All Glossary

Medium Temperature Glaze

These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.

Details

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 for potters, whereas industry uses zinc for fast firing). Typically frits are employed to supply the B₂O₃ or ZnO. Historically Gerstley Borate and Colemanite were very common sources of B₂O₃. 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, glaze chemistry is normally needed to determine how to best incorporate boron-sourcing materials.

A cone 6 fluid iron glaze has a completely different surface when cooled slowly

The mug on the left has been cooled slowly (and crystallizes). On the right it was cooled quickly (and is glossy).

Transform the yellow-white of cone 6 to blue-white of cone 10R

Adding a little blue stain to a medium temperature transparent glaze can give it a more pleasant tone. Some iron is present in all stoneware bodies (and even porcelains), so transparent glazes never fire pure white. At cone 10 reduction they generally exhibit a bluish color (left), whereas at cone 6 they tend toward straw yellow (right). Notice the glaze on the inside of the center mug, it has a 0.1% Mason 6336 blue stain addition; this transforms the appearance to look like a cone 10 glaze (actually, you might consider using a little less, perhaps 0.05%). Blue stain is a better choice than cobalt oxide, the latter will produce fired speckle.

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By Tony Hansen