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.
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.
C.         Decoration

A pot can be decorated before or after firing. When the clay is partially dry and somewhat stiffened (“leather hard”), bits of clay can be pressed into the pot; the body of the vessel can be incised, stamped, or pressed with lines and other patterns; or clay can be cut out and the body pierced. The vessel walls can be smoothed by burnishing, or polishing, so that rough particles are driven inward and the clay particles are aligned in such a way that the vessel surface is shiny and smooth. (Some clays can be polished after firing.) Slip (liquefied clay strained of coarse particles) may be used: The bone-dry (completely dry) or partially dry pot can be dipped into slip of creamy consistency (to which color is sometimes added); or the slip can be brushed on or trailed on with a spouted can or a syringe. Designs can be drawn with a pointed tool that scratches through the slip to reveal the body, a technique known as sgraffito.
D.         Glazes

Historically, unglazed pottery has always been more common than glazed pottery. Glaze is a form of glass, consisting basically of glass-forming minerals (silica or boron) combined with stiffeners (such as clay and fluxes) and melting agents (such as lead or soda). In raw form, glaze can be applied either to the unfired pot or after an initial unglazed, or biscuit, firing. The pot is then glaze fired; the glaze ingredients must melt and become glasslike at a temperature that is compatible to that required for the clay. Many kinds of glazes are used. Some heighten the color of the body; others mask it. Alkaline glazes, popular in the Middle East, are shiny and frequently transparent. These glazes are composed mostly of silica (such as sand) and a form of soda (such as nitre). Lead glazes are transparent, with traditional types made of sand fused with sulfide or oxide of lead. These glazes were used on earthenware by Roman, Chinese, and medieval European potters and are still used on European earthenware. Tin glazes, opaque and white, were introduced by medieval Islamic potters and were used for Spanish lusterware, Italian majolica, and European faience and delftware. Eventually the Chinese and Japanese made such glazes for the European market.

Metal oxides give color to glazes. Copper will make a lead glaze turn green and an alkaline glaze turquoise; a reduction kiln will cause the copper to turn red. Iron can produce yellow, brown, gray-green, blue, or, with certain minerals, red. Feldspars (natural rocks of aluminosilicates) are used in stoneware and porcelain glazes because they fuse only at high temperatures. The effects of specific glazes on certain clay bodies depend both on the composition of each and on the potter’s control of the glaze kiln.
E.         Underglaze and Overglaze Decoration

Pottery can also be painted before and after firing. In Neolithic times, ochers and other earth pigments were used on unglazed ware. Metal oxides used in or under glazes require somewhat higher temperatures in order to fix the colors to the glaze or body—they include copper green, cobalt blue, manganese purple, and antimony yellow. If enamels (fine-ground pigments applied over a fired glaze) are used, the pot must be refired in a muffle (covered, indirect-flame) kiln at low temperatures to fuse the enamel and glaze. Decals and transfer prints (designs printed on paper with oxides and, while wet, transferred to the pot, the paper burning away in the firing) are often used to decorate commercially manufactured pottery. In the 18th century the print plate was hand engraved, but now lithography and photography are used.

Potters’ marks have been used to identify ware in China since the 15th century, and in Europe since the 18th century, and famous pottery marks have always been easily forged. Greek potters and painters signed their work, as is true of a few Islamic potters and most 20th-century potters.
Source: “Pottery,” Microsoft® Encarta® Online Encyclopedia 2007 © 1997-2007 Microsoft Corporation. All Rights Reserved.