Sanford- The cup happened instantly this time (and other times, if I recall correctly), so my assumption was internal stress rather than moisture, as you suggested. It’s possible the moisture issues could cause further movement, so I’m letting it sit for a bit. Since the movement arose from tension, I don’t think I’m going to try the moisture tricks because I think the piece will return to equilibrium and (reverse) cup if I do this, at least in this case. I realize you were suggesting this only for when the cupping came from moisture, not from stress.
Edmund- That’s interesting. Does he try to go right to the desired thickness or close to it? I suppose that would minimize the difference in stress across the thickness and reduce the amount of strain (movement). On the other hand, if there is movement, you have nothing to work with. I resawed down the middle of 6/4 with the idea of having plenty to work with to get back to dimension. Maybe what I will do is focus on removing material from just the exterior surface with the hope of reducing the stress difference between the two faces. Maybe that will help it relax a bit and, at the same time, I’ll be taking away the hump. I’ll measure the sagitta of the concave side and see if it lessens when I remove material from the hump.
I’ll bet the lesson here is to cough up the cash and buy quartersawn, but walnut is scarce these days and quartersawn that will yield my needed 9 inches is rarer still. Another lesson is to just avoid this in the design, if possible. Unfortunately, I’m building a reproduction, so there aren’t many options. (Harvey- That’s why veneer isn’t an option, partly).