The two clay bars were fired side-by-side at cone 01. The back bar is of a raw clay dug from a creek bed in Alberta, Canada. Notice how it puffs up inside and eventually splits open the outer layer (which has sealed in the gases of decomposition). The front bar is that same clay, but mixed 50:50 mix with Redart. It is stable and strong as a stoneware. You can see all the lab tests I did on this in my insight-live account at http://goo.gl/KiUoU0
When clay materials and bodies bubble as they melt or over fire. This normally happens in raw materials that contain particulates that produce gases during firing.