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Malinda, May 2 in Clay and Glaze Chemistry
Black coring is caused when there is insufficient oxygen in the firing. Increased ventilation is needed to enable the carbon and sulphur to burn out of the clay and to prevent the iron in the clay body from fluxing. A long slow firing in oxidation is needed up to 900C, especially since your tiles are thick, to allow gasses to escape and avoid reducing the iron in the clay. How you load your kiln is important too, stacking work makes it more difficult for the organics to burn off.
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