The green corrosion on copper artifacts, sculptures and buildings is so aesthetically pleasing that countless recipes exist in books and online so that do-it-yourselfers can create the same look on anything made from the metal.
But depending on the recipe or the environmental conditions, that pretty green color could be any one of a handful of different corroded copper chemicals, such as nantokite (CuCl) or paratacamite (Cu2(OH)3Cl).
Or, in the case of the Statue of Liberty, the green patina is a copper sulfate.
Copper can also corrode into other colors besides green, such as browny-red cuprite or black tenorite.
Scientists don’t actually know precisely which conditions produce the different corrosion chemicals—but they should, especially when the authenticity of an artifact is in question.
For example, museum researchers need to know if the corrosion chemicals on a possibly fake copper artifact came from natural aging processes or are the result of a quick-aging forged process.
First they started reading do-it-yourself patina manuals, such as, The Colouring, Bronzing and Patination of Metals.
Then the scientists used intense X-rays to spy on the formation of one of copper’s green corrosion products, nantokite, as they made the patina according to different recipes.
Among their discoveries, the team found that some steps in the recipe, such as rinsing the newly produced patina in water, produced side-products such as browny red cuprite.
“Most protocols for producing the green copper patina were not developed in a systematic way,” Dowsett says. We want to figure out what’s actually going on in these different processes, he adds.
Next the team is working out the conditions for making other green copper patinas, beyond nantokite.
Watch out copper artifact forgers…