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