Patina on metal is a thin layer that variously forms on the surface (tarnish produced by oxidation or other chemical processes). The most common example of which is rust which forms on iron or steel when exposed to oxygen. Another, is verdigris, a green or blue powdery patina that forms on copper and bronze. Resulting from the slow chemical reaction of copper with carbon dioxide, producing copper carbonate. A patina takes many years to develope under natural weathering conditions.