Training Material

Physical stages of clay

Clay ware takes on varying physical characteristics during the making of pottery.

Pugged clay: refers to clay that has been processed through a pug mill or obtained from a supplier in this state. It is in its plastic form and ready to be used on the wheel or for other projects such as hand building.

Leather-hard: refers to a clay body that has been partially dried. It has the consistency of cheddar cheese and is still pliable and can be pierced with your finger nail. You can trim a foot into your pot at this stage and also attach a handle.

Greenware:  refers to unfired objects. Finished work at this stage is in the process of drying and at its most fragile state when totally dry ready for firing.
Biscuit/bisque: refers to the first firing of pottery where clay is fired to a certain temperature high enough to change the body of the clay structurally but still leaving the fired piece porous.

Glaze firing: is the final stage of some pottery making. A glaze may be applied to the biscuit form and the work can be decorated in several ways. After this the work is glazed fired, usually to a higher temperature than the bisque firing. Stoneware and porcelain clays can be fired to vitrification which produces dense durable work.
Pottery shrinks approximately 10-15% during the process from drying to the final firing.
Pottery can be shaped by a range of methods that include:

Handbuilding: This is the earliest forming method. Wares can be constructed by hand from coils of clay, combining flat slabs of clay, or pinching solid balls of clay or some combination of these. Parts of hand-built vessels are often joined together with the aid of slip, an aqueous suspension of clay body and water. Hand-building is slower than wheel-throwing, but it offers the potter a high degree of control over the size and shape of wares. The speed and repetitiveness of other techniques is more suitable for making precisely matched sets of wares such as tablewares although some studio potters find hand-building more conducive to creating one-of-a-kind works of art.

The potter's wheel: In a process called "throwing" (coming from the Old English word thrawan which means to twist or turn), a ball of clay is placed in the centre of a turntable, called the wheel-head, which the potter rotates with a stick, with foot power or with a variable-speed electric motor.

During the process of throwing, the wheel rotates rapidly while the solid ball of soft clay is pressed, squeezed and pulled gently upwards and outwards into a hollow shape. The first step of pressing the rough ball of clay downward and inward into perfect rotational symmetry is called centring the clay--a most important skill to master before the next steps: opening (making a centred hollow into the solid ball of clay), flooring (making the flat or rounded bottom inside the pot), throwing or pulling (drawing up and shaping the walls to an even thickness), and trimming or turning (removing excess clay to refine the shape or to create a foot).

Considerable skill and experience are required to throw pots of an acceptable standard and, while the ware may have high artistic merit, the reproducibility of the method is poor. Because of its inherent limitations, throwing can only be used to create wares with radial symmetry on a vertical axis. These can then be altered by impressing, bulging, carving, fluting, and incising. In addition to the potter's hands these techniques can use tools, including paddles, anvils and ribs, and those specifically for cutting or piercing such as knives, fluting tools and wires. Thrown pieces can be further modified by the attachment of handles, lids, feet and spouts.