In the past manufacturers used pure manganese dioxide as a body addition to create a black fired surface. Up to 5% is required. This practice obviously puts workers at risk to the toxic effects of the dust. But it also put potters and manufacturers at risk to the fumes produced during firing. Today, manufacturers use (or should use) manganese-containing raw materials (e.g. clays like Raw Umber or its calcined equivalent - Burnt Umber). These contain iron or other metals that, as a combination, are very effective at staining a clay body. Only 10% raw umber is needed and that material contains only 10% manganese, so this puts the user at far less toxicity risk. Still, potters and users should be vigilant in deciding whether to use such clay bodies, since the risk is obviously not zero. Umber ores can be a who's who of the periodic table, containing are wide range of potentially toxic compounds like barium, lead, albeit in small percentages. Much denser black colors can be produced by employing a black engobe colored using a black ceramic stain (stains are man-made kiln-sintered powders that are stable).
Manganese granular is used in bodies also (to create a speckled fired surface). But before classifying bodies containing it as dangerous it is good to put it 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.
In Raku: Among potters and hobbyists, the most significant use of manganese is in metallic raku fired glazes (20% or more). Standing downwind or close to outdoor raku kilns laden with manganese-saturated glazes is be a serious hazard (and has poisoned some potters).
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.
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.
This bag will give you a clue as to what manganese dioxide, MnO2, is mainly used for. Staining bricks.
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).
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.
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.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.
|Hazards||Manganese and Parkinsons by Jane Watkins|
|Hazards||Manganese Inorganic Compounds Toxicology|