These two cone 6 glazes are being compared in a melt-flow tester. Their calculated oxide chemistries are essentially the same, but G2934Y4 sources KNaO from Ferro Frit 3110 rather than feldspar, and much of its MgO from Ferro Frit 3249 rather than the talc used in G2934 (talc is so refractory that it is a key ingredient in making kiln shelves!). The Y4 recipe even sources Al2O3 from calcined alumina (another kiln shelf material!). Yet the fritted glaze flows much farther.
How can the frit make such a difference in the melt, despite the burden of the Al2O3? The feldspar and talc require decomposition/reaction before the oxides become available to the melt, whereas the fritting process has already combined the oxides into a glassy, highly reactive homogeneous form.

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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 in glazes. When oxides are then supplied by a different material, a new "system" 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, as a whole, 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. |
| 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 |
Melt Fluidity
Ceramic glazes melt and flow according to their chemistry, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them. |
| 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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