What are they?

Arsenic is an element sharing properties of metals and non-metals. It naturally occurs as a dark grey / black or yellow solid. Two notable compounds of arsenic found in the environment are the yellow and red sulphide minerals orpiment (As2S3) and realgar (As4S4).

Arsenic was used for centuries as a pesticide and preservative, as a pigment in its red and yellow mineral forms and, since the late 18th century, as a vivid green coloured copper-arsenic pigment and dye. Arsenic can be alloyed with metals such as copper to make bronze.