A Simple Test Determines the Best Ceramic Firing Temperature for a Clay Body

How to Determine How High to Fire Your Pottery Clay and Learn Your Ceramic Firing Temperature!

ceramic firing temperature

Figure 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 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.

Peak 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.

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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).

ceramic firing temperature

Figure 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.

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.

**First published in 2016
  • I am always surprised that so many people do not understand this. It is BASIC. Clay changes by heat work, if it didn’t we would not have ceramic. Clay dries and is brittle it is subjected to heat where eventually it sinters i.e. begins to degrade; that process continues until the whole thing mealts into a puddle. depending on the boday and temp that pudle may be bubbly like lava or flatter like glass. The higher the iron content the lower the temperatur at which it degrades into a fused or vitreous body that we may well want or if blating throw out. So testing as in all ceramic processes is important. If you dig your own clay this testing is imperative. Suppliers give accurate information but you are in control of your firing processes and quick or slow firing will affect the resulting body. Heat work, the result of temperature and time, is most important that is why cones are used and electric pyrometric indicators are not as accurate. So check what the supplier tells you and use that as a guide, different packing densities, thickness of pots and position in kiln all affect the heat work so always rely on cones not pyrometers.

  • I am a beginner in ceramics and about to glaze my first pieces. I bisquere them at 900°c and have Amaco cone 6 glazes. I heard that tableware has to be bisqued to higher temperatures than I did and use lower glazes to become strong. I am really confused whether I can go ahead or not.

  • I recently discovered this bloating effect much by accident when I attempted to make some neriage style mugs colored with copper carbonate and a questionable Mason stain mixed with some reclaimed cone 6 porcelain. An interesting effect and one that I plan on experimenting with further.

  • Stacey M.

    I would guess that rather than the age of the clay, it was a change in the recipe ingredients since clay materials vary at the source and from one source to another. That is why, ideally, each batch of ceramic materials should be tested to make sure they behave as expected.

  • Katherine F.

    I learned about bloat when I fired pieces made from Imco 1C clay that had sat unopened in a plastic bag on the shelf for a period exceeding two years alongside pieces that were from a new bag of the 1C clay. The older pieces bloated making for interesting art, but questionably functional. The younger ones did not, though they were on same shelves and of a similar nature. What is it about age that causes clay to be more susceptible to bloat?

  • Peter H.

    >Ellen says: Testing density?

    I worried about this, then saw that the graph axis is marked shrinkage.
    So I suspect that you just plot linear shrinkage versus temperature/cone.

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