Copper's Natural Weathering Characteristics
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.
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.
the initial weeks of exposure, particularly in a humid atmosphere or
in areas of frequent rainfall, radical color changes often take place
with iridescent pinks, oranges and reds interspersed with brassy yellows,
blues, greens and purples. During continued exposure, these interference
colors fade and are replaced by relatively uniform russet brown shades
referred to as statuary or oxidized finishes.
to varying fabricating procedures, some mills may coat coiled or flat
sheet stock with a thin coat of anti-stain oil film. This film may give
rise to dark purple or black surface colorations soon after installation
and exposure. This is a temporary color phase caused by the thin oil
film, which is quickly washed off by rain allowing the natural weathering
of Copper to proceed.
industrial and seacoast atmospheres, the natural patina generally forms
in from five to
atmospheres, where the quantity of air-born sulfur dioxide is relatively
low, patina formation may not reach a dominant stage for 10 to 14 years.
In arid environments, the basic sulfate patina may never
form due to the lack of sufficient moisture. Similarly, exposed horizontal
surfaces develop the patina more rapidly than sloping surfaces which,
in turn, patina more rapidly than vertical surfaces. The critical variable,
in all instances,
is the dwell time of moisture on the exposed surfaces.
progressive oxide, sulfide and sulfate films which develop on Copper
exposed to the atmosphere are quite thin two to three thousandths of
an inch highly adherent, but with relatively low abrasion resistance.
Neither the oxide nor sulfide films are particularly corrosion resistant.
The sulfate patina, on the other hand, is highly resistant to all forms
of atmospheric corrosion, once it has had an opportunity to form completely.
It thus significantly increases the durability and, hence, the service
life of Copper roofing and flashing.
weathering of Copper will reach a final equilibrium with its local environment.
This state of equilibrium is very stable and no further weathering will
occur after this state is reached. However, the final equilibrium color
will vary depending on orientation, slope, and local weather conditions.
information provided by The
Copper in Architecture Website.