Characteristics: Silver-gray brittle, crystalline solid that darkens in moist air. Allotropic forms: black, amorphous solid (beta-arsenic); yellow, crystalline solid. Mohs hardness 3.5; insoluble in water, caustic and nonoxidizing acids. Attacked by HCl in presence of oxidant. Reacts with nitric acid. Low thermal conductivity; a semiconductor.
Derivation: Flue dust from smelting of copper, gold and lead, from which white arsenic (arsenic trioxide) is obtained as a byproduct, in varying degrees of purity. This is reduced with charcoal. Arsenic is no longer produced in the U.S.
Hazard: Arsenic and inorganic arsenic compounds are toxic by ingestion and inhalation. A known carcinogen. Exposure limits: 0.01 mg/m3 of air.