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The difference between sourcing fluxing oxides from frits vs. raw materials is significant to say the least. The oxides MgO and CaO normally come from natural mineral that melt high. But common frits that source them soften low. The chemistry in the two cone 6 glazes (compared on this melt flow tester) are the same. But Y4 sources the KNaO from Ferro Frit 3110 instead of feldspar and alot of the MgO from Ferro Frit 3249 instead of talc. Even though Y4 has 10% calcined alumina it still flows much better! Alumina, as a source of Al2O3, is a super refractory material (compared to kaolin, the normal source), the glaze should flow less - but the frits overcome even that to create this amazing fluidity in a matte glaze. The lesson: All glazes have a chemistry, but that cannot be taken in isolation from the materials that source it. Glaze chemistry is a relative, not absolute science.
Ceramic material powders can be crystalline or amorphous (or a combination) and they can be natural or man-made and they can have very different particle physics. Ceramic materials are classic case studies of the sciences of solid state physics and solid state chemistry. Mineral sources of oxides impose their own melting patterns - when one is substituted for another, to supply an oxide, a different system with its own relative chemistry is entered. An extreme example of this is sourcing Al2O3 to a glaze using calcined alumina instead of kaolin. Although the chemical formula of the glaze may remain unchanged, the fired result would be completely different. This is because very little of the alumina would dissolve into the glaze melt. At the opposite extreme, a different frit could be used to supply a set of oxides (while maintaining the overall chemistry of the glaze) and the fired result would be much more chemically predictable. Why? Because they readily release their oxides to the melt.
Materials |
Ferro Frit 3110
High sodium, high thermal expansion low boron frit. A super-feldspar in clay bodies. Melts a very low temperatures. |
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Materials |
Ferro Frit 3249
A magnesia borosilicate frit. Very low thermal expansion and melting point. Invaluable in pottery to increase the MgO in glazes and thereby prevent crazing. |
Glossary |
Frit
Frits are used in ceramic glazes for a wide range of reasons. They are man-made glass powders of controlled chemistry with many advantages over raw materials. |
Glossary |
Glaze Chemistry
Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc. |
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