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  • Johannes Scott

CLAY: introduction

POTTER’S CLAY is used by potters for building and wheel throwing. It is prepared following a formula made up of various minerals. Primary recipes are formulated for respective kiln firing temperatures and minor recipes for clay texture and colour.

The potter’s preparation of clay includes wedging the clay as if it is dough for baking. The wedging removes air pockets that could cause cracks later. Later, with drying and kiln firing, the body would shrink, and trapped air would expand. This tension in air pressure causes clay to crack, or later during firing, to explode.

The longer clay rests, the more its plasticity matures. This is a molecular development in which water and clay particles come to rest in a cohesive, geometrical direction. The clay molecules orientate themselves offering least resistance for longevity – hereafter, in nature, clay transform, for example, into sandstone and other forms of permanent solidity such as geological reference to natural fossils.

The Japanese ceramic tradition of maturing clay relied on cooperation between ancestral generations. The grandson would unearth the clay immured by his grandfather (the tradition was gender orientated), in turn, he would later in his life prepare and bury clay for his grandchild to be unearthed decades later.

Matured clay pockets can be recognised by a layer of mould from surplus organic matter growing on the surface.

GREEN WARE is drying clay that had lost plasticity and is in the utility process of making or constructing a ceramic product. It can be described as cheese-like or leather hard because it is ideal for carving and other decorating techniques. It is the ideal stage of drying for joining separate firm sections of clay together, such as with slab work and adding pulled handles on to a wheel-thrown teacup.

Green ware goes through stages of drying. It changes from cold to room temperature. This temperature difference, by tradition, registers well against the potter’s cheek – it registers the dryness of the item for biscuit firing.

DRY GREENWARE is brittle and has changed colour – it can be crushed for making slip with vinegar, or for developing new clay formulas. Clay recipes often consists of specific clay types that had been dried and crushed before mixing a new formula. For example, china clay, ball clay, kaolin and talc. Silica, on the other hand, is like crushed powdered glass and, milled with clay, would increase the refractory level in order to formulate a clay body that would vitrify at a higher firing kiln temperature. This combination was a crucial recipe ingredient in the Chinese invention of porcelain. It took the West centuries to capture this secretive formula.

BISCUIT WARE is modelled clay that had been fired in the kiln at a temperature of about 1000 degrees Celsius. Biscuit ware cannot be crushed to remake clay because its molecular structure had been altered permanently and therefore it can no longer chemically bond with water molecules. Biscuit ware is recognised by its porous quality. This absorbent stage of the ceramic object is best for the application of glaze. Glaze can be brushed onto the object by layering or the item can be dipped into glaze. The longer the submersion into the liquid glaze, the thicker the glaze application – a too long submersion and the fired item loses its permeability, decreasing the proper layering of glaze.

GLAZE WARE, or stoneware is the additional, higher firing of glazed biscuit ware in a kiln reaching a temperature that would again alter the ceramic molecular structure, reaching vitrification. Glazed ware, by definition, is no longer permeable and can therefore be hygienically utilised as DINNER WARE. Earthenware, or glaze ware that had not been vitrified is often employed as ornamental utility in the creative design industry.


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