Medium Temperature or Mid-Fire Glaze
In functional ceramics this term generally refers to glazes that mature from cone 4 to 7. At these temperatures it is difficult to compound glazes that will melt well without the need for powerful melters like zinc and boron. Thus a medium temperature glaze contains mostly the same kinds of ingredients as a high temperature one, but additionally it needs a source of zinc or boron (boron is by far more popular and less troublesome). Typically frits are employed to supply the B2O3. Historically Gerstley Borate and Colemanite were very common sources also. Boron has a low thermal expansion and thus is an ideal additive since it reduces the tendency of glazes to craze. Since there are no practical insoluble sources of pure boron, ceramic chemistry is normally needed to determine how to best incorporate boron-sourcing materials.
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