Specialist Crafts Which Clay Guide

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Finding the right clay:

Confused about which clay and glaze to use? The information below will help you make the right choice for the type of work you are doing and the finish you want to achieve.

Considerations prior to clay and glaze selection:

- Some clays have very wide firing ranges, enabling them to be used for earthenware (1100∘ C–1150∘C), middle temperature (1180∘C–1220∘C) or stoneware (1220∘C–1300 ∘C).

- The properties of each clay type help determine the outcome of the work. For example, clay which contains a coarse grog has a low shrinkage rate and good warp resistance which is better for slab and coil work. Smooth body clays without grog are easier on the hands and so are better for thrown work.

- The choice of clay colour and its properties will have an effect on the glaze finish. Some clays contain iron, for example, and specks may show through the glaze. This can produce some nice effects and should be considered. Select a glaze with a firing range which exceeds the lower temperature given for the clay. If this is not done, the clay will be underfired and the glaze may crack. See which glaze you need on page 376.

- Use the chart below to help you fire your biscuit and glazed pieces at the right temperature.

- It's more traditional to biscuit fire work prior to glazing, but glazes can be applied directly to greenware. The advantages of biscuit firing work first are: work becomes durable and can be handled more easily; glazes will not introduce moisture back into the clay which may cause thin or unsupported areas to distort; glazes tend to brush on more easily to bisque fired surfaces giving more even colour coverage, and can also be easily wiped off if mistakes are made.

- Clays fired to higher temperatures (stoneware) become less absorbent. This can be an advantage for work which is intended for outdoors. Moisture is less likely to penetrate into a ceramic piece which can cause algae growth and frost blowouts. If clays are fired above their recommended maturing point then bloating is likely and work distorts.