The glass phase that forms during the firing of a ceramic material can be thought of as the glue that holds the finished work together. The fancy name for this is vitrification–or the ceramic chemistry that transforms a clay body into a hard, non crystalline glass.
Clay bodies have ranges of temperature that they can be fired to. In today’s post, Dave Finkelnburg explains how to examine the results of test firing a clay body to varying temperatures and determine the best ceramic firing temperature. – Jennifer Poellot Harnetty, editor.
P.S. Learn more about vitrification and the glass phase that forms during the firing of ceramic materials in the May 2016 issue of Ceramics Monthly.
Vitrification, from vitreum, Latin for “glass” is the most important, and perhaps the most poorly understood, process in ceramics. A glass formed in the process of vitrification, even in tiny amounts, is what holds ceramic materials together.
It isn’t necessary to know the particular chemistry of a given clay body to determine the vitrification temperature. The temperature can be determined quite accurately by examining samples of the body fired to different peak temperatures.
All clay bodies shrink during firing. As peak firing temperature increases, that shrinkage continues until a clay body reaches its maximum density. However, if the firing temperature is raised even higher, the body will begin to expand again. The shrinkage curve shown, for a hypothetical cone 3 clay body (Figure 1), illustrates what happens when any clay body is fired. As the body is heated it shrinks and becomes denser. If heating continues after peak density is reached, gas bubbles in the glass phase expand and the body expands too, becoming less dense again. However, there is an important difference in the body before and after full density is reached. The pores early in the firing are open and can absorb moisture. Late in the firing those pores are only in the glass phase so they are closed, sealed in by the glass. While in this case the body shrinks linearly from cone 012 to cone 2, the body begins to bloat above cone 4 and expand again. A similar situation with a cone 04 clay body can be seen in figure 2 (see above).
The bloating occurs because the glass phase contains bubbles of vapor that form during the firing. As the kiln temperature increases, the vapor inside the bubble is heated and its pressure inside the bubble increases.
At the same time, the glass phase becomes less viscous. That permits the vapor bubble to expand. If firing proceeds to a high enough temperature, the body will either slump from formation of an excessive quantity of glass phase or bloats will burst, rupturing the surface of the ware.
Firing should be confined to a temperature that does not exceed that observed along the horizontal portion of the shrinkage curve. Better clay bodies will exhibit a larger span of peak firing temperature with little change in volume. Few bodies exist, however, that are capable of firing to acceptable maturity over a full 5 cones of temperature difference.