MADI
A manganese oxide used to colour glaze, slip and clay · Brown through plum and purple-brown; metallic at heavy loadings · It gives up oxygen twice as it heats, once early and once in the melting range · A purple needs oxidation and is easily lost to reduction or to iron
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Manganese dioxide colours a glaze brown, through plum and purple-brown, and at heavy loadings the surface goes metallic. It releases oxygen twice on the way up, and the second release lands where a glaze is sealing.
Manganese dioxide gives up oxygen twice as it heats, once between about 500°C and 600°C, again between about 900°C and 1000°C. The first passes before most glazes begin to melt; the second lands where a stoneware glaze is softening and sealing, which is why a thick wash can blister where a few parts in a hundred will not. Colour runs brown, through plum and purple-brown, and at a fifth of the recipe or more the surface goes metallic — though the metallic recipes in circulation carry other colourants as well. A purple wants a high-alkali, low-alumina glaze fired in oxidation, and it is easily lost: a reducing pass, or a base carrying iron, pulls the colour back to brown.
