Pottery

I

Introduction

Pottery, vessels and other articles made of clay that is permanently hardened by firing in a kiln. The nature and type of pottery is determined by the composition of the clay and the way it is prepared; the temperature at which it is fired; and the glazes used. The main types of clay fall roughly into the categories of earthenware, stoneware, and porcelain.

II

Types, Procedures, and Techniques

Earthenware is porous pottery, usually fired at the lower kiln temperatures of c. 900°-1150° C (1652°-2102° F). Depending on the clay used, it turns buff, red, brown, or black when fired. To be made waterproof, it must be glazed. Nearly all ancient, medieval, Middle Eastern, and European painted ceramics are earthenware, as is much contemporary household dinnerware. Stoneware, water-resistant and much more durable, is fired at temperatures of 1200°-1300° C (2191°-2372° F). The clay turns white, buff, grey, or red and is glazed for aesthetic reasons. (Pottery fired at about 1150°-1200° C (2102°-2191° F) is sometimes called middle-fire ware; its earthenware or stoneware traits vary from clay to clay.) Stoneware was made by the Chinese in antiquity and became known in northern Europe after the Renaissance. Porcelain, first used by the Chinese, is also high-fired at 1280°-1300°C (2336°-2372° F) and becomes white, vitreous, and translucent.

A

Preparing and Shaping the Clay

The potter can remove some of the coarse foreign matter natural to secondary clays, or it can be used in various quantities for different effects. A certain amount of coarse grain in the clay helps the vessel retain its shape during firing, and potters using fine-grained clays often “temper” the clay by adding coarser materials such as sand or grog (fired and pulverized clay) before kneading the clay into workable condition. The plasticity of clay allows pottery to be shaped in several traditional ways. The clay can be flattened and then shaped by being pressed against the inside or outside of a mould—a stone or basket, or a clay or plaster form. Liquid clay, known as casting slip, can be poured into plaster moulds. A pot can be coil built: clay is rolled between the palms of the hands and extended into long coils, a coil is formed into a ring, and the pot is built up by superimposed rings. Also, a ball of clay can be pinched into a desired shape. The most sophisticated pottery-making technique is wheel throwing.

The potter's wheel, invented in the 4th millennium bc, is a flat disc that revolves horizontally on a pivot. Both hands—one on the inside and the other on the outside of the clay—are free to shape the pot from a ball of clay that is placed at the centre of the rotating wheel head. Some wheels are set in motion by a stick that fits into a notch in the wheel (often activated by an assistant); called a handwheel, this is the classical wheel of Japanese potters. In 16th-century Europe, with the addition of a flywheel separate from the wheel head and mounted in a frame, the potter could control the wheel by kicking the flywheel. A kick bar, or foot treadle, was added in the 19th century. In the 20th century the electric wheel with a variable-speed motor allowed greater and more regulated rotating speed.

B

Drying and Firing

To fire without breaking, the clay must first be air dried. If the clay is thoroughly dry, porous, relatively soft pottery can be baked directly in an open fire, at temperatures of 650°-750° C (1202°-1382° F); primitive pottery is still made in this way. The first kilns were used in the 6th millennium bc. Wood fuels—and, later, coal, gas, and electricity—have always required careful control to produce the desired effect in hardening the clay into earthenware or stoneware. Different effects are achieved by oxidizing the flames (giving them adequate ventilation, producing a great flame) or by reducing the oxygen by partially obstructing the entrance of air into the kiln. For example, a clay high in iron will typically turn red in an oxidizing fire, whereas in a reducing fire it will turn grey or black; in reduction firing the clay's red iron oxide (FeO₂, or with two molecules, Fe₂O₄) is chemically converted to black iron oxide (Fe ₂O₃) as the pot gives up an atom of oxygen to the oxygen-starved fire.