Terry Wright

Glaze development using triaxial blends

As part of her diploma studies, Hornsby TAFE student Anna Choi was interested in exploring bronze-like glaze qualities and set about testing recipes containing large quantities of manganese dioxide, several of which were recipes from well known British ceramist Colin Pearson. Some of the most interesting results came from a simple triaxial test with potash feldspar, manganese dioxide and powdered red clay at the three respective corners. Several of the glazes from this triaxial had collections of large interestingly shaped chocolate brown matt crystals dispersed though a deep rich metallic brown glaze.

The crystals that develop in this glaze are not round and are very irregular and the surface has an appearance not unlike a dark handmade paper with confetti-like bits in it. The glaze produces these qualities only in reduction - fired to cone 10. It appears to be quite temperature sensitive and sometimes runs. Thinning out the glaze towards the exterior base and putting work on an appropriate setter will help to minimise this problem. This glaze has no clay in it so the addition of 4% bentonite and a drop of calcium chloride will help to keep ist suspended. The glaze has been tested over a range of clay bodies and it seems to make no difference whether the clay body is dark or light but some porcelains seem to increase its fluxing properties.

Occupational, Health and Safety considerations

Manganese dioxide has been classified as hazardous to health and potash feldspar contains silica. Ensure that you have read the relevant material safety data sheets before using these raw materials and adopt appropriate safety precautions. Complex reactions which occur during glaze firing make it difficult to predict what fumes may be produced. Ensure that there is appropriate ventilation during the firing.

The recipe that worked best of all was very simple:

Potash feldspar 80
Manganese dioxide 20

Terry Wright is Head of Ceramics Hornsby TAFE, NSW

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