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Manganese in Clay Bodies

Manganese materials (powder and granular) can be ground from a variety of ore materials (e.g. raw umber), 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 produce metal oxide fumes that can be very harmful. Unventilated indoor kilns pose a significant threat. Standing downwind or close to outdoor raku kilns laden with manganese saturated glazes 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 particles that actually bleed up through the glaze to either near or at the surface have been significantly diluted and stabilized against leaching by the glaze melt that surrounds them (they bleed and diffuse into it). Thus the total area of leachable manganese glass on a functional surface is extremely small. Notwithstanding this, there is a perception that must be considered. Some potters have used engobes and slips containing manganese granular so that these can be applied, under the glaze, to non-food surfaces only.

High-manganese body at cone 6. Practical and safe to use?

High-manganese body at cone 6. Practical and safe to use?

This type of clay is made by a number of North American manufacturers. Generally, raw or burnt umber are used to stain the color (10% or more). Umber contains a high percentage of manganese. The umber also fluxes the body and some manufacturers make the mistake of adding it to a cone 6 body and this is the result: bubbling of the glaze (in this case it is a transparent) and bloating and warping of the body. If the manganese is added to a cone 10 clay, the umber will flux it to mature around cone 6 without these problems. These bubbles are happening because the umber is decomposing and potentially gassing metal fumes. A functioning kiln vent is a must to fire this type of body.

Mug made from a cone 6 black-burning stoneware body

Mug made from a cone 6 black-burning stoneware body

Black burning bodies are popular with many potters. They are normally manufactured by adding around 10% burnt or raw umber to an existing buff-burning cone 6 stoneware. Umbers are powerful colorants, they have high iron and also contain manganese (the latter being the primary source of the color). But these clays can be troublesome. First, good kiln venting is needed to avoid breathing the dangerous manganese metal vapors. Micro-bubble clouding/gloss-loss in the glazes and blistering/bloating of the bodies are common. But this mug fired perfectly. Why? The umber was added to a cone 10 stoneware instead (and it has fluxed the body to mature at cone 6). The mug has been white engobed on the inside and partway down the outside during leather hard stage. After bisque it was clear glazed on the inside giving a flawless surface (using G2926B) and dipped in GA6-A Alberta Slip base amber-clear. The GA6-A over the black clay produces a very deep, rich, almost black ultra-gloss surface.

An original container of manganese dioxide

 An original container of manganese dioxide

This bag will give you a clue as to what manganese dioxide, MnO2, is mainly used for. Staining bricks.

A body containing manganese bubbles the glaze

A body containing manganese bubbles the glaze

Laguna Barnard Slip substitute fired at cone 03 with a Ferro Frit 3195 clear glaze. The very high bubble content is likely because they are adding manganese dioxide to match the MnO in the chemistry of Barnard (it gases alot during firing).

Decomposing manganese granular particles in a buff stoneware causing it to bloat

Decomposing manganese granular particles in a buff stoneware causing it to bloat

A cone 6 stoneware with 0.3% 60/80 mesh manganese granular (Plainsman M340). Fired from cone 4 (bottom) to cone 8 (top). It is normally stable to cone 8, with the manganese it begins to bloat at cone 7. The particles of manganese generate gases as they decompose and melt, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.

Links

Hazards Manganese and Parkinsons by Jane Watkins
Hazards Manganese Inorganic Compounds Toxicology
Materials Manganese Carbonate
Materials Manganese Dioxide
Materials Manganese Oxide
Materials Manganese Granular
Glossary Toxicity
The vast majority of materials used in ceramics are insoluble. But many still present hazards. And you can add hazards (to you and customers of your ware) by the way you use them. Still, there is a need to be realistic about it.

By Tony Hansen


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