Copper in architecture offers high corrosion resistance, minimum maintenance and has an attractive appearance. The longevity of copper roofing is unsurpassed, a well installed and cared for copper roof can last for hundreds of years and due to the long lasting and reusable nature of copper, a copper roof is actually a very environmentally friendly option. Copper and its principal architectural alloys are relatively active metals which, when left unprotected, tend to oxidise (weather). Long term atmospheric exposure generally results in the formation of the naturally protective gray-green patina.

Because copper and its alloys afford a broad spectrum of both natural and weathered colours, much effort is expended to either hasten the natural weathering by chemical means or to preserve the bright natural colors through the application of clear protective coatings.

Natural Weathering

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 sulphurous compounds.

As natural weathering proceeds, the metal exposed to the atmosphere changes in hue from the natural salmon pink colour through a series of russet brown shades to light and dark chocolate browns and finally to the ultimate blue-green or gray-green patina.

During the initial weeks of exposure, particularly in a humid atmosphere or in areas of frequent rainfall, radical colour changes often take place with iridescent pinks, oranges and reds interspersed with brassy yellows, blues, greens and purples. During continued exposure, these interference colours fade and are replaced by relatively uniform russet brown shades referred to as statuary or oxidized finishes.
Due 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.

In industrial and seacoast atmospheres, the natural patina generally forms in from five to seven years. In rural 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, patinate more rapidly than vertical surfaces. The critical variable, in all instances, is the dwell time of moisture on the exposed surfaces.
The 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. The natural weathering cycle of copper is illustrated by the 12 sequential colour plates in the Weathering Chart.

Although the plates represent a typical sequence, the weathering of any installation will depend on local environmental factors, orientation and amount of residual lubricants.

The 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 colour will vary depending on orientation, slope, and local weather conditions.

Chemical Colouring

Because of the time required for copper to weather to the ultimate blue-green or gray-green patina, men have sought for centuries to hasten the process by chemical means.

Colouring however is an art, mainly a matter of craftsmanship and experience. Chemical coloring techniques depend upon time, temperature, surface preparation, humidity and other variables which influence the ultimate result. A wide range of colored finishes may be produced on architectural copper-base alloys by conversion coatings that are chemical in nature. The purpose is to hasten the natural weathered effect that generally results from exposure to the elements.

Clear Coatings

Frequently, it is desired to preserve the original salmon or gold tones of copper or brass surfaces. This can be achieved by applying clear protective coatings. The coatings create a weather barrier and prevent the further oxidation of copper, brass and bronze surfaces. However, as with any coating, degradation over time will require the stripping and replacement of the clear coating.