Thermal Shock Cracking Whilst Slumping

[Richard Carder]

I have just slumped a dish 11" square. The total thickness of the dish is 6mm, Bullseye glass. The kiln that I am using is a Paragon Fusion 8. The firing schedule whilst annealing is 522c for 1 hour. When I opened the kiln there was a clean crack along the centre of the dish. There must be something wrong with my firing schedule, can anyone help?

[Bert Weiss]

Most often, this crack will come from heating up too fast. Slow down. There have been a constant stream of people who post, "This schedule has always worked for me before" Sometimes you have to be more conservative during the heatup. Slumps are particularly difficult because the glass contacts the mold in places and is open to the air in others.

I don't know your kiln, but many kilns will anneal 6mm to a point where it won't crack in the kiln, with a simple shut off. I am not saying don't run an anneal schedule for this thickness.

Slump firings can be difficult to determine if the cracked edge is sharp or rounded. If the crack happens during the heatup, the edge will be exposed to the highest heat in the cycle. If it is hot enough for long enough, the edge will round. Fuse firings are easy to tell the difference.

[Richard Carder]

Thanks for your reply. The edge of the glass where it cracked is rounded, so it make me think that it cracked during heating up, so I will slow it down.

[Morganica]

Slow down is probably good advice, but what *is* your schedule? Your complete schedule, start to finish? And what are you slumping into?