What unity formula factors indicate that a glaze may be clear when fired?

Unit 7 Topic 9
Using Unity Formula

All oxide values in the unity formula should be within the parameters shown in the Limit Formula tables for the maturing temperature of the glaze.
The alumina : silica ratio should be in the vicinity of 1 : 9 for stoneware glazes.

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Ideally for a clear glaze all oxides need to be dissolved and remain in the glass matrix on cooling.

Glazes that are not clear either

  • contain oxides that were not dissolved or
  • have oxides which precipitated out of the molten glass during the cooling process or
  • have a fine pattern of gas bubbles trapped in the glass.

A clear glaze balance will vary depending on the mix and level of individual flux oxides in the glaze for the chosen firing temperature.

For each flux group pattern and firing temperature there will be a certain level of alumina and silica which will create clear glass providing the flux oxide content is not excessive.
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