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.
Cooper weathering chart.
Because copper and its alloys afford 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 bright, natural colors through the application of clear, 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.
During the initial weeks of exposure, particularly in a humid atmosphere or 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.
Due to varying fabricating procedures, some mills may coat coiled or flat sheet stock with a thin coat of antistain 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 to proceed.
In industrial and seacoast atmospheres, the natural patina generally forms in five to seven years. In rural atmospheres, where the quantity of airborne sulfur dioxide is relatively low, patina formation may not reach a dominant stage for 10-14 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 that develop on copper exposed to the atmosphere are quite thin (0.002-0.003 of an inch) and 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 service life of copper roofing and flashing.
Figure 1 shows the age process from the initial unexposed copper to 30 years of weathering process later. The natural weathering cycle of copper is illustrated by the 12 sequential color plates. 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 color will vary depending on orientation, slope and local weather conditions.
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