Glaze is coffee-staining and leaching after two years. Is it toxic?

Monday 16th April 2018

This is a insight-live.com/glossary/119">cone 04 terra cotta piece. The coffee stain cannot be removed because the coffee has also leached off the surface gloss. Glazes are glass. Glass is leachable if the chemistry is out-of-balance. So is this glaze poisoning the user? No, it has an insurance policy. It is transparent, made from a mix of two frits (Ferro 3124, 3134) plus kaolin and silica. The recipe contains no heavy metal colorants or pigments and no toxic fluxes like lithium or barium. But the body is red, how can the glaze be white? A white porcelain-like engobe layer was applied at the leather hard stage and it was clear-glazed after bisque. The fix: The predominant frit, 3134, has almost no Al2O3 (the oxide most important in producing durability). So I increased the Al2O3 (doing the chemistry in my Insight-live.com account). I also began firing one cone higher, at cone 03.

Pages that reference this post in the Digitalfire Reference Library:

Are Your Glazes Food Safe or are They Leachable?, Leaching, Liner Glaze

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