Q. After we finish kiln drying and go through stress relief (conditioning), what should we do next?
A. Because stress relief involves adding moisture back to the surface of the lumber at high temperatures (170F or so), when we shut the kiln off, there is likely a small difference between the core moisture content (MC) and the shell MC.
Therefore, as you might already appreciate, there is a rule that says, “Do not run hot lumber into the rough mill.” The obvious reason is that there is a moisture gradient, which the shell being too high in MC. As the moisture adjusts to the ambient conditions in the shop or even the customer’s home, the wood will shrink or swell, stresses flue joints and causing warp.
So, after the kiln drying is complete, we need to wait a few days for the moisture in the lumber to adjust, especially for the shell to lose the moisture we added during stress relief. With storage in a warm location, maybe two days is sufficient, but three is better. In a colder location, perhaps five days is best. This storage is best when done outside the kiln if possible as each extra day the lumber is in the kiln costs about $25 per day per MBF in lost profit.
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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