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.

By Tony Hansen Follow me on

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