Most problems in fusing are caused by rushing the firing. The glass must heat slowly during the critical temperature range of 100 – 500 degrees F.
The second critical temperature range is annealing, which is the cooling range of 950 – 750 degrees F average. Cool the glass slowly in this range so the stress in the glass will have time to dissipate.
If you become impatient after the glass has fused and you crack open the lid of the kiln for a few seconds to peek inside, you may hear a “ping,” which is the sound of glass cracking. Avoid the temptation to open the lid. Wait until the kiln has cooled to room temperature. Some artists schedule their fusing so that it is completed before they go to bed. That way they will be asleep while the glass cools and they won’t be tempted to open the lid.
After each firing, examine the shelf. Recoat if the kiln wash is chipped. When glass sticks to a bare section of the shelf, the glass cracks.
If the cracks appear along the edge lines of glass pieces that are fused together, then the cracking may be due to incompatible glass. This may happen if you fuse different brands of glass.
If the edges of cracks are rounded, then the glass probably cracked as it heated up. If the edges of cracks are sharp, then the glass probably cracked during cooling.