1 Shrinkage curve for a cone 3 clay body. Note that the shrinkage reverses as the body begins to bloat when fired above cone 4.
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.
Defining the Terms
Bloat: A ceramic fault caused by an excessive quantity of glass phase produced by severe over-firing. As a result, gas bubbles trapped in the glass expand, reducing the density of the body and in extreme cases rupturing the body.
Frit: A mixture of ceramic materials that has been melted to a glass, then cooled, crushed and ground to a fine powder for use as a glaze or clay-body ingredient.
Porosity: A measure of the total volume of space between or within particles and phases within a ceramic material.
Vitrification: Transformation by heat and fusion of a mixture of materials into a brittle, hard, non-crystalline glass.
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. This glass is a liquid at the peak firing temperature and solidifies as the piece cools after the firing.
The temperature at which the glass forms, the vitrification temperature, depends entirely on the chemistry of the clay body. For the clay in our studios that chemistry is always some combination of silica, alumina, and flux oxides. In low-fire bodies, frits may be used to introduce some of the ingredients including boron as an additional glass former. Bone china contains phosphorous as a second glass former.
In all traditional ceramics, though, silica still forms the larger portion of the glass network. Alumina modifies the glass network, making the glass harder and more chemically durable. Flux elements lower the temperature at which vitrification begins. That is the melting point of the eutectic composition within a given mixture of ceramic materials.
In some low-fire clay bodies, large quantities of talc in the recipe introduce magnesium oxide as a flux. So-called talc bodies, which typically mature in firings between cone 08 and 04, are often used as casting slips since they may lack plasticity.
Low-fire bodies may also be composed of naturally occurring deposits of clay rich in flux elements. Only such deposits that are free of iron will produce bodies that fire white. These are rare and iron-red clays are far more common world wide.
Fired porcelains typically contain about one-third glass phase by volume. Bone china is even glassier. This helps explain why these materials are so strong for their weight. Low-fire ware is typically porous, with a smaller volume of glass phase present in fired work and consequently a weaker structure. There are, of course, exceptions regarding strength of ceramic work governed by the many possible combinations of clay-body recipe and firing temperature.
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 (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.
2 These tests were all made from the same terra-cotta body and fired to increasingly higher temperatures. Note that the samples are at or very near full density at the two lowest firing temperatures. However, at 2057°F (1125°C) a very slight bloat is evident. Each sample fired to a higher temperature becomes more vitrified. By 2192°F (1200°C) the body is seriously bloated, glassy, and obviously overfired. Photos: Matt Katz.
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.
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