Custom Patinated Copper

THE SCIENCE BEHIND PATINATED METALS

Copper alloys, whether they are prepatinated or not, will continue to transform as the surfaces age. This happens more rapidly when exposed to moisture, sun, and pollutants. Over time, this transformation will occur at a progressively slower rate as the copper surface reaches a chemical equilibrium. For example, bronze statues, exposed to the weather for centuries, often develop a darkish “bloom” in the form of a spot or streak. These localized changes are the product of natural pollutants and the further aging of the surface.

The predominant oxide to develop on the surface of copper alloys exposed to the atmosphere is cuprous oxide, Cu2O. This oxide is essentially the mineral cuprite. The color is reddish brown, but often exhibits a range of color from orange to yellow, even purples, as can be seen in the Dirty Penny copper material developed by Zahner.

The minerals of copper undergo a very slow aging process, though the process is somewhat faster when near the sea. Another somewhat common mineral formation on copper alloys combines carbon dioxide and forms carbonates over the initial cuprous oxide layer. The carbonate mineral forms are malachite and azurite. These relatively uniform corrosion products are difficult to artificially create.

All copper and copper alloy surfaces exposed to the atmosphere undergo changes. On a molecular level they seek various compounds from the atmosphere and readily combine with them. They actually remove pollutants from the air, albeit very slowly, by combining with sulfur dioxide and carbon monoxide to form mineral compounds that effectively trap the pollutants.