Gray Oxide

Lead forms several nearly stoichiometric oxides with narrow phase limits, namely PbO, Pb3O4 and PbO2, as well as several compounds with oxygen having relatively larger phase limits. In commercial production by thermal method molten lead is directly oxidized by a mixture of air and water vapour. In the cupellation process in which a hearth is used or in kettles in the Barton Process both modifications Red Tetragonal PbO and Yellow Rhombic PbO form in varying proportions depending on temperature and cooling rate.