CLAY notes by P.E. Maddalena

by

Most clays may be vapour glazed with a various degree of success. As a basic principle the more silica in the clay the smoother the glaze will be, but with less silica and more alumina, the more distinctive “orange peel” texture will result.

The colour of the clay after firing is governed by the nature of the firing and by the iron content. From pale gold (1%) to pale tan (1.5%) to medium brown (1.8 -2 %). More than 3% of iron will cause the glaze to be progressively duller.

Not all clays are well fluxed by the soda because many contain insufficient free silica, e.g. china clays, ball clays and stoneware clays with alumina/silica molecular ratios between 1 to 2 and 1 to 4. The best clays for vapour glazing are those fireclays which are high in silica. These can have alumina/silica ratio of 1 to 5 or higher.

Extra silica is sometimes added to clay as quartz sand but silica in isolation does not make a glaze, some alumina is necessary. Small amounts of calcia and magnesia help to stabilise the glaze.

When the kiln is soaking at high temperatures, various forms of quartz develop in the body of the pots: some of this quartz is very unstable, the most unstable form being cristobalite.
As already mentioned, the more silica in the body, the better it will salt. However, the more silica in the body the higher the danger of cristobalite to develop in quantity and pots to shatter unless due care is taken.

The most stable of quartz forms is mullite which forms if the kiln is cooled very rapidly. Therefore fast cooling down to around 1000 °C has developed thus stabilising the pernicious effects of excessive silica.

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