Natural Weathering

Copper naturally weathers to a characteristic blue-green or gray-green patina, which is a direct result of slight corrosion from sulfur compounds in the air.

As natural weathering progresses, the hue of the exposed metal transitions from a natural salmon pink to a series of reddish-browns, then to light and dark chocolate browns, ultimately resulting in the final blue-green or gray-green patina.

In the initial weeks of exposure, especially in humid air or frequently rainy areas, the color can change dramatically, showcasing a rainbow of colors including pink, orange, and red interspersed with brass-like yellows, blues, greens, and purples. During prolonged exposure, these interference colors fade, replaced by a relatively uniform reddish-brown hue, known as the statue or oxidized finish.

Due to variations in manufacturing processes, some factories may apply a thin layer of protective oil film on coils or sheets. Shortly after installation and exposure, this oil film can temporarily color the surface deep purple or black. This temporary discoloration is caused by the thin oil film, which is quickly washed away by rain, allowing the copper to weather naturally.

In industrial and coastal atmospheres, natural patina generally takes 5 to 7 years to form. In rural atmospheres, where the concentration of sulfur dioxide is relatively low, the formation of patina may take 10 to 14 years to reach a dominant stage. In arid environments, the lack of sufficient moisture may prevent the formation of basic sulfate copper patina altogether. Additionally, horizontal surfaces weather faster than inclined surfaces, while inclined surfaces weather faster than vertical surfaces. In all cases, the key variable is the duration of moisture retention on the exposed surface.

The oxides, sulfides, and sulfate films that gradually form on exposed copper are very thin, measuring only two to three thousandths of an inch. They have strong adhesion but relatively low wear resistance. Both oxide and sulfide films are not particularly corrosion-resistant. In contrast, once fully formed, sulfate copper patina possesses a strong resistance to all forms of atmospheric corrosion. Therefore, it significantly enhances the durability of copper roofs and waterproofing sheets, extending their service life.

While sheets represent a typical sequence, any installed weathering will depend on local environmental factors, orientation, and the amount of residual lubricant present.

Copper weathering will reach a final equilibrium with its local environment. This equilibrium state is very stable, and once reached, further weathering will not occur. However, the final equilibrium color will vary based on orientation, slope, and local weather conditions.

Twelve consecutive color plates illustrate the natural weathering cycle of copper, showing the aging process from initially unexposed copper to its weathered state after 30 years.