Neither the oxide nor sulfide films are particularly corrosion resistant.
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. The critical variable, in all instances, is the dwell time of moisture on the exposed surfaces. Similarly, exposed horizontal surfaces develop the patina more rapidly than sloping surfaces which, in turn, patinate more rapidly than vertical surfaces. In arid environments, the basic sulfate patina may never form due to the lack of sufficient moisture. 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 industrial and seacoast atmospheres, the natural patina generally forms in from five to seven years. 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. This film may give rise to dark purple or black surface colorations soon after installation and exposure. During continued exposure, these interference colors fade and are replaced by relatively uniform russet brown shades referred to as statuary or oxidized finishes.ĭue to varying fabricating procedures, some mills may coat coiled or flat sheet stock with a thin coat of anti-stain oil film.
#Does brass patina green series#
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.Īs 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.ĭuring 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. Long term atmospheric exposure generally results in the formation of the naturally protective gray-green patina.īecause 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. Copper and its principal architectural alloys are relatively active metals which, when left unprotected, tend to oxidize (weather).
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