Surface Oxidation of Copper

Most Copper alloys will develop a bluegreen
patina when exposed to the elements
outdoors. Typical of this is the colour of the
Copper Statue of Liberty in New York. Some
Copper alloys will darken after prolonged
exposure to the elements and take on a
brown to black colour.
Lacquer coatings can be used to protect the
surface and retain the original alloy colour.
An acrylic coating with benzotriazole as an
additive will last several years under most
outdoor, abrasion-free conditions.