A glaze is a smooth, glassy coating applied to ceramic objects in order to add color and decoration to the surface or to vary its texture. The glaze forms a hard, nonporous surface that is easily cleaned.

Glazes are usually made from powdered glass combined with colored oxides of such elements as cobalt, chrome, manganese, or nickel. The mixture of powders is suspended in water and applied to the ceramic surface by spraying, brushing, or dipping. The glaze is then dried and fixed onto the ceramic surface by firing. During firing, the glass softens and flows over the ceramic surface to a greater or lesser extent, and reacts with the ceramic substrate to form a strong, adherent bond to it. Various components such as alkali oxide, borates, and lead oxide can be added to the glaze to make it soften at a lower temperature, so that it flows more easily during firing and smooths out roughness and defects in the ceramic surface.

If a glaze is applied to a fired ceramic substrate, a second firing is necessary to melt and bond the glaze to the substrate. It is also possible to apply a glaze to an unfired ceramic and fire both the glaze and substrate together. Two firings make possible a greater range of colors and textures.

The suspension, or slip, in which the glaze is applied to the ceramic surface must have particular properties to ensure that the glaze is easy to apply, does not run as it is drying, and adheres well both when wet and after drying. These slip properties are often obtained by adding a small amount of clay to the suspension, and by controlling the amount of water in the slip and the size of the powder's particles. Organic surface active agents (detergents) also can be added to a slip to improve its properties.

Often fine crystals grow in the glaze, making it more translucent or opaque. Crystals also give a dull, or mat, surface finish. The crystals nucleate and grow in the glassy glaze in much the same way as they do in glass ceramics. Crystal growth can be encouraged by heating the fired glaze for a time at a temperature somewhat below the firing temperature.

Glazes craze, or develop fine cracks, if the thermal expansion of the glaze varies significantly from that of the substrate. Thus the glaze composition must be designed with an expansion coefficient close to that of the substrate. On the other hand, an artist may wish to induce crazing for its different appearance and texture (crackle glaze); the larger the difference in expansion the finer the craze.

Colors in glazes are controlled by adding coloring agents to the glassy components of the glaze. A wide variety of colors are possible, depending upon the agent added, the base composition of the glaze, the color of the substrate, and the state of oxidation in the kiln. . Temperature fluctuations in the atmosphere, and other anomalies will cause variations in the glaze colors from one batch to another and even in the same firing depending on where the tiles are located relative to the hottest and coolest areas of the kiln.

Special effects in glazes can also be produced. If salt is added to the kiln during firing, the glaze develops a fine orange-peel texture, which can be uniform or spotty depending upon conditions. A glaze that froths during firing gives a rough surface of broken bubbles known as a blister glaze.

Robert H. Doremus
Bibliography: Chappell, James, Potter's Complete Book of Clay and Glazes (1977); Green, David, Pottery Glazes, rev. ed. (1973); Norton, Frederick H., Elements of Ceramics, 2d ed. (1974); Parise, C. J., ed., Science and Technology of Glazing Systems (1990); Ruscoe, William, Glazes for the Potter (1974); Wood, Nigel, Oriental Glazes (1978).
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