A thin layer of corrosion products with a distinctive colouration that forms on a metal surface exposed to air and water. Patina usually refers to the greenish coating that forms on copper alloys over time. In most non-polluted environments it is basic copper carbonate but in industrial and urban areas it is mainly basic copper sulphate.
You can treat the surface of metal with heat, paint, beeswax or oil quenching to provide a virtual rainbow of effects and colours.
See also: Copper.