Mapping corrosion with hyperspectral imaging

For more than a century, a cast of Auguste Rodin’s “The Man with the Key” stood watch at Solli plass, a traffic-congested area in Oslo’s city center. Exposure to sulfur dioxide pollution and acid rain extensively damaged the bronze statue, and it was recently relocated to the Vigeland Museum’s courtyard for conservation. But assessing the damage to the sculpture is complicated because bronze in outdoor urban environments typically degrades into two types of basic copper (II) sulfates: brochantite [Cu₄SO₄(OH)₆], and antlerite [Cu₃SO₄(OH)₄]. Both compounds appear green, which makes it impossible to tell by looking which form of corrosion is present on the statue—information that’s important for preservation. To gather that information, researchers led by Norwegian University of Science & Technology’s Emilio Catelli and Lise Lyngsnes Randeberg used short-wavelength infrared hyperspectral imaging—a technique that combines spectroscopy with imaging—to create a map of the brochantite and antlerite on the statue (J. Spectral Imaging 2018, DOI: 10.1255/jsi.2018.a10). Because the technique allowed them to gather data without moving or damaging the statue, the researchers say it offers conservators a useful tool in assessing the condition of bronze statuary. Next, they plan to see if they can detect other corrosion products using the technique.