Verdigris is a green or blue-green PATINA that forms on copper and brass surfaces due to oxidation. It is caused by the reaction of copper with air and water, and is often seen on old monuments and sculptures. The term 'verdigris' comes from the Middle French word verde gres, which means 'green of Greece'. PATINA is a thin layer of discoloration or corrosion that forms on the surface of metal over time. It is often used to give an aged look to metal objects, and can be found on many old coins and sculptures.