Bullseye Glass COE Coefficient of Expansion Kiln Academy kiln-education Tips

COE Does Not Equal Compatibility!

The compatibility of glass is always going to be a hot topic as we see many questions asking... Is Bullseye glass COE 90? Or, can I fuse two bottles together? Unfortunately, the answer to this question is not simple.

The word compatibility refers to the properties of glass that allow it to be heat-bonded with another glass such as in a kiln. For many years, it was believed compatibility was solely determined by the properties of expansion in different glasses. Due to this, an overemphasis has been placed on the numerical value of the COE (coefficient of expansion) of glasses.

The intent of most glass fusers who ask this question is to find out if glass from one company is compatible with glass made by another manufacturer. Regrettably, matching COE's does not ensure compatibility between glasses. No single piece of information will tell you if they are as there are many factors besides COE that determine compatibility. The only true way to confirm compatibility is to to test them.
If you are looking for more detail, Bulleye's Technote 3: Compatibility of Glasses, is a great resource and explains this complex and important matter in greater depth.

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