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Eutectic

In ceramics, this term refers to certain chemistries that melt at much-lower-than-expected temperatures.

Details

The lowest temperature at which a simple mix of oxides will melt and react to form a transparent glass. Mixtures of SiO2 and the alkalis and alkaline earth oxides are well studied. The temperature at which melting occurs is often an anomaly, that is, it is lower than the melting temperatures of similar mixtures. Lead-tin solder is an example. Lead melts at 327C, tin at 231C. The lowest melting combination is 67 lead, 33 tin (180C).

Generally, eutectics are a matter of academic study, reliance on them is not something designed into ceramic glazes. In fact, the opposite, non-eutectic mixtures, are the rule. All ceramic frits we know of behave in a non-eutectic way, most having a wide melting (or softening) range. This softening phenomenon is beneficial, it is what makes melting glazes hang onto the ware over a wide range of temperature (many must be heated well past the softening range to get mobile melts). Thus, being armed with a carefully calculated recipe to produce a eutectic at a specific temperature does not likely equip one to produce a better transparent base glaze than just using frits. This is because frits have a fundamental advantage: They have been premelted. That fact alone would likely trump the eutectic mix-of-materials (especially if the powder mix is not ball milled well to maximize particle surface area). The frit approach also has the advantage that gases of decomposition have already been expelled, creating a better potential for a transparent glass.

An novel method of glaze chemistry is promoted by Bob Magnuson (see link below). He explains how, in eutectic mixtures, all species solidify simultaneously to a clear glass, whereas in non-eutectic ones, excess or uninvolved oxides precipitate and solidify out-of-step. He demonstrates how knowing about eutectics makes it possible to formulate more transparent glazes and addresses the concepts of "combining eutectics" to build better transparent glazes. But the most exciting concept is mathematically subtracting eutectics from the unity molecular formula of stoneware glazes to isolate their mechanisms, or unique characteristics.

We have linked some eutectic mixtures, however these are not verified by any testing we have done. If this area is of interest to you it might be better to seek other sources of information (e.g. phase diagrams).

Related Information

Links

URLs http://en.wikipedia.org/wiki/Eutectic
Eutectic at Wikipedia
URLs https://ceramicartsnetwork.org/ceramics-monthly/ceramic-glaze-recipes/glaze-chemistry/techno-file-using-eutectics/#
Using Eutectics article and calculation workship by Bob Magnuson
URLs https://www.youtube.com/watch?v=FVUuhjnjAoA
German potter Cornelius Breymann investigates limit formulas, eutectics
On this Youtube video Cornelius will take you on a slow and deliberate journey. If you stick with him you will discover how, by industrious blending of feldspar, calcium carbonate and silica we can see what ratios of CaO, SiO2 and Al2O3 (and the materials sourcing them) produce a well-melted high temperature glaze. You will see how the process demonstrates where feldspar comes up short as a glaze by itself and what it needs to be one. And you will see the CaO:SiO2:Al2O3 eutectic demonstrated.
Typecodes Eutectic
These are theoretical mixtures of oxides, not real materials. Use these to learn about low melting chemistries.
Materials Eutectics
Materials Zinc and Cadmium Metasilicate Eutectic
Materials Ca Fluoride & Ca Metasilicate Eutectic
Materials Na & Ca Metasilicate Eutectic
Materials Na & Ba Metasilicate Eutectic
Materials Na & Sr Metasilicate Eutectic
Materials Sodium Metasilicate Eutectic

By Tony Hansen


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