Black iron oxide black, Magnetite, Fe3O4; an iron ore primarily used to make pig iron in blast furnaces for further conversion to steel. As a glaze colorant, its spinel crystal structure resists dissolution in glaze melts somewhat, producing fired effects that are different and darker (because of high iron content) from other forms of iron oxide. As a body stain in clay bodies, magnetite ore can produce fired colours ranging from light grey to taupe-grey to dark brown to near-black depending on concentration.
For more information on black iron oxide, see the Digitalfire.com Reference Library of ceramic materials.