Mineral
Oropimente
As₂S₃ · Sulfuros
Orpiment is a toxic arsenic sulphide of golden-yellow colour, historically used as a pigment known as auripigmentum.
Orpiment, realgar and pararealgar are low-temperature arsenic sulphides, fragile and with a characteristic yellow–red colour range. It commonly occurs together with realgar, sometimes showing orange transitions within the same specimen. It shows a silky, platy and stratified structure. At close scale it displays exfoliated crystals, delicate plates or fibrous masses with a lustre between resinous, greasy and silky.
It is notably dense: it weighs much more than it looks, nearly twice that of the same volume of quartz. Handle with care: do not inhale dust and wash hands immediately after handling. As a pigment it had a chemical problem beyond toxicity: when mixed with lead-derived pigments, such as white, the paint eventually blackened. Many old paintings lost golden details due to this incompatibility.
Etymology
From Latin "auripigmentum" (golden pigment). It appealed to artists for its solar sheen, but hid a chemical pitfall: it is incompatible with lead. When mixed with colours such as lead white, its sulphur reacted and blackened the work. Thus many details intended to remain luminous eventually succumbed to a silent chemical struggle between pigments.