The Collaborative International Dictionary of English v.0.48:

massicot \mas"si*cot\, n. [F. massicot; E. masticot is a
   corruption.] (Chem.)
   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.
         [1913 Webster]