The tendency of cinnabar to darken has been ascribed to conversion from red ?-HgS to black ?-HgS. However ?-HgS was not detected at excavations in Pompeii, where originally red walls darkened, and was attributed to the formation of Hg-Cl compounds (e.g., corderoite, calomel, and terlinguaite) and calcium sulfate, gypsum.

As the mercury cell as used in the chlor-alkali industry (Castner-Kellner process)