All Glossary

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 in the area. The temperature at which melting occurs is often an anomaly, that is, it is much 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. Most frits behave in a non-eutectic way, having a 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).

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.

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By Tony Hansen

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