α-HgS is used as a red pigment when it is known as vermilion. Vermilion is known to darken and this has been ascribed to conversion from red α-HgS to black β-HgS. Investigations at Pompeii where red walls when originally excavated have darkened has been ascribed to the formation of Hg-Cl compounds (e.g., corderoite, calomel, and terlinguaite) and calcium sulfate, gypsum, rather than β-HgS, which was not detected.