open in the customer's dry home.
Who dries your wood? I would be glad to work with the drying people, as a consultant, to correct the problem.
If you have any questions, please feel free to call and discuss this further. I imagine that it is quite an expensive problem. Feel free to fax any kiln records or other pertinent data.
Let me know what I can do further to help.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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