Verdigris

Verdigris is the greenish-bluish film that is created on the surface of copper by the application of a dilute acetic acid. It has a long history of use as a pigment, in dyeing, in the arts, and in medicine.

The term can also refer to the greenish film that forms on the surface of brass, bronze, and copper as a natural result of weathering by exposure to air or sea salt.  However, the chemical compositions of the natural verdigris (copper carbonate or copper chloride) and artificial are different, and only artificial verdigris (copper acetate) has the uses listed above.

The use of verdigris as a pigment and medicine dates back at least to its mention by Pliny the Elder (AD 23/24 -79) in his encyclopedic Naturalis Historia, but in all likelihood goes back further than that.  In the 15th through 17th centuries, verdigris in oil paint was a popular lightfast and intense green used by many well-known artists, including the Early Netherlandish master Jan van Eyck.  In the city of Montpellier in France, the manufacture of verdigris was a common household industry.  Montpellier verdigris, renowned for its quality as a pigment, was made by exposing copper strips to the vapors arising from distilled wine.

Through its history, verdigris has also been touted as a treatment for a wide variety of illnesses, from leprosy to infections of the skin.

In modern times, however, verdigris — mostly due to its toxicity and relative instability as a pigment — has largely fallen out of use.  But not entirely: today it is found in industrial fungicides, as a dye, and architecturally as a patina for copper and bronze.

Featured photo credit: The Magdalen Reading, by Rogier van der Weyden (after 1483). Courtesy of Wikimedia Commons.