Pottery

I

Introduction

Pottery, clay that is chemically altered and permanently hardened by firing in a kiln. The nature and type of pottery, or ceramics (Greek keramos, “potter's clay”), 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.

II

Types, Procedures, and Techniques

Earthenware is porous pottery, usually fired at the lowest kiln temperatures (900°-1200° C/1652°-2192° F). Depending on the clay used, it turns a buff, red, brown, or black color when fired. To be made waterproof, it must be glazed. Nearly all ancient, medieval, Middle Eastern, and European painted ceramics are earthenware, as is a great deal of contemporary household dinnerware. Stoneware—water-resistant and much more durable—is fired at temperatures of 1200°-1280° C (2191°-2336° F). The clay turns white, buff, gray, or red and is glazed for aesthetic reasons. (Pottery fired at about 1200° C/2192° 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 (14th century to 17th century). Porcelain is made from kaolin, a clay formed from decomposed granite. Kaolin is a white primary clay—that is, a clay found in the earth in the place where it was formed and not transported there by rivers; secondary clays, borne by rivers to the site of deposit, contain impurities that give them various colors. Porcelain is fired at 1280°-1400° C (2336°-2552° F); it is white and often translucent. Porcelaneous ware was first made in China, hence its common name china. Chinese porcelain is less vitrified (and therefore softer) than its modern European counterpart, which was developed in Germany in the early 18th century. European imitations of Chinese porcelain are also made; called soft-paste or frit porcelains, they are fired at about 1100° C (about 2012° F). In the mid-18th century, English potters invented bone china, a somewhat harder ware that gained whiteness, translucency, and stability through the inclusion of calcium phosphate in the form of calcined (fired, chemically altered) ox bones.

A

Preparing and Shaping the Clay

The potter can remove some of the coarse foreign matter natural to secondary clays, but coarse matter can also be used in varying quantities to achieve particular effects. A certain amount of coarse grain in the clay helps the vessel retain its shape in firing, and potters using fine-grained clays often “temper” the clay by adding coarser materials such as sand, fine stones, ground shells, or grog (fired and pulverized clay) before kneading the clay into a 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 mold—a stone or basket, or a clay or plaster form. Liquid clay can be poured into plaster molds. 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 superimposing rings. Also, a ball of clay can be pinched into the desired shape. The most sophisticated pottery-making technique is wheel throwing.

The potter's wheel, invented in the 4th millennium bc, is a flat disk 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 upward from a ball of clay that is thrown and centered on 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 better regulated rotating speed.

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B

Drying and Firing

To fire without breaking, the clay must first be air dried. If the clay is thoroughly dry, porous and relatively soft, the 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. Various effects are achieved by oxidizing the flames (giving them adequate ventilation, to produce a great flame) or by reducing the oxygen through partially obstructing the entrance of air into the kiln. For example, a clay high in iron will typically burn red in an oxidizing fire, whereas in a reducing fire it will turn gray or black; chemically, in reduction firing the clay's red iron oxide (FeO₂, or with two molecules, Fe₂O₄) is converted to black iron oxide (Fe ₂O₃) as the pot gives up an atom of oxygen to the oxygen-starved fire.