Last year, I had tried to be creative for Christmas and create a piece using irregular edges. I was so excited to see it when it came out of the kiln and alas, it had a great big crack in it. I wrote Stacy Smith (one of my favorite glass people!) at Bullseye and after answering many questions because she was going to have to assess via mail rather than seeing it in person, Stacy wrote me that I had too many disparate layers and this makes it hard for the glass to heat up and cool evenly. She suggested that I try it again and take the ramp up and the ramp down very slowly.
Short description of this piece: In the center it was 5mm thick (2 sheets – one 2mm and the other 3mm). The edges were then laid down so that they would overlap the sides but not be completely on the center section, so the sides ranged from 3mm to 8mm. And since I also overlapped the corners, in some places the corners were actually 11 mm thick.
As a side note, if you want to know whether your glass cracked on the way up or down, look at the crack. If it has smooth edges like this one does, then it cracked on the way up because the edges softened when it hit the peak temperature. If it cracked on the way down either during annealing or after, it would be a sharp crack.
Took me awhile to get around to Try 2, but I made another one earlier this week and this time, while I did some overlap as before, I tried to have less variation in thicknesses ranging from 3mm to 8mm. And instead of using a normal fusing schedule, I took this one very slowly ramping at 150 degrees per hour up to 1225 degrees and held for 1.5 hours, then 250 degrees per hour (to get through the devitrification range quicker) to 1410 degrees and then annealed it on the way down for 2 hours. If I had added more thickness to my piece, I would have actually annealed the piece for 3 hours. This time no crack!