Understanding Drying Stress in Wood

A complaint about cupping in mahogany door stiles after machining leads to a discussion of what causes drying stress and related movement in kiln-dried wood. June 23, 2006

Whilst making some mahogany storm doors, I jointed the face and edge of the stiles as the rest of the stock like normal. Then I sent it all through the planer as normal. The stock was 6/4 and the doors were to finish at 1". When I got to just under 1 1/4", I noticed that some of the stiles had cupped, so I jointed the faces again, not worrying about the edges just now as the stock was still over width. Then I planed them down to 1". It took me 3 passes, so I turned the wood over after the first complete pass so as not to hog all the material off the one side and chance cupping again. Yes, you guessed it - some of them were cupped again when I was done. Is there anything else I should have done to prevent the cupping?

Forum Responses
From Professor Gene Wengert, forum technical advisor:
This wood has transverse drying stress, which is also called casehardening. In other words, the drying stresses were not relieved in the drying process using a stress relief process that is called conditioning. Can you return the material? A reasonable supplier should give you a 100% refund, as the wood is not properly dried. Proper drying includes conditioning.

From contributor S:
For passage doors we try to pull stock several days before milling and stand on long edge with air allowed on both faces. Typically, most movement that was prevented from being flat stacked will occur during this time. Then joint/plane, assemble, leaving 1/16-3/32 more to be taken off during final sanding. Occasionally, you get a board here and there that will be a problem, but luckily never had a whole batch be bad. Hopefully your supplier will man up and replace it all 100%.

From Professor Gene Wengert, forum technical advisor:
Moisture content changes take time, so warp that occurs over time is moisture related. Drying stresses are seen immediately.

From contributor S:

Gene, what causes the drying stresses? Is it dried too quick or slow? Or is it some other factor?

From Professor Gene Wengert, forum technical advisor:
Drying stresses are normal events in all typical drying processes. The stresses result when the outside begins to dry and tries to shrink, but the inside is still wet and does not shrink. The outside then goes into tension and eventually will be stretched out and dry to a larger size than if it had been free to shrink. This is (in a few words) the cause of casehardening or drying stresses. Such stress can be avoided by extremely slow drying, taking perhaps a year for 1" stock and drying it so slowly that there is little difference in MC between the outside and the inside. Very impractical to do, so we will condition the lumber at the end of drying to remove any stress. Special note: the high humidity at night in air drying and solar drying serves to relieve most of the stresses daily, so AD or solar dried lumber is often free of stress or has very little.

From the original questioner:
Thanks for the replies. The stiles bent immediately, but not all of them. Less than half. The amount of bend was exaggerated because the stiles were together with opposite bends. Still was not as straight as I would like.