(MnO2) A metal oxide that produces blacks, browns and violets. In terra-cotta bodies, around 5 % can be added to produce dark grey to black colors. In glazes, large amounts (over 20%) can produce metallic surfaces, which are NOT dinnerware safe. The fern like stains in Mocha ware are achieved with the combination of tobacco juice and manganese dioxide. It does release oxygen at temperatures under 2000°F, which may cause pin holing in glazes. Above 2000°F it becomes a flux which reacts with silica to produce violet colors, or, if alumina is present, browns that have a softer more pleasant tone than those produced by irons.