Page 24 - Decorative Arts, Part II: Far Eastern Ceramics and Paintings, Persian and Indian Rugs and Carpets

Glazed vessels undergo one of two basic types of firing. In the first type, liquid glaze can be applied
directly to leatherhard ware and fired. Here, the body and glaze vitrify together in a single firing. In the second
type, vessels are fired more than once. The first firing is the bisque (sometimes called biscuit) firing. The objects
are heated to a minimum of 600 degrees C. (though more commonly between 900 and 1,000 degrees C), which
serves to strengthen and prepare the body for glazing. Liquid glaze is then applied to the cooled bisque ware, and
the object undergoes a second firing, the glaze and major firing, the temperature of which can be higher or lower
than that of the bisque firing. Sintering (a stage in which component particles begin to adhere but not yet fuse)
occurs at temperatures below the melting point and progresses to vitrification as the temperature increases. An
additional low-temperature firing, ranging between 600 and 900 degrees C., is required when overglaze decora-
tion, such as gilding and enamels, is applied.
Kiln atmospheres tend to be either oxidizing or reducing; oxygen is either given to or taken away from
the oxides in a ceramic body or glaze. In an oxidizing atmosphere, oxygen is allowed to flow freely into the kiln.
Volatile components, such as sulfur and carbon, burn away, so that essentially only oxides remain. As the heating
advances, the remaining components begin to fuse and vitrify. Oxidation firing is a somewhat less exact process
than reduction firing, in which the proportion of gases entering the kiln must be more carefully maintained.
In a reducing atmosphere, the amount of oxygen introduced into the kiln is purposely minimized. The
oxygen readily combines with carbon to form carbon dioxide. Prompted by the dearth of atmospheric oxygen,
the by-product carbon monoxide seeks oxygen from other sources, that is, oxides in clays and glazes, in an effort
to form carbon dioxide. This reduction process has been succinctly described by Nigel Wood:
The effect of reduction-firing is straightforward to bring about, and must have happened
naturally in most developing ceramic traditions. By cutting down the amount of air entering the
fireboxes of the kilns, or by overloading the fireboxes with fuel, less efficient burning occurs and
"reducing" gases begin pouring through the kiln chamber. These are mainly carbon monoxide and
hydrogen—both "oxygen-hungry" gases that actively convert to the more stable forms of carbon
dioxide and water. 7

As Kerr and others have shown, the atmosphere could vary considerably from one part of a kiln to
another, enabling the potters to fire different types of wares and glazes at once. 8 The iron and other oxides pre-
sent in most clays influence the final color, in both oxidizing and reducing atmospheres. The iron component is
profoundly affected by reduction. Even in a high-fired body, with its minimal iron content, white or light colors
become slightly cooler in tone.
Reduction firing produces a rich range of colors. Although the range was slightly more limited than that
possible in oxidation, it was successfully exploited by the Chinese to achieve a great variety of effects. That glazes
were nearly always fired in reduction attests to the potters' skill in manipulating a potentially unpredictable process.

TYPES OF GLAZES
The chemistry and technology of the many colored glazes developed from the fifteenth through the eighteenth cen-
turies is complicated and not yet fully understood. In addition to the composition and interaction of the glaze mate-
rials, the clay body color, kiln atmosphere (reducing or oxidizing), length of firing, and cooling all affect the color.
Glazes are generally classified according to their constituents, which influence the temperature range at
which they mature. Glazes that vitrify below about 1,100 degrees C., such as those with lead or alkalis as primary
constituents, are considered to be low-temperature glazes. Their characteristics include a relatively soft texture, a
distinct glassy appearance, and a tendency for the oxides to produce bright colors.
Lead-based glazes mature within very low temperature ranges, from a minimum of approximately 886
degrees C. (the temperature at which lead oxide alone will melt) up through about 1,190 degrees C. The lower

8 D E C O R A T I V E A R T S