Mixing Frozen and Green Lumber in a Kiln

It's best to separate the loads. Here's some insight into why. December 9, 2010

Question
Is it true that if I have air dried lumber (hard maple) in my yard that is now about 25-30% and I am putting it in the kiln with dead green lumber 50%, that the partially air dried lumber will not warp as it will be somewhat rewetted? Also, we have our lumber stacked on sticks and put into our yard which is subject to all elements (snow, rain, sun, wind). It is air drying for anywhere from two weeks to two months - will this lumber warp? I have tried to explain this to the higher up, but I’m getting nowhere.

Forum Responses
(Sawing and Drying Forum)
From contributor Y:
If your lumber is hard maple and winter cut, it has probably been frozen most of the time since being cut. The moisture content will not have decreased much since being cut. If your lumber was properly stickered, immediately after cutting then covered with a lid or under cover it should not warp. I like to get my maple kilned as soon as possible before the weather gets warm. This keeps the nice white color, reduces sticker staining and fungus discoloration.

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From Gene Wengert, forum technical advisor:
Just as ice evaporates (sublimes), and because much of the water in wood is not frozen, you will indeed get a lot of drying in an air yard during the winter. In fact, the wood can dry so fast (certain species like oak and beech) that you will get air drying checks. What is preventing warp is the dry, strong outside fibers. If they are wetted, then these strong fibers become weak and warp can happen. Therefore, do not mix wet and dry in the same load or you will see warp in the drier lumber. You might consider running the kiln with the wet lumber and then adding the drier material when the kiln's EMC is at or below the AD lumber's surface MC (probably around 16% MC but maybe as low as 12% MC). It is also possible to dry wet lumber too slowly and get warp. Over-drying also encourages warp.