Mercadium Red. An extinct pigment which deserves a side note as a super-lightfast red. We'd call it very toxic (artiscreation gives it their most extreme rating, a D meaning, "Extremely Toxic, only attempt working with these pigments (especially the dry form) in laboratory like conditions with proper safety equipment " which frankly sounds scary and would make it a terrible pigment to work with. More information can be found in the Artist's Guide to Health and Safety on pigments containing cadmium and mercury (and possibly barium depending on the formulation). Author Monona Rossol’s updated work can be found through her site.
While cadmium reds have excellent lightfastness in general, this one is famous for having even more lightfastness in a test of extended light exposure done by Virgil Elliott. At one time Vasari offered this as a color, which they called Cadmium Vermilion Red Light, however they no longer carry it.
