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Google searches on ceramic stain toxicity generally confirm the relative safety of these products, compared to raw metal oxides, as powders in use in production. Of course, saturating glazes with stains, especially reactive fluid melt glazes, can invite leaching issues the same as with oxide colors.
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A search for "ceramic stain toxicity" shows 19 digitalfire hits out of 32. Pretty well all of these pages refer to the increased toxicity of metal oxides over stains. No alarm. However, a search for "manganese dioxide toxicity" (lower right) is alarming (with disturbing words like neurotoxicity, movement disorders, Parkinson’s, distonia, liver disease, iron depletion, etc.). So then, why are so many potters still using recipes for black that contain high percentages of manganese dioxide? Although many black glazes use a combination or iron oxide and cobalt (e.g. 10%/2%), that does not work as well so manganese is still commonly employed (along with other metal oxides). There is a better way: Black stain in a proven base recipe (like GA6-B, G2934, G2926B), as little as 4% is possible.
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