Any time multiple pieces of glass are fused or melted together it is critical that the glass is compatible. The simplest explanation of compatibility in glass is that when the glass has cooled (annealed) properly there is little or no stress between the different glasses.
Why would there be stress? Different glass expands and contracts at different rates. Consider two fused pieces of glass that are cooling where one contracts more than the other. The greater contraction will cause that piece to pull at the piece to which it is fused - creating stress.
Many artists use "COE" or "coefficient of expansion" to identify compatible glass. Although COE has become a convenient shorthand, it is important to understand that COE only tells part of the story. COE, for example, is usually measured at temperatures well below fusing or annealing temperatures. Just because the COE of two pieces of glass is equal at 300F does not mean the COE is equal at 800F. Other qualities of glass, for example the viscosity (resistance to flow), also come into play.
Spectrum Glass refers to their fusing glass as System 96 because of the 96 COE. Many people refer to Bullseye's fusible glass as 90 COE. Bullseye Glass resists the COE label and prefers "Tested Compatible" -- asserting (correctly) that COE is not a guaranteed inidicator of compatibility. Uroboros makes fusible glasses that are either System 96 or "90 COE". Moretti glass, a favorite among lampworkers, is typically labeled 104 COE. Borosilicate has a COE of 33.
It is unlikely that the glass art community is going to abandon COE as a label for compatible glasses. And it is probably okay to use COE as shorthand -- so long as artists understand that just because two glasses have the same COE label there is no guarantee they are compatible. When in doubt, read the manufacturers definition of compatible. And test, test, test.
