Q. We have some white oak lumber that has a dark outside color, but the inside is whiter in color, which creates a big issue. Do you know what is going on?
A. I am familiar with this defect where the core is much lighter in color than the shell. It happens with other species as well as with white oak.
This color difference is caused by partial air drying (rather than complete air drying) followed by normal kiln drying. In slow air drying, the shell develops the darker colors on the outside regions that are drying.
The wet core however is not yet drying much, so the color is not yet established. Then, when at rather high MCs, when the lumber goes into the kiln, the wet core is dried quickly at higher temperatures, which gives a lighter color, compared to air drying colors.
It is important, and a basic drying principle, to either kiln dry green from the saw, or air dry more thoroughly in air drying so that the incoming MC into the kiln is in the 20-30 percent MC range, rather than over 35 percent MC.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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