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Blue lead line on gums after working with red lead for 12 days

Red lead (lead[II,IV] oxide, also called minium) is an inorganic compound with the formula Pb3O4. A bright red or orange solid, red lead has been used as pigment for centuries. In more modern times, it has been used in the manufacture of batteries and rustproof primer paints. It is an example of a mixed-valence compound, being composed of both Pb(II) and Pb(IV) in the ratio of two to one. (Source: Oliver T. Lead poisoning in its acute and chronic forms: The Goulstonian Lectures, delivered in the Royal College of Physicians, March 1891. Edinburgh and London: Young J. Pentland, 1891. Public domain.)