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These two sets of SHAB test bars are fired from coldest to hottest (bottom to top) through the range from cone 04 up to cone 4. The graph shows the decreasing porosities and increasing shrinkages as temperature rises. Note the terra cotta reaches maximum shrinkage and minimum porosity at 2000°F (above which it begins expanding and melting). The talc body, by contrast, slowly matures, reaching a minimum porosity of 7% at 2150°F. It thus only sinters, never reaching a state of anywhere near the maturity and strength of the terra cotta. Clearly, the latter is not only high in iron oxide, but also in fluxes (like K2O, MgO, Na2O, CaO, etc) - these are responsible for how fast it vitrifies. The point where maximum fired strength is produced is the third bar up, 1970°F on the graph. However, there are multiple issues: Ware will warp in the kiln, the terra cotta red color has been lost and LOI gases of decomposition will disrupt glazes (with bubbles, blisters and pinholes). The most practical temperature is the second bar up, adequate density, good color, better quality glaze surfaces. The talc body? Although it fires to poor maturity across the range, it has one trump card: It provides a good base for glazes, producing bright colors and no bubbles.
Glossary |
Artware
Potters know artware as pottery firing at low temperatures with brightly coloured glazes and decorated using decals, underglazes, lustres, etc. |
Glossary |
Terra Cotta
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve. |
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