Q: I am working with solid lumber cores that are yellow-poplar lumber, edge-glued together and then laminated with a decorative veneer. What is the maximum difference in moisture content you would want between the pieces in the core? I have been told that as long as they are within 2 percent of each other you should not get telegraphing. Thanks for your help.
A: The 2 percent maximum MC difference between pieces is indeed correct, but is only part of the requirements. The pieces also need to be very dry - 6.5 to 7 percent MC on the average. If they were 8 to 10 percent MC and then dried to 6 percent MC in use, they could shrink different amounts as they dry and the end result would be telegraphing.
For readers who do not know what the term telegraphing means, it is the phenomenon where the wood pattern from the core shows through, usually as bumps or lines, in the finished surface. It is the result of the core not being perfectly smooth. Even though a core may be smooth when sanded and smooth when laminated, if the MC changes and the pieces shrink differently, then the pattern can become obvious, especially with a gloss surface. Often, when laminating, a crossband (an extra sheet of veneer, wood or synthetic) will be used between the core and decorative veneer to moderate (not 100 percent corrective in all cases) any shrinkage difference.
Telegraphing can also occur between the particleboard (or fiberboard) core and the solid wood edgebanding. These materials shrink at different rates when the MC changes. The key is to have all materials at the same MC and at a low MC that will be in equilibrium with the in-use conditions in the home or office, about 35 percent RH average in most cases.
Special note: Sometimes, due to the drying method used for composite products, they will have an MC that is 1/2 percent lower than solid wood, even though both are in equilibrium with the same RH. So, a composite core can be 6.0 to 6.5 percent MC when the lumber bands are 6.5 to 7.0 percent MC.
Not all moisture meters will work really well with different wood products and at these low MCs, double check your meter with a few oven tests to be certain.
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