The distinctive colors of copper and copper alloys make
them prized for architectural and consumer items and
objects of art. Their natural metallic tones range from red to
yellow to silvery gray (Figure 6). A number of other colors
can be obtained by chemical or electrochemical processing
of their surfaces. Copper and its alloys are extremely resistant
to atmospheric corrosion, but over time a superficial
discoloring or tarnish layer eventually forms. All metals
tarnish or form an oxide layer when exposed to the atmosphere.
The thickness and chemical content of the layer
varies as a function of exposure time, atmospheric conditions
and base alloy chemistry.