Page 23 - Decorative Arts, Part II: Far Eastern Ceramics and Paintings, Persian and Indian Rugs and Carpets

Firing conditions must be precisely controlled. When fired at temperatures between 1,200 and 1,400 degrees C., the
ware attains a vitrified body that is extremely hard and brittle and is translucent when thin. Porcelain vessels can be
either wheel-thrown or shaped in earthenware molds and templates.
Porcelain was first developed in China, not as a result of sudden discovery, but through an evolution of
technology. The transition from stoneware to porcelain was dependent upon three factors. First, improvements
in kiln design made higher firing temperatures possible. The earliest Neolithic horizontal cave-type kilns, which
produced temperatures generally below 1,000 degrees C., gave way to those with chimneys, which brought the
temperatures up to 1,200 degrees C., and eventually progressed to the so-called dragon kilns (built into the slope
of a hill) that maintained firing temperatures above 1,300 degrees C. Second, the potters of the Shang (c. i6oo-c.
1050 B.C.) and Zhou (c. iO50-c. 256 B.C.) dynasties understood very well the raw materials they used. Through
conscious selection of materials, they eventually were able to devise a clay with a decreased iron oxide content
(resulting in a whiter body) and an increased alumina content (strengthening the body). The third factor was the
development of glazes in the Shang dynasty. The early glazes were created when the coating formulations con-
tained a higher proportion of flux and the firing temperature was increased. 4
The whiteness and translucency valued as the ware's defining characteristics by Westerners 5 are not
included in the Chinese standard, which demands resonance when struck, extreme hardness, impermeability to
liquids, and imperviousness to acids, even when unglazed. Vitrification was the criterion for classification. The
Chinese word for pottery in general and earthenware specifically is tao. Vitrified wares, both stoneware and
porcelain, are described by a single word, ci.
European ceramic technology had developed only to the point of stoneware production before the mass
importation of Chinese porcelain beginning in the sixteenth century. The rush of emulation that followed this
influx of Chinese vessels first produced a ware in Europe called "soft paste," superficially resembling Asian porce-
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lain in color and glaze texture but differing from "Chinese soft paste." With the eighteenth-century discovery of
kaolin deposits in Europe, Western production of "true" porcelain developed rapidly.

FIRING PROCESSES
Although several types of kilns are used in the production of ceramics, the principle for the various designs is the
same. The kiln is designed to house the ceramics, control the amount of oxygen and heat introduced, and hold
the heat that enters the firing chamber. Throughout most of Chinese history the usual fuel was wood with straw,
although some areas used coal in certain kilns.
From the sixth century, and perhaps earlier, the Chinese fired many wares using saggers inside the kilns.
These fire-clay boxes protected the ware from flame and turbulence-carried materials (e.g., ash), and provided
some additional control over the firing atmosphere. Saggers held stacks of dishes or individual pieces. The sag-
gers themselves were stacked inside the kiln in ways that made for efficient firing.
Ceramics can undergo a single or "through" firing or multiple firings, during which the clay body and
glaze undergo a series of complex chemical changes, here briefly described.
After the ceramic vessel is fully formed, it is allowed to air dry. Prepared clay can contain up to twenty-
five percent water. This large proportion of water means that a certain amount of shrinkage is inevitable. Slow,
even drying is important to prevent cracking of the body. This initial drying stage allows the water between the
clay particles to evaporate, resulting in ware that is in a leatherhard state.
Water continues to be driven off in the initial stages of firing. As the temperature rises, changes in the
body progress: organic material decomposes, clay particles begin to adhere, carbon and sulfur oxidize, and at very
high temperatures maturation begins. The temperature and firing conditions are determined by the body com-
position and the desired appearance.

C E R A M I C T E C H N I Q U E S
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