George, I read the chapter on drying and dimensional change in the reference you pointed to. I found nothing inconsistent with what I said.
My family has been in the millwork/commercial cabinet business for 50 years; we used a 4" maximum as a rule of thumb for a glued up top with alternating orientation to limit the total amount of washboard like cupping.
One source of cupping is a result of imposing a differential moisture gradient across an originally flat, uniformly dry board. This can be caused by a temperature drop across the wood (a temperature difference results in a relative humidity difference that in turn results in a moisture content difference.)
Another common cause is running boards that have a moisture gradient through the planner; they'll cup after the gradient goes away.
Anyhow, if the root cause for cupping deformation is a change in dryness gradient thru the thickness of the board, the resultant curvature will involve the moisture difference times the expansion coefficient divided by the thickness of the board.
A 4" wide x 1/8" thick board moist on one side and dry on the other cups a lot more than 4" wide x 1" thick board under the same swelling conditions (the cup depth of the thin board will be about 8 times that of the thicker board.)
Using the dimensional strategy I mentioned in my previous message (glue-up board width proportional to thickness) keeps the relative washboard effect the same after equilibriation.
Do you have evidence to the contrary? Or perhaps you thought I was refering to thermal expansion due to my mentioning temperature gradients?
or maybe we were wrong all along?
Dave