Black iron oxide

Black iron oxide is a fine powdered pigment. Iron oxides are synthesized to avoid the presence of impurities found in naturally occurring iron oxides such as ferrous or ferric oxides, arsenic, lead, and other poisonous substances.
In ceramic applications, black iron oxide is used as a source of Fe. After being fired at high temperatures, iron oxide provides the color in glaze.
Black iron powder is also used to color a variety of non-ceramic products.
For corrosion resistance, black iron oxide or magnetite is also used. Black iron oxide is also used in anti-corrosion paints (which are widely used on bridges).