Oops! There were all sorts of issues here. First I made the panel too long as I was hoping it would slump on this wave mold with the area over the curve on the left coming down first and then on the right the panel would not have ended up on the kiln shelf. So... Lesson 1 - I suspect it was to heavy on the right side. Second lesson is I goofed up the firing schedule. The slump finished long before I thought it would but when I checked the kiln the panel was still in one piece. That by itself wouldn't have been a deal breaker. But third lesson --- My impatience did me in I think. I cracked the kiln lid just a bit in order to cool down the slump from around 1200° to 900° to start the anneal process. I think that was just enough to let some air in and the glass being on a stainless mold as opposed to flat on a shelf and stainless cools faster than glass and PING! Thermal shock. That's my analysis anyway. Next step is to decide how I want to fix it OR not.