Is my kiln schedule not long enough?

[admin]

When re-sawing 8Q cedar, high moisture is in the very center causing cupping of the boards. Is my kiln schedule not long enough? How can I correct this?

Question by Kevin S. from Carl Diebold Lumber in Troutdale, OR

[MichaelM]

Kevin,

High MC in the center is not the root cause of the cupping. 

There is stress in kiln dried lumber.  The stress is relieved during a conditioning process during which the wood is exposed to an equilibrium moisture content about 4% greater than the target moisture content.  This is easily accomplished when the target MC is 10% or less.

It is difficult to get the EMC in the kiln high enough if the wood is greater than 10% moisture content.  So the cupping in the wet wood is likely due to stress.

I would also guess that the cupping has some time delay - it might take 6 to 12 hours for it to fully cup.  This is partly due to the MC change as you suggest, but it is also due to the viscoelastic nature of the wood.  There would be a time delay even if there was a uniform MC throughout the piece.

Hope this helps.
Mike M.