Information from the Getty AAT
litharge
A crystalline form of lead monoxide, the oxidized product of molten lead that has been stirred or atomized to incorporate air then cooled and ground to form the yellow powder. It is used as a drier in paint and varnish, for the fluxing of earthenware, and in the manufacture of glass. Litharge tends to be slightly more orange than massicot, another crystalline form of lead monoxide, due to the presence of small amounts of red lead oxide.