Copper is not only tough, sturdy, highly adaptable metal, it is also - with the exception of gold the only primary metal that possesses color. Common applications for copper (or lead coated copper) are roofing, steeples, cupolas, and flashings.
Copper and its principal architectural alloys are relatively active metals which, when left unprotected, tend to oxidize (weather). Long term atmospheric exposure generally results in the formation of the naturally protective gray-green patina.
Because copper affords a broad spectrum of both natural and weathered colors, much effort is expended to either hasten the natural weathering by chemical means or to preserve the natural colors through the application of protective coatings.
The natural weathering of copper to the characteristic blue-green or gray-green patina is a direct consequence of the mild corrosive attack of airborne sulfur compounds. As natural weathering proceeds, the metal exposed to the atmosphere changes in hue from the natural salmon pink color through a series of russet brown shades to light and dark chocolate browns and finally to the ultimate blue-green or gray-green patina.