1. It sounds like "dunting" which is especially awful when perfectly flat pieces split in firing. It seems the larger the slab, the more likely it is to happen. Some think it's because of friction on the kiln shelf. You might try a dusting of dry kiln wash under the plaque on the shelf. 2. It's important to make sure to keep them flat, since clay has a memory. If the slab bends, the distortion can pop up later, even after it has been laid flat again. i sandwich tiles between two "crezon" boards (this is sign painters board with a smooth, very slightly absorbent surface. It resists warping even with continued wetness) and flip them every few hours so they can be dried evenly top and bottom. 3. Another point is not to let them dry too quickly. I lightly cover with thin plastic, not fully opening to the air until after leather hard. Even then i flip regularly. 4. The carved areas on your pieces will dry faster and that could cause warping and/or cracking too. one thing i try to do is carve out the thicker areas from the back side to keep all areas approximately the same thickness. but sometimes it just works out and sometimes it doesn't. hope this helps! I love bas relief.