In clay body testing it is about the data.

It reveals how a 20% feldspar addition affects this clay

Foundry Hill Creme (FHC) is used in North America as a stoneware body base or addition. It fires like a ball clay but it is less plastic. How much feldspar would it take to make it into a vitreous cone 6 stoneware? We fired SHAB test bars from cone 4 to 10 (bottom to top) and 10R (the soluble salts discolor it in reduction). The data collected (in our Insight-live.com group account) produces firing shrinkage and absorption data that calculates to the red columns (and plots to the blue lines). Notice how the porosity drops steadily from cone 3 to 10 (the firing shrinkage rises steadily over the same period). A 20% nepheline syenite addition plots to the orange lines. Notice 2200F (cone 6): The porosity drops from 8% to vitreous (zero) while the firing shrinkage increases 2%. The FHC drying shrinkage (the first DSHR red column) averages 0.3% lower than the mix, this is counterintuitive but a pleasant surprise. While the mix did have a higher water content (which would increase drying shrinkage) another factor appears to be how well this clay hosts the feldspar addition without compromising its own plasticity.

Related Pictures

How to decide what temperature to fire this clay at?

Test bars of a terra cotta clay fired from cone 06-8

This picture has its own page with more detail, click here to see it.

This is an extreme example of firing a clay at many temperatures to get wide-angle view of it. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware warps during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, watertight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low-fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low-fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low-fire transparent base that works fine on this up to cone 2: G1916Q.

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Links

Materials	Foundry Hill Creme
Articles	Setting up a Clay Testing Program in Your Company Set up a routine testing pipeline and start generating historical data that will enable your staff to understand your source materials and maintain, adjust and troubleshoot your clay body recipes.

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