Lead plumbate, also called red lead, minium or Mennige (in German), is a mineral showing colors from light red to brown/yellow tints. As a pure chemical it shows a vivid red. Minium is rare and occurs in lead mineral deposits that have been subjected to severe oxidizing conditions. It also occurs as a result of mine fires. It is most often associated with galena, cerussite, massicot, litharge, native lead, wulfenite and mimetite.
Lead plumbate is obtained by heating lead monoxide (\(PbO\)) to 450-480°C in air:
\[3 PbO + 1/2 O_2 \rightarrow Pb_3O_4\]
or by oxidative annealing of lead white:
\[3 Pb_2CO_3(OH)_2 + O_2 \rightarrow 2 Pb_3O_4 + 3 CO_2 + 3 H_2O\]
Lead plumbate decomposes into lead monoxide and oxygen above 550°C.
\(Pb_3O_4\) can be seen formally as a lead(II)plumbate(IV), \(Pb_2[PbO_4]\), or \(2PbO\cdot PbO_2\). In nitric acid the lead(II) oxide reacts forming lead nitrate, while the insoluble lead(IV) oxide is left unchanged:
\[Pb_3O_4 + 4 HNO_3 \rightarrow 2 Pb(NO_3)_2 + PbO_2 + 2 H_2O\]
Lead plumbate is virtually insoluble in water. However, it dissolves in hydrochloric acid (which is present in the stomach), and is therefore toxic when ingested. Lead plumbate (in a mixture with linseed oil or other organic adhesives) has been used as an anti-corrosion paint for iron. It forms insoluble iron(II) and iron(III) plumbates when brought into contact with iron oxides and with elementary iron. However, its use as a protective undercoat paint is limited due to its toxicity.
Lead plumbate was used as a red pigment in ancient and medieval periods for paintings and the production of illuminated manuscripts (the term miniature is connected to the name of the substance).
Contributors and Attributions
- Hans Lohninger (Epina eBook Team)