Zahner has developed proprietary techniques to preweather copper and copper-alloyed sheets with deep, rich patinas not unlike those that formerly took several decades to develop naturally. Some of these techniques can develop the rich green and blue tones of cupric nitrate on the copper surface. Other techniques will develop browns, yellows, and oranges in various hues.
Creating patinated surfaces on metal requires requires safety and ecology protocols to be set in place. The use of proper safety equipment, eye protection, rubber gloves, and a respirator is essential. If producing these on a larger scale or in a factory environment, the patina process must be performed in a well-ventilated space with careful attention to waste disposal in accordance with EPA and safety regulations.
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.
Natural Weathering of Copper Alloys
Copper is a fascinating metal to use when considering color. No other metal comes close to offering the variety of color that copper can develop. Copper seeks other elements to develop compounds that possess unique color attributes. Essentially, the copper surface, like most metal surfaces, seeks components in the atmosphere and surroundings to combine with. Over long periods of time, the metal surface develops compounds that resemble closely their mineral equivalent. Once this occurs, the copper compounds are defiant and stubborn. Further change is inhibited.
For instance, copper roofing, exposed for a century or more, develops a thick, beautiful green patina. This patina is typically the result of sulfur and oxygen from the air combining with the surface copper atoms. Water intermixes the reaction, and, over time, the surface develops a hydrated copper sulfate not unlike that of the mineral bronchantite.
Today, the development of these green copper sulfate patinas has greatly diminished. The reason is that the atmosphere is cleaner and less polluted with sulfur compounds necessary to form the green patina. Thus, it takes considerably longer today to naturally develop the green patina in many areas world. In dry climates, the green patina may never develop.
Learn more about the patina development at Zahner by signing up for the Zahner newsletter, or contact a member of the Zahner sales team to see how we can develop a custom surface for your design.