Pottery is made by forming a clay body into objects of a required shape and heating them to high temperatures in a kiln to induce reactions that lead to permanent changes, including increasing their strength and hardening and setting their shape. There are wide regional variations in the properties of clays used by potters and this often helps to produce wares that are unique in character to a locality. It is common for clays and other minerals to be mixed to produce clay bodies suited to specific purposes.
Prior to some shaping processes, air trapped within the clay body needs to be removed. This is called de-airing and can be accomplished by a machine called a vacuum pug or manually by wedging. Wedging can also help to ensure an even moisture content throughout the body. Once a clay body has been de-aired or wedged, it is shaped by a variety of techniques. After shaping it is dried before firing. There are a number of stages in the drying process. Leather-hard refers to the stage when the clay object is approximately 75-85% dry. Clay bodies at this stage are very firm and only slightly pliable. Trimming and handle attachment often occurs at the leather-hard state. Clay bodies are said to be "bone-dry" when they reach a moisture content at or near 0%. Unfired objects are often termed greenware. Clay bodies at this stage are very fragile and hence can be easily broken.
A History of Pottery
The production of pottery is one of the most ancient arts. The oldest known body of pottery dates from the Jomon period (from about 10,500 to 400 BC) in Japan; and even the earliest Jomon ceramics exhibit a unique sophistication of technique and design. Excavations in the Near East have revealed that primitive fired-clay vessels were made there more than 8,000 years ago. Potters were working in Iran by about 5500 BC, and earthenware was probably being produced even earlier on the Iranian high plateau. Chinese potters had developed characteristic techniques by about 5000 BC. In the New World many pre-Columbian American cultures developed highly artistic pottery traditions.
After general sections on basic pottery types and decorating techniques this article focuses on the development of Western pottery since the beginning of the Renaissance. For detailed treatment of ancient Western and non-Western pottery, see Chinese art and architecture; Egypt, ancient; Greek art; Islamic art and architecture; Japanese art and architecture; Korean art; Mesopotamia; Minoan art; Persian art and architecture; pre-Columbian art and architecture.
TYPES OF WARES
Pottery comprises three distinctive types of wares. The first type, earthenware, has been made following virtually the same techniques since ancient times; only in the modern era has mass production brought changes in materials and methods. Earthenware is basically composed of clay--often blended clays--and baked hard, the degree of hardness depending on the intensity of the heat. After the invention of glazing, earthenwares were coated with glaze to render them waterproof; sometimes glaze was applied decoratively. It was found that, when fired at great heat, the clay body became nonporous. This second type of pottery, called stoneware, came to be preferred for domestic use.
The third type of pottery is a Chinese invention that appeared when feldspathic material in a fusible state was incorporated in a stoneware composition. The ancient Chinese called decayed feldspar kaolin (meaning "high place," where it was originally found); this substance is known in the West as china clay. Petuntse, or china stone, a less decayed, more fusible feldspathic material, was also used in Chinese porcelain; it forms a white cement that binds together the particles of less fusible kaolin. Significantly, the Chinese have never felt that high-quality porcelain must be either translucent or white. Two types of porcelain evolved: "true" porcelain, consisting of a kaolin hard-paste body, extremely glassy and smooth, produced by high temperature firing, and soft porcelain, invariably translucent and lead glazed, produced from a composition of ground glass and other ingredients including white clay and fired at a low temperature. The latter was widely produced by 18th-century European potters.
It is believed that porcelain was first made by Chinese potters toward the end of the Han period (206 BC-AD 220), when pottery generally became more refined in body, form, and decoration. The Chinese made early vitreous wares (protoporcelain) before they developed their white vitreous ware (true porcelain) that was later so much admired by Europeans.
Regardless of time or place, basic pottery techniques have varied little except in ancient America, where the potter's wheel was unknown. Among the requisites of success are correct composition of the clay body by using balanced materials; skill in shaping the wet clay on the wheel or pressing it into molds; and, most important, firing at the correct temperature. The last operation depends vitally on the experience, judgment, and technical skill of the potter.
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