All Hazards

Manganese in Clay Bodies

A number of minerals source manganese in very high percentages, these can be used to manufacture pure manganese dioxide which is used in the manufacture of stains and as a raw colorant in glazes. But clay body manufacturers do not need to use pure manganese dioxide because the element manganese can be found in the materials umber and ochre, a benefit is that both are clay-like. It takes about 10% raw umber to make a buff burning clay body fire black. The umber does not add any handling hazards beyond those common to clays, the main of which is the presence of quartz.

What about the MnO in black clay bodies generating fumes during firing or leaching into food and drink? Obviously, if a producer adds 5-10% pure manganese powder to a body that is not a good idea (why even do it since a body can be stained to black using umber, it contains only 10% MnO). But put this in perspective. First, if a black body is covered by a safe glaze then it presents no food contact issue. Pure manganese is employed in thousands of glaze recipes in common use, often at levels far above 5%, that is where the attention should be. Consider also that manganese begins to volatilize just beyond cone 6 (black bodies will start bloating and then melt making that abundantly clear). If you are firing kilns full of MnO sourcing materials (in the bodies and/or glazes) it only makes sense to have a ventilation system attached.

A way to completely avoid the above-mentioned problem is the use an engobe that is colored using a black ceramic stain. Our L3954B is a good base for cone 6 and L3954N for high temperatures.

Among potters and hobbyists, another significant use of manganese is in metallic raku-fired glazes (20% or more is common in recipes). Standing downwind or close to outdoor raku kilns laden with manganese saturated glazes is an obvious hazard. However, adequate temperatures must be reached for the fuming to occur.

What about granular manganese used to add speckle to bodies? Before classifying these as dangerous the situation must be put into perspective. Such bodies contain only about 0.2% of 60-80 mesh manganese granular. We have found that at cone 6 oxidation, granular manganese can be tolerated without body bloating in vitreous (near zero porosity) bodies, this indicates that decomposition is not occurring, otherwise bloating would occur. Also, the vast majority of manganese particles are encapsulated within the clay matrix. Most of the tiny percentage of particles exposed at the body surface are engulfed by the glaze. All of the particles that actually bleed up into 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.

Related Information

High-manganese black-burning 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 as the stain (10% or more). The umber is a clay that matures at low temperatures, so it acts as a flux in the body. 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. The umber is decomposing and potentially gassing metal fumes. However, if the umber is added as a stain to a cone 10 clay, it fluxes it to mature the resultant body at around cone 6 and these gassing problems do not occur.

This cone 6 black-burning stoneware is safe. Why?

Black burning bodies are popular with many potters. This one is stained by adding 10% raw umber to a buff-burning stoneware. Umbers are powerful natural clay colorants, they have high iron and also contain manganese oxide. This mug fired perfectly. For both aesthetic and safety reasons. A white engobe, L3954B, was applied during leather hard stage, on the inside and partway down the outside. After bisque, transparent G2926B glaze was applied inside and GA6-B outside (over the engobe it fires amber but over the black clay it produces a deep brown). What about safety? Notice the clay has not bloated, the glaze has no bubble clouds or blisters, that means no manganese fumes were being generated. This happened because the top temperature on the firing was closely guarded to not exceed cone 6. And the body is a cone 10 stoneware, not cone 6, there is insufficient feldspar to trigger release of the MnO from the umber (and its subsequent decomposition to metallic state).

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

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 are causing this stoneware to bloat

This is 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 at the same time as the body is vitrifying, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.

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