Orpiment, As2S3, is a common monoclinic arsenic sulfide mineral. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.46. It melts at 300 °C to 325 °C . Optically it is biaxial (-) with refractive indices of a=2.4, b=2.81, g=3.02.
Orpiment is an orange to yellow mineral that is found worldwide, and occurs as a sublimation product in volcanic fumaroles, low temperature hydrothermal veins, hot springs and as a byproduct of the decay of another arsenic mineral, realgar. It is often found in association with realgar. It takes its name from the Latin auripigmentum (aurum - gold + pigmentum - pigment) because of its deep yellow color.
It was an important item of trade in the Roman Empire and was used as a medicine in China although it is highly toxic. It was also used as a fly poison and to poison arrows. Because of its striking colour, it was also a favourite with alchemists searching for a way to make gold, both in China and the West.
Orpiment was ground, processed and used for centuries as a pigment in painting, being one of the few clear, bright yellow pigments available to artists up until the 19th century. Orpiment presented problems, however, such as its extreme toxicity and its incompatibility with other common pigments like lead and copper-based substances such as verdigris and azurite. The use of orpiment as a pigment matter ended almost entirely with the advent of the cadmium yellows and the various dye-based colors of the 19th century.
It is presently used in the production of infrared-transmitting glass, oil cloth, linoleum; in semiconductors and photoconductors, as a pigment and in fireworks. Mixed with two parts of slaked lime, orpiment is still very commonly used in rural India as a depilatory. It is also used in tanning industry to remove hair from hides.