First of all I need to explain some of the terms used in glass fusing such as COE, compatibility, annealing, and devitrification.
Compatible, the glass used in fusing must be compatible or having the same COE. If your glass is not compatible the stress of incompatibility may cause the glass to literally pull itself apart. Different manufacturers have different glass formulas. Just because the glass is made by the same manufacturer does not mean it is compatible. Although if you fuse glass together from the same piece or sheet it should be compatible with itself. Compatible glass is made specifically for fusing and is tested and marked as such.
COE, co-eficency of expansion is a fancy way of saying what rate the glass expands and contracts at as it is heated and cooled. In glass fusing there are two main COE's Bullseye's 90 and Spectrum96. Personally I use mostly Spectrum96. Bullseye and Spectrum are both glass manufacturers that are located here in the Pacific Northwest.
Glass that is fused must be annealed. Annealing is the process of letting the glass "soak" at the proper temperature for a while. This allows all the
molecules to settle down into the correct alignment. Then the glass is slowly cooled, keeping the molecules in alignment until it has cooled to the safe point. The times and temperatures for annealing may vary depending on the glass used and the size of the project.
Devitrification simply means that the glass has "burned" in the kiln ( this is not a technical explanation). Devit leaves a matte to cloudy surface on the top of the glass and can be impossible to correct.