I have 12" copper skillet I picked up this summer. It's a Sur La Table piece made by Mauviel. 1.5mm copper with stainless steel interior.
It's a HEAVY pan and I've been pleased with how it's performed but I took it out this morning to saute some apples for a dessert I'm making and it seems to have warped!
I use it on a ceramic cooktop...I never cook above med/med high(#6 setting of 10) and haven't had a problem with any of my other cookware warping.
I'm really surprised. I'll take it back under warranty. Warping shouldn't be a regular occurence with a properly made copper pan right? I was thinking about investing in a couple more pieces of copper(though was already thinking about a thicker layer of copper)but if I have to worry a lot about warping maybe not.