In ceramics, it is used primarily in clays and glazes to achieve fired speckle. This is the same material as manganese dioxide powder, it is simply not ground to a fine powder. Typically a 60-80 mesh material is used in amounts around 0.2-0.3% in bodies firing from cone 4-6. The granular particles do not melt at cone 6 but they will bleed into glazes. They begin to decompose at cone 7, and thus will bloat bodies containing them at that temperature and up.
Pyrolusite ore is a source of manganese used widely in industry for the manufacture of manganese steel, alkaline batteries, decolorizing glass agents, and in ceramic bodies and glazes. Pyrolusite can contain small amounts of quartz (e.g. 3%) as well as barium compounds (e.g. 2%) and trace amounts of lead (e.g. 0.2%). It is typically about 75% MnO2.
G2934Y glaze on Standard #112 body at cone 6
This silky matte glaze produces an appearance very similar to dolomite matte glazed ware fired in cone 10 reduction. The degree of matteness can be controlled by the cooling rate of the firing. Although this body is made by Standard Ceramics, the effect would be similar using speckled bodies made by other manufacturers also. These pieces made by Tom Friedman.
The penetrating power of granular manganese specks
Firing: Cone 6 oxidation. Glaze: G2934 matte. The body contains 0.2% manganese 60-80 mesh. Multiple layers of the underglaze are not able to prevent the manganese from bleeding through to stain the surface.
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.
Are manganese speckled clay bodies a toxicity hazard?
Before jumping to conclusions consider all the factors that relate. This is M340S, it is fired at cone 6. That temperature is a "sweet spot" for this effect, high enough for the particles to bleed and low enough they do not bloat the body. Such bodies contain only about 0.2% of 60-80 mesh granular manganese (compare this to many glazes that employ 5% powdered manganese as a colorant). Further, the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function the glaze thickness and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal. Consider also that the total area of manganese-stained glass on a functional surface is extremely small for this effect.