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EXHIBITION

  • Johannes Scott

CLAY: bodies

Standardised, ready-made clay bodies do not need additional preparations. The pugmill process of manufacturing had removed air pockets that would otherwise be necessary to knead from the clay.

Storing bagged clay for an extended period would mature it and increase its plasticity for easier throwing on the wheel. Often, clay arrives from the factory too wet for immediate usage. The clay can be de-bagged and put out overnight to reach a desired state of firmness. ‘Weathered’ clay would need to be wedged by hand before usage in order to disperse the lumpy outer layer that formed overnight.

Different clay bodies contract at different rates, both during drying and firing. From wet to fired, respective clays shrink between 5 – 15%. Once a new clay is introduced to the studio, it is good practice to make a sample to record its shrinkage percentage. Roll a coil and cut to 10cm; dry, and fire. Measure again after exit from the kiln and record the shrinkage percentage – the rate will always be consistent. A similar clay sample can be made for testing glaze application on different clay bodies.

Clay bodies can be categorised by the four principle wares: Raku; Earthenware; Stoneware; Porcelain. The four principle wares are distinguished from one another by the temperature at which the desired degree of vitrification occurs. Vitrification is the melting or fusion of the heterogenous particles of which the clay is composed into one homogenous, solid mass. Any clay overfired – beyond vitrification – would turn to slag, and eventually liquify.

RAKU

Raku fires at red heat temperature of a bonfire – about 750 degrees Celsius and originated in Persia and Egypt.

Raku requires the clay item being places inside a dull red, heated furnace, preferably a kiln with a spy hole. After about an hour when a brilliant red heat temperature is reached and the glaze becomes visibly molten, the pottery item is removed with long-handled tongs. From here, according to some traditions, it may be placed inside sawdust for further reduction or in water to stimulate the glaze to graze.

The raku firing process requires a clay that can withstand sudden and violent changes in temperature. The Japanese tradition employs a plastic white clay body to which grog is added, one-third of mass by bulk. Contraction is lessened and the grog opens pores to allow water chemically trapped in the clay to escape fast and not burst its imprisoning walls in the sudden heat rise.

In addition, at the low temperature at which raku fires, it never reaches vitrification. While the thick, porous clay vessel is a bad conductor of heat, it is also this fragility that is the principle characteristic of the raku tradition.

EARTHENWARE

Also known as terra cotta, earthenware fires at orange heat, an intermediate temperature of approximately 1000 degrees Celsius. On tapping, the ring is always dull, but not as dull as with raku.

Pit fired building bricks date back to 27 000BC – for millennia, earthenware was the only pottery made.

This type of non-vitreous pottery originated in the Near East and Asia, from where it migrated to ancient Greek and Roman Europe, and in the late Middle Ages to Spain as Hispano-Moresque ware. Most recognised is the earthenware tin-glazes maiolica pottery of the Italian Renaissance and Dutch Delftware.

Characteristically permeable, water absorption is 5 – 8% and therefor glazing is necessary for it to be watertight.

The average clay body formulation is 25% kaolin, 25% ball clay, 35% quartz, 15% feldspar.

STONEWARE

The name signifies the quality of melted stone. It fires at white heat – a temperature range between 1200 and 1400 degrees Celsius. On tapping, it has a clear ringing tone.

Stoneware was developed 5000 years ago, after which the technology disappeared for a few millennia, only to reappear in China between the 2nd and 6th centuries.

Total vitrification distinguishes fired stoneware; the clay body has no porosity, even unglazed. Its strength is superior to earthenware and close to Chinese porcelain. Early Chinese pottery is often referred to as proto-porcelain and porcelaneous stoneware

PORCELAIN

Invented in the 7th or 8th century during the Sung dynasty in China, porcelain fires at white heat, a temperature of about 1400 degrees Celsius.

The distinguishing hallmark of vitrified porcelain is that its white clay body turns translucent. Also referred to as hard-paste porcelain, the clay body has no porosity.

Soft-paste porcelain, later developed in Europe, is an imitation. Fired at lower temperature, it lacks the same strength, but its white translucent clay body has an attractive, glassy feel.

Medici porcelain was the first to successfully produce soft-paste porcelain. It was a non-commercial venture; its experimental manufactory was established in 1575 at the Casino of San Marco in Florence. Production was limited to a few hundred pieces and served as diplomatic gifts. The underside emblem carried the arms of Philip II of Spain.

A commercial venture followed with Chantilly soft-paste porcelain. The manufactory was established at Chantilly in Oise, France and operated between 1730 and 1800. Workers modelled delicate rococo teapots, moulded table and tea ware, small vases, figures of Orientals, and imitations of Japanese porcelain in Kakiemon designs.

In 1710, hard-paste, or true porcelain was developed for the first time in Europe, at Meissen, Germany.

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