Here a list of the major types of cracks in fused glass and some information about why they occur and how to prevent them.
Curved cracks across the middle of the piece.
Improper annealing causes this kind of crack. It will show up as gentle curves (sometimes as straight lines) that break the pieces into two or three pieces. Often the crack will curve sharply as it nears the edge of the glass. This kind of crack is the piece's way of relieving stress. The solution is to spend longer annealing the piece.
Cracks where two different glasses come together.
Glass incompatibility causes these cracks, which often show up as curved cracks around the edges of the two types of glass. The cracks can be very small or they can cause the pieces to break apart, but they will always be along the edges of the piece of incompatible glass. Conduct your own compatibility tests or use "tested compatible" glass to keep this from reoccurring.
Small, interconnected cracks (like a spider web).
These cracks generally extend from a single spot on the underside of the glass. They aren't usually severe enough to cause the item to split into pieces. Sometimes shelf primer will also be stuck to the underside of the glass. Most likely, this kind of crack is caused by glass sticking to the kiln shelf. A close examination of the shelf may even reveal small pieces of glass that are stuck to the shelf. The obvious solution is to scrap the shelf clean and apply fresh kiln wash.
Pie-shaped pieces, with smooth edges.
These cracks, which usually occur with such force that they split the piece into five to ten pieces, are caused by thermal shock. The edges of the pieces are often rounded because these tend to happen early in the firing cycle (around 300 to 500 degrees Fahrenheit) and the edges round during later phases of the firing. The cure for thermal shock is to fire more slowly in the early part of the heating phase. You might also try cutting very large pieces into smaller ones before firing. Finally, it's a good idea to peek in the kiln around 400 degrees or so just to make sure the piece is still OK.
Cracks that occur long after firing.
Sometimes a glass piece will just be sitting on a table when you hear a sharp ping. It might be quite loud, and perhaps there's a second (or even a third) ping. When you check it out, you discover that the glass piece you thought was beautifully finished has cracked. (This cracking can even be severe enough to shatter the piece, leaving the artwork in pieces on the table.)
The reason for this disaster is undoubtedly stress that has built up in the glass piece. Stress can come from many factors: improper annealing, thermal shock, incompatible glass, and even "normal" wear and tear. Improper annealing is the most likely culprit. If you can access the firing log for the piece, check to make sure it was properly annealed.
If you used the same schedule you've always had success with, then perhaps this piece of glass was a bit thicker or larger than normal. Perhaps it was a different glass than you usually use. Perhaps your "normal" annealing schedule needs to be adjusted to anneal just a bit longer and slower.
For more excellent information visit the website, Warm Glass, and their link here.