 The natural weathering of copper is a direct consequence of the corrosive attack of airborne sulfur compounds. This leads to a gradual change in the surface color until equilibrium is reached and the change is stabilized. The final color is different and depends on the location and local weather. Salt, moisture, sunlight and orientation can affect the final color. In general, copper changes in hue from a natural salmon through a progression of russet browns, grays and finally to a blue-green or gray-green patina.
Because copper appears low in the reactivity scale of metals, the formation of patina takes an extended period of time, usually 8 to10 years. Compare this with a matter of hours for the formation of rust (iron oxide) on a cleaned steel surface.
The formation of patina also slows down further oxidation once it is established, and this is why there are buildings throughout Europe with copper sheeted roofs—some of which are hundreds of years old.  |
