Red oxide, or minium, is the tetraoxide of lead, formula Pb₃O₄. It is also called red lead. Not commonly occurring in nature, lead tetraoxide can be prepared by a number of simple reactions, one example being the oxidation of the commonly occurring Pb⁺² oxide, litharge:
6 PbO + O₂ ' Pb₃O₄
Pb₃O₄ can be written 2PbOּPbO₂, which indicates it consists of a mixture of oxidation states +2 and +4.
Paint and Primer
Red oxide is used in paints and primers as a rust preventive. If traces of rust are present on an iron surface, red oxide paint will still adhere, because it interacts with the surface by forming chemical bonds.
Red oxide, also called red lead, reacts with iron and iron oxides to form insoluble compounds called plumbates, in which lead is part of the anion, (PbO₃)⁻². For instance, ferrous plumbate has the formula Fe(PbO₃), in which the cation is Fe⁺².
Falling Into Disuse
Red oxide paint is falling into disuse due to concern over lead poisoning.
Red oxide is used in some glass formulations, where there exists no significant health threat to the public.