The Collaborative International Dictionary of English v.0.48:
massicot \mas"si*cot\, n. [F. massicot; E. masticot is a
Lead monoxide (also called Lead protoxide), PbO, obtained
as a yellow amorphous powder, the fused and crystalline form
of which is called litharge; lead ocher. It is used as a
pigment. It is also called lead oxide yellow, as opposed to
red lead, which is lead tetroxide Pb3O4.
[1913 Webster +PJC]
Note: Massicot is sometimes used by painters, and also as a
drier in the composition of ointments and plasters.