http://secondwindworkshop.blogspot.com/2012/02/upended.html will take you back to all the gory details.
After tapping into the vast experience, knowledge and generosity of the online woodworking community it looked like the problem indeed boiled down to the temperature and humidity variations between Minnesota and Colorado. I was following that trail myself, but some further detective work by my online brothers and sisters pointed to the fact that the snug fit of the lids was not allowing moisture to escape from the lids evenly in this drier Colorado climate. The lids were such a good fit (not bragging here…well, not too much) that the bottom of the lid (which is a separate piece of wood) was completely contained, so as the top of the lid shrunk slightly, the ends running parallel to the edge faces began to curl up.
Some folks advised removing the lids for a while to let the bottoms of the lids acclimate to the new climate. After several months off the boxes, the return movement was very little, if any. Another piece of advice was to sand away the finish on the bottom of the lid so that any needed moisture loss would be uninhibited. I sanded those surfaces a couple of months ago and I’m pleased to say that the lid that showed the least amount of cupping had almost returned to its original state. It looked like the second lid showed some improvement, but the cupping was still quite noticeable. Thanks again for everyone’s input!
With a couple extra days off this weekend to play in the shop, I decided to do some tuning of the lids to get them back into shape. For the first lid, with the least amount of movement, I was able to sand away at the “high” point on the bottom of the lid and get it to lay flat across all the top surfaces of the box walls again. The fit also remains nice, with no slop or side-to-side wobbling.
The second lid required a bit more extensive surgery…
With the lid clamped between two pieces of scrap, to protect the handle piece, I tuned up the scalpel (my Stanley #92 shoulder plane) and sedated the patient.
I took a few passes at a time along the cupped edges, at the “high” points, and tested the lid’s fit with the box.
Here are both boxes all gussied up and NOT smiling anymore, which is what we wanted to accomplish. J