If perfect permanence was the only characteristic of a pigment that is valuable, we would all be painting with Titanium white instead of Flake, and using Cobalt or Cerulean Blue rather than any organic blue. Reasonable permanence is acceptable. This is because Cadmiums and Vermilion are very similar chemically. They are both metallic sulfides. The free sulfur or polluting sulfides may react with lead white.
Many people strongly warn against mixing cadmiums with lead white.
I m not the only one who thinks its day as an artist s pigment is long past. True, but there are plenty who say the same thing about Flake White, and the cadmiums. Of course, people need to know what they're doing. Mercuric sulfide is insoluble in water. This makes it far less dangerous than soluble mercury compounds.