The GBMF test compares Custer Feldspar with 15% Frit 3110 and 15% Ferro Frit 3134 (when this feldspar is fired without additives at cone 10, it barely starts to flow). This tester is fired at cone 6. This shows how much help the feldspar needs to melt at cone 6. And it illustrates how effectively the boron-rich frit promotes flow in this feldspar at cone 6. Frit 3134 supplies much more B2O3 and CaO, and less SiO2, than 3110. The latter supplies more alkalis.

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This melt-flow test compares two feldspars at cone 10, with no additives. Feldspars are among the most common flux sources in glazes, yet by themselves they flow surprisingly little. How is this possible?
Feldspar supplies potassium and sodium (K2O, Na2O), but these come packaged with substantial alumina and silica (Al2O3 and SiO2). The result is a viscous melt: These specimens have softened and become glossy, but barely moved. In a glaze, feldspar interacts with other materials to create a different oxide balance, one that can melt more readily and flow much farther. Its usefulness as a flux source depends on the chemistry of the whole recipe—not simply on how far it runs by itself.
| Materials |
Ferro Frit 3110
High sodium, high thermal expansion low boron frit. A super-feldspar in clay bodies. Melts at very low temperatures. Useful to make crackle glazes. |
| Materials |
Ferro Frit 3134
A frit with 23% B2O3. The most common of frits used in pottery in North America. Around the world, other companies make frits of equivalent chemistry. |
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