Iron Oxide, Black – (m.p. 2818o F/1548o C) Red: Fe2O3 Iron Oxide is the most commonly used form of Iron as a colorant in glazes and clay bodies. It is used most often to produce tan to brown colors. When used in amounts greater than 4% it can react as a flux and increase the fluidity of the glaze. Black: FeO Iron Oxide as a colorant will create a variety of colors depending on the glaze base used. Its effect is most often the same as Red Iron Oxide. Black Iron Oxide is preferred for producing celadon glazes in reduction firing.