Glass Compatibility Conditions

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“Glass Compatibility Conditions” refer to the physical conditions required when combining (fusing or layering) glass from different manufacturers or with different compositions within a single piece (treatise §14.4). While Chikaoka (2021, p. 142) explicitly states that “the rule of fusing is to select glass from the same manufacturer,” the Q&A section (ibid., p. 170) conditionally accepts the practice, noting that “theoretically, glass from different manufacturers can be fused together as long as their coefficients of expansion match.” The position taken in this paper is to propose “matching COE and viscosity-temperature curves” as practical conditions for mixing different types of glass.

Simply having the same COE does not guarantee safe mixing. This is because different compositions result in different viscosity-temperature curves; even at the same firing temperature, this can lead to inconsistencies—such as one material over-melting while the other remains under-melted—which in turn generates internal stress. For example, Bullseye COE90 and Oceanside System96 (which actually has a COE of 96) cannot be mixed, but even if both are labeled as COE90, there is a risk if they are from different manufacturers. The only reliable method to verify compatibility is to measure residual stress after firing using a polariscope test, confirming through actual measurement that the retardation value is within the acceptable range (≤50 nm/cm).

Related Concepts

COE（coe）・粘度-温度曲線（viscosity-temperature-curve）・偏光板テスト（polariscope-test）・残留応力（retardation）・Bullseye COE90（bullseye-coe90）・ガラス転移（glass-transition）