What is Porcelain?

A Collector's Guide to the Craft

Porcelain is a type of ceramic characterized by its strength, impermeability, and translucence. Like all ceramics, porcelain is made by firing (heating) clay mixed with other materials. Uses of porcelain include purely decorative objects such as figurines and jewelry, as well as more utilitarian objects such as tableware. Additionally porcelain's various properties have made it useful in wide-ranging applications including electrical insulation, plumbing, tiles and medical devices.

People have been making ceramic pottery since at least 4,000 B.C. when the first kilns (ovens) capable of achieving temperatures of between about 450 to 750 degrees Celsius were developed. Firing causes the minerals in the clay to fuse bringing about a permanent hardening along with other desirable characteristics such as reduced porousness. In tracing the historical development of ceramic manufacture, improvements are observed in three areas: the raw materials used; the techniques to prepare those materials for firing; and the ability to achieve higher and better controlled firing temperatures.

The precise origins of the first porcelain are lost to history, but in China sometime during the T'ang dynasty (618-906 A.D.) potters arrived at a method of producing porcelain by heating a mixture of two forms of granite, china-clay (kaolin) and china-stone (petuntse, felspar or feldspar), to about 1200-1400 degrees Celsius. By firing those materials at those temperatures the potters produced a fully vitrified ceramic, meaning that the component particles were completely melted and fused into a glass-like material.

Porcelain craft spread slowly through the years appearing ultimately in both Korea during the 12th century and Japan in the 16th. Porcelain became a significant trade commodity and the techniques of its manufacture a closely guarded secret. Europeans struggled to develop their own methods. In the late 16th century, Italian potters produced a ceramic made with white clay and glass, known as Medici or frit porcelain. The first European effort considered to be "true" porcelain was produced at Meissen in Saxony in the very early 18th century.

There is no universally agreed to definition for porcelain, and the actual composition of porcelain varies significantly. Within this variety are three main porcelain types: hard paste, soft paste and bone. The term paste refers to the mixture of clay and other materials out of which porcelain is made.

Hard paste porcelain is distinguished from the others by the use of kaolin. As noted above, the classic formulation in China contained kaolin and felspar. The earliest European hard paste contained alabaster rather than felspar, but the results were less than ideal. Finding and using sources for felspar ultimately yielded a better product. Additionally hard paste will contain quartz or silica sand.

Soft paste porcelain dates back to the early Italian attempts to imitate Chinese porcelain using white clay and glass. Numerous formulations were developed which better emulated the results of hard paste, and successful manufacturing efforts took place in France and England. However well done and highly collectible these "artificial" porcelains may be, they do lack some of the prized characteristics of true porcelain.

As the name implies, bone porcelain (more commonly bone china) is made with ash from animal bone, along with felspar and quartz. Bone china was first used in England in the late 18th century although the earliest efforts were not very satisfactory. By the 19th century the techniques were greatly improved with the result being a strong, white and translucent porcelain used in English tableware to this day.

Glazing is a layer of glass which coats a ceramic. In pre-porcelain ceramics, glazing was essential to creating pottery that was non-porous and would hold liquids. In porcelain, glaze is used primarily in decoration and to strengthen and protect the piece. Glaze begins as glass which is ground to a powder and then mixed with clay and water to form a creamy liquid that can be dipped or sprayed on.

One of the advantages in working with hard paste porcelains is that after an initial firing called the biscuit (or bisque) firing, glaze can be applied and the paste and glaze can be fired together, and the porcelain is vitrified with the glaze. In soft paste porcelains the glaze does not become fully one with the paste and the result is a thicker, glassy surface.

Beyond the glaze itself are three basic types of porcelain decoration, underglaze, overglaze, and gilding. These decorations are made using various combinations of metallic and glass compounds which melt and fuse with the porcelain and glaze when fired. The nature of the materials involved and the result desired require different firing temperatures and therefore multiple firings in a specific order. It should not be surprising that control of this process is elusive and requires the highest artistry.

The names of these decoration types are indicative of their application. In underglazing the design is applied prior to the glaze, most classically the cobalt blue and white of China patterns with other colors attainable using iron and copper. Overglaze is an enamel applied atop the glaze in subsequent firings which cause the enamel to fuse with the glaze. All early decorations were hand painted but hand painting was ultimately supplanted by various transfer and printing techniques. In gilding, a gold decoration is fused to the surface of the glaze.

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