What Is Hard Porcelain Clay?
- Chinese ceramicists developed porcelain clay during the Yuan dynasty, between 1279 and 1368 AD. These pieces were made primarily in blue and white, eventually expanded into five colors during the Qing dynasty. Chinese porcelain shipped to Europe commanded enormous prices. European ceramicists were unable to duplicate this type of clay, but invented a number of similar-looking, but structurally inferior, types of clay, including soft-paste porcelain and bone china. True European porcelain developed in the 18th century, and has been the predominate form of this ceramic since then.
- Porcelain clay is composed of kaolin, flint and feldspar. Traditionally, a special type of feldspar, called petuntse, was used to give this material its glassy texture, but other types work just as well. According to the University of Manitoba, some porcelain clay may also contain white grog, pyrophillite (hydrated aluminum silicate), and light-colored ball clay, but purists argue that these are merely a form of fine white stoneware.
- The clay used to make porcelain is difficult to work with. It tends to have little strength before firing, is prone to extensive warping and cracking, and has little plasticity. However, once fired, porcelain can produce very thin, translucent pots and sculptures. Glazed porcelain produces a distinctively luminous-looking surface. The high temperatures at which this clay is fired cause the glaze to fuse tightly with the clay, producing no clear line between the two.
- Porcelain clay varies significantly in its properties and appearance, depending on the proportions of the materials used to mix it. High percentages of kaolin produce a "severe" porcelain. This material is very white, and tends to be bright and shiny. It may feel cold or harsh, however. A high feldspar content produces "mild" porcelain, which has a more satiny, mellow look and gentler feel.