COE 90 and COE 96 are the two dominant glass systems in fusing, and the number one rule is simple: never mix them. But beyond compatibility, the two also fire a little differently, and knowing how saves you a lot of trial and error.
COE stands for Coefficient of Expansion, a measure of how much glass expands when heated and contracts when cooled. For a piece to survive the kiln, every component, base sheet, frit, stringers, must share the same COE. Fuse COE 90 to COE 96 and the mismatch in contraction creates stress that cracks the piece, sometimes in the kiln, sometimes weeks later.
| COE 90 | COE 96 | |
|---|---|---|
| Common brand | Bullseye | System 96, Wissmach |
| Maturity | Slightly higher temps | Matures a bit lower / softer |
| Full fuse (typical) | ~1490°F | ~1460°F |
| Anneal point | ~900°F | ~950°F |
Because COE 96 is softer, it generally needs slightly less heat work to reach the same result. That's why the two systems have their own published firing schedules, and you should always start from the one that matches your glass.
A frequent beginner mistake is trying to fuse leftover stained glass. Most stained glass isn't COE-tested and isn't made to survive kiln temperatures. You might get lucky on a tiny piece, but it's a gamble that usually ends in cracks. Stick with glass clearly labeled COE 90 or 96.
Whichever glass system you choose, I can point you to a kiln and controller that handle its schedules with ease. Spencer — (801) 839-5882.
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