During the initial weeks of exposure, particularly in a humid atmosphere, 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.
In industrial and seacoast atmospheres, the natural patina generally forms in about five to seven years. In rural atmospheres, where the quantity of air-born sulphur dioxide is relatively low, patina formation may not reach a dominant stage for 10 to 14 years. In arid environments, the basic sulphate 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, changes faster than vertical surfaces.
The critical variable, in all instances, is the dwell time of moisture on the exposed surfaces. The progressive oxide, sulphide and sulphate 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 sulphide films are particularly corrosion resistant. The sulphate 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.