jiggering
A potter throwing a plate on a wheel shapes one face with the spinning mould and the other with a steady hand. Jiggering is that idea turned into a repeatable machine so a factory can make thousands of identical plates. A lump of plastic clay body is slapped onto a rotating profiled plaster mould; a shaped tool is then lowered against the spinning clay, and between the mould below and the tool above the clay is squeezed and sheared to its final thickness and profile. One face is formed by the mould, the other by the tool.
The names split by shape. Jiggering proper makes flatware — plates, saucers, platters — where the mould forms the upper (eating) face and the descending tool shapes the underside and foot. Jolleying is the same operation for hollowware like cups and bowls, where the clay is pressed into (or over) a mould and the tool works the inside. In both, the plaster mould does double duty: it forms one surface and, being porous, it wicks a little water from the clay so the piece firms up and releases cleanly. Modern lines replace the fixed profile tool with a heated spinning roller (roller-head or roller forming), which irons the clay more smoothly and faster.
Jiggering is the backbone of mass-produced ceramic tableware precisely because it is fast, automatic, and gives consistent parts from a plastic body. Its limits are the limits of the geometry: it makes bodies of revolution — round, axially symmetric plates and cups — and not much else, and like all plastic forming it works a wet body that must then be dried and fired with the usual shrinkage and warping risks. A clay that is off in water content or plasticity shows up immediately as tearing, an uneven foot, or an S-crack in the plate base.
A dinner plate is jiggered by throwing a flat clay bat onto a spinning plaster mould that forms the top face, then lowering a profile tool (or heated roller) to shape the underside and foot ring to exact thickness. The plate firms as the plaster wicks water, releases from the mould, and moves on to drying — thousands per hour, all alike.
A rotating plaster mould forms one face and a profiled tool the other, mass-producing round tableware from a plastic clay body.
Jiggering only makes axially symmetric shapes — round plates and cups — because the part is formed against a spinning mould. Wrong water content or plasticity in the clay shows up at once as tearing or an S-crack in the base.