3.2 Ceramics Processes

Ceramics, the art of making objects from earthy materials heated to high temperatures, bridges the gap between functional craft and fine art sculpture. Understanding ceramics requires a firm grasp of material science, thermodynamics, and physical technique. The process is entirely dependent on the transformation of clay—a hydrated silicate of alumina—into a permanent, stone-like material through the application of intense heat.

Types of Clay Bodies

Clays are formulated into specific "bodies" designed for different firing temperatures and structural properties.

• Earthenware: Fired at relatively low temperatures (Cone 06 to 04, roughly 1830°F - 1940°F). It remains porous after firing and is not waterproof unless glazed. Terra cotta is a common reddish earthenware heavily used in historical pottery and sculpture.
• Stoneware: Fired at high temperatures (Cone 5 to 10, roughly 2160°F - 2350°F). The heat causes the silica in the clay to vitrify (turn to glass), making the resulting ceramic dense, durable, and non-porous even without glaze. It is the standard for functional tableware.
• Porcelain: Fired at the highest temperatures (Cone 10+, above 2350°F). Made from highly refined kaolin clay, porcelain is characterized by its stark white color, fine texture, and translucency when thin. It is notoriously difficult to work with due to its lack of plasticity.

The Stages of Clay

Clay undergoes radical physical changes as it loses water. An artist must time their techniques to correspond with these specific stages:

1. Slip: Clay suspended in water to form a liquid slurry. It is used as a "glue" to join pieces of clay together and in slip-casting (pouring into plaster molds).
2. Plastic: The ideal working stage. The clay is pliable, easily molded, and holds its shape without cracking or sagging.
3. Leather-Hard: The clay has lost most of its moisture but is not entirely dry. It is stiff, cold to the touch, and can no longer be molded. This is the perfect stage for carving (sgraffito), incising sharp lines, trimming wheel-thrown pots, and joining structural slabs.
4. Bone Dry (Greenware): All physical moisture has evaporated. The clay is chalky, room temperature, and incredibly fragile. It is at its most vulnerable state and is ready for the first firing.
5. Bisqueware: Clay that has been fired once to a low temperature. It is now permanently hard and cannot be reclaimed (turned back into slip). It is porous, making it highly absorbent and ready to accept liquid glaze.
6. Glazeware: The final stage, where bisqueware has been coated in glaze and fired a second time to a high temperature, melting the glaze into a glassy surface coating.

Forming Techniques

Ceramic objects can be formed through handbuilding or mechanical means.

• Pinching: The most basic method, shaping the clay by compressing it between the thumb and fingers.
• Coiling: Building up walls using long, rope-like rolls of clay. Coils must be meticulously scored and slipped to ensure structural integrity.
• Slab Building: Rolling clay into flat sheets of uniform thickness. Slabs are typically allowed to reach the leather-hard stage before being joined to create hard-edged geometric forms or cylinders.
• Wheel Throwing: Using a mechanized potter's wheel to create symmetrical, cylindrical forms. The process involves centering the spinning clay, opening the center, and pulling up the walls.

Scoring and Slipping

Whenever joining two pieces of clay (unless they are extremely soft and plastic), the artist must score (scratch the surfaces to create texture) and apply slip (liquid clay). This acts as a velcro-and-glue mechanism, ensuring the pieces fuse together rather than popping apart as they shrink during drying.

Glaze Chemistry

A ceramic glaze is essentially a custom-formulated glass coating designed to melt at a specific temperature and adhere to the clay body without running off. All glazes consist of three core components:

1. Silica: The glass former (usually flint or quartz). Silica alone melts at an impractically high temperature.
2. Flux: The melting agent (like feldspar, whiting, or frit). Fluxes lower the melting point of the silica so it can turn to glass within the limits of a ceramic kiln.
3. Alumina: The stabilizer or refractory (usually clay). Alumina stiffens the molten glass, preventing it from sliding completely off the pot during firing.

Color in glazes is achieved by adding heavy metal oxides, such as cobalt (blue), copper (green or red), iron (brown or yellow), and chrome (green).

Kilns and Firing Atmospheres

The firing process is not merely about reaching a temperature; the atmospheric chemistry inside the kiln profoundly affects the outcome.

• Oxidation Firing: Typically achieved in electric kilns. There is an abundant supply of oxygen throughout the firing. Glazes develop bright, clean, predictable colors (e.g., copper oxide turns a brilliant green).
• Reduction Firing: Typically achieved in gas or wood kilns. The artist restricts the oxygen supply by dampening the flue. Starved for oxygen, the fire seeks it out by stripping oxygen molecules from the clay and the metallic oxides in the glaze. This radical chemical change alters colors dramatically (e.g., copper oxide in a reduction atmosphere turns a deep, oxblood red).
• Raku: A specialized, rapid firing technique where porous pieces are removed from the kiln while glowing red hot and plunged into combustible materials (like sawdust). The resulting localized reduction creates metallic lusters and heavy crackle patterns.

Safety in Ceramics

Dry clay and glaze ingredients contain high amounts of free silica, which causes silicosis. Floors should never be swept dry; they must be mopped to prevent kicking up dust. Many glaze colorants (barium, lead, manganese) are highly toxic and must be handled with appropriate respiratory protection and avoided entirely for functional ware intended for food use.