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Foundry Hill Creme (FHC) is used in North America as a stoneware body base or addition. It fires like a ball clay, being quite refractory. But it is much 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). We mixed it with 20% nepheline syenite and collected the data on that also, that plots to the orange lines. Notice 2200F (cone 6): The added nepheline drops the porosity from 8% to zero (and increases the firing shrinkage about 2%). Notice also the FHC drying shrinkage (the first DSHR red column) averages a little under 6.5%, it is enough to tolerate the non-plastic addition of nepheline syenite and still maintain good plasticity (notice the mix averages a little higher, that is because it was softer).
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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Foundry Hill Creme
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