I slumped a beautiful 8" bowl, but the bottom came out rounded, I put it back in the kiln and slumped at a slower rate, it came out ok, not as rounded, but not like the mold. I tried it a second time with another 8" piece of glass, but set my kiln at a really slow mode, it came out the same way the first bowl came out, a rounded bottom. How can I slump a bowl and have it come out right the first time?? Thanks!! Lola