How Copper Gets Its Green Patina

Most people wandering past a building with copper siding or roofing have noticed that the metal corrodes into a potpourri of pretty green minerals, one of which is cuprous chloride (CuCl), called nantokite. Now, researchers led by Annemie Adriaens of Ghent University, in Belgium, have used time-lapse synchrotron X-ray diffraction to take a first glimpse of the growth of nantokite on copper (Anal. Chem., DOI: 10.1021/ac300457e). The research will help architects and do-it-yourselfers who want to purposely elicit the green corrosion in copper-clad buildings, as well as conservation scientists who want to understand how copper objects corrode, Adriaens says. The team produced nantokite in a synchrotron sample cell via two known methods: The first technique coats cupric chloride (CuCl₂) on copper in the absence of air, whereas the second technique simply drops cupric chloride on copper in the presence of air. The researchers found that the rustic green patina appears whether or not the reaction is exposed to air. However, they noticed that rinsing nantokite in water after the reaction was complete—a typical protocol in do-it-yourself recipes—caused side reactions that produced red cuprite (Cu₂O) and greenish paratacamite [Cu₂(OH)₃Cl].

GOING GREEN: A copper ingot from Crete displaying corrosion. Chris 73/Wikimedia Commons