Information from the Getty AAT
orpiment (pigment)
A soft, yellow mineral composed of arsenic trisulfide that occurs naturally in volcanic fumaroles, hydrothermal veins, hot springs, and as a decomposition product of realgar. It changes to a red crystalline form at 170 C. Orpiment was used from ancient times as a pigment in painting, in manuscript illumination, and polychrome sculpture, having good tinting strength, but not permanent and toxic. King's yellow is the synthetic counterpart.