The blended expansion is the weighted average of the two values. Compatibility is judged by the difference between the blended value and each component: a gap of one point or less is generally safe, up to about two points is risky and needs testing, and beyond that the pair should not be combined.
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Enter two COE values, then calculate.
About this calculator
The coefficient of expansion, or COE, describes how much a glass expands per degree of temperature change. Two glasses with different COE values shrink at different rates as they cool, and if they are fused together one of them will crack. This is the single most common cause of a piece that fails after it looked finished.
Enter the COE of glass A, the COE of glass B, and the proportion in which they are mixed. The calculator returns the blended COE and a compatibility verdict.
Things to keep in mind
- Common reference values: soda-lime around 90, borosilicate around 33, lead crystal around 92. Values vary by manufacturer.
- Always fuse test pieces before committing a large piece to a new colour combination.
- A difference of two points can survive a small bead and fail a thick sculpture, because stress scales with thickness.
- COE is reported over a specific temperature range. Values from different sources are not always directly comparable.
Reference only. Results are based on published ranges for common glass families and are intended for learning. Always verify against your own materials and equipment, and follow the manufacturer's instructions.