Glass COE Compatibility Calculator
Check coefficient of expansion (COE) compatibility between glasses for fusing and blowing.
Avoid cracking from mismatched COE in soft and borosilicate glass.
Coefficient of Expansion (COE) Compatibility
COE measures how much a glass expands per degree of temperature increase. When two glasses with different COE values are combined and cooled, they shrink at different rates — creating internal stress that causes cracking, hours to weeks after the piece is finished.
The compatibility window: Acceptable: COE difference within 5 units (and ideally within 2)
Common COE families (×10⁻⁷ /°C):
| Glass System | COE Range |
|---|---|
| Soft glass (Effetre, Moretti) | 104 |
| Bullseye / System 96 | 96 |
| Spectrum 96 (now Oceanside 96) | 96 |
| System 96 fusing | 96 |
| Uroboros 96 | 96 |
| Bullseye 90 (older) | 90 |
| Float glass (window) | 86-92 |
| Pyrex / Borosilicate | 33 |
| Quartz | 5-7 |
Why COE matters in practice:
- Two glasses at 70°F (21°C): no stress visible
- Heated to 1100°F (593°C): both expand, but at different rates
- Cooled back to 70°F: each contracts at its own rate
- The mismatch is “frozen” into the joint as permanent stress
- Result: crack within hours, days, or weeks (sometimes years)
Industry rules:
- Within 1 COE unit: safe for any application — full fuse, mix freely
- Within 2 COE units: safe for tack fuse, decorative drape, slumping
- Within 5 COE units: marginal — small accents only, expect some failures
- Over 5 units: WILL crack eventually — never combine
Common mistakes:
- Mixing 96 and 90 COE without testing — 6 units apart, will crack
- Using float glass with 96 frit — 6-10 units apart, will crack
- Using “compatible” labeled glass without verifying — labels can be wrong
- Borosilicate (33 COE) with anything else — never compatible with any non-boro
The polariscope test: The only definitive way to verify COE compatibility is the polariscope test:
- Make a small test bar combining the two glasses
- Fire normally and anneal
- View through cross-polarized filters
- Visible iridescence at the joint = COE mismatch
- Clear/black at the joint = COE match
Stress visible in the field:
- Hairline cracks at color boundaries
- Spontaneous breaking of glass days later
- Sound of “ticking” from a cooled piece (stress relieving slowly)
Safe glass families:
| You’re Working In | Use ONLY These |
|---|---|
| Bullseye 90 | Bullseye 90 series |
| System 96 / Bullseye 96 | Any 96 COE family glass |
| Effetre 104 (soft glass beads) | All Effetre, Moretti, Vetrofond 104 |
| Borosilicate (boro) | Northstar, Glass Alchemy, Trautman — all 33 COE |
Cross-COE projects to AVOID:
- Float glass + Bullseye/System 96 (commonly attempted, fails)
- Pyrex + soft glass (massive 70+ unit difference)
- Lead crystal + any other family (lead’s COE varies)
- “Mystery glass” from estate sale + your studio glass
For lampworkers (boro vs soft):
- Boro 33 and soft 104: 71 units apart — IMPOSSIBLE to combine
- This is why lampworking studios are typically dedicated to one OR the other
- Switching tools and torches between systems requires careful contamination cleanup