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Manganese in Clay Bodies
Manganese materials (powder and granular) can be ground from a variety of ore materials, thus the powder is not a pure manganese oxide. While they are often considered a nuisance dust the ores often also contain significant amounts many other compounds like barium, lead, quartz.
Among potters and hobbyists, the most significant use of manganese is in metallic raku fired glazes (20% or more). However high amounts of manganese dioxide can also be used in bodies for dark grey and black colors (up to 10%). These can likewise produce metal oxide fumes that can be very harmful. Unventilated indoor kilns pose a significant threat but standing downwind or close to outdoor kilns can also be a serious hazard.
Before classifying bodies containing manganese granular (to create a speckled fired surface) as dangerous the situation must be put into perspective. Such bodies contain only about 0.2% of 60-80 mesh manganese granular. The vast majority of particles are encapsulated within the clay matrix. Most of the tiny percentage of particles exposed at the surface are engulfed by the glaze. All of the tiny number of particles that actually bleed up through the glaze to either near or at the surface have been significantly diluted and stabilized by the glaze melt that surrounds them. Thus the total area of leachable manganese glass on a functional surface is extremely small.
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By Tony Hansen