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Medium Temperature 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 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.

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

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.

A cone 6 wood ash glazed planter

A cone 6 wood ash glazed planter

A highly variegated cone 6 cobalt rutile ash glaze

A highly variegated cone 6 cobalt rutile ash glaze

Example of a variegated wood ash glaze at cone 6 oxidation. It contains a small amount of cobalt as well as some rutile.

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

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).

Out Bound Links

  • (Glossary) Glaze Chemistry

    Glaze chemistry is learning what each oxide does in a fired glaze and the relative advantages and disadvantages of each material supplying it. The chemistry of a glaze is expressed in a manner similar to its recipe, except that the items are oxides and the amounts can be by weight (an analysis) or n...

  • (Articles) G1214M Cone 5-7 20x5 Glossy Base Glaze

    This is a base transparent glaze recipe developed for cone 6. It is known as the 20x5 or 20 by 5 recipe. It is a simple 5 material at 20% each mix and it makes a good home base from which to rationalize adjustments.

  • (Oxides) B2O3 - Boric Oxide
  • (Articles) Reducing the Firing Temperature of a Glaze From Cone 10 to 6

    Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.

  • (Glossary) Boron Frit

    This term is very generic, referring of course to frits that contain boron. Unfortunately that is 80-90% of available frits! Boron frits may have 1% boron or 50% boron. Even though the boron in the frit is no longer in the borax form it is still customary to refer to such as "borax frits". Since man...

In Bound Links

  • (Glossary) Borosilicate

    A silicate is an SiO2-centric solid (crystalline or glass). A borosilicate simply is a silicate with boron. The term 'borosilicate' is synonymous with medium and low fire glazes because boron is not employed at high temperatures (CaO, Na2O, MgO, etc flux silica and bond with it to form crystalline o...

  • (Typecodes) 3: MTR - Medium Temperature Glaze Recipes
  • (Videos) Reducing the Firing Temperature of a Glaze From Cone 10 to 6

    A key lesson because it explains the difference between cone 10 and 6 glazes, demonstrates how to evaluate frits to choose the best one to source boron to a glaze, how to determine how much frit to ad...


By Tony Hansen




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