Another of the secrets of Leonardo’s Mona Lisa has been revealed: it is plumbonacrite, a very rare and unstable mineral compound that was discovered in the background layer of the famous portrait preserved in the Louvre as well as in the Last Supper painted in Milan. Produced by mixing oil and lead oxide, its presence demonstrates the painter’s willingness to experiment, anticipating by centuries what other artists such as Rembrandt and Vincent van Gogh later did. This was revealed by the analyzes conducted by a team of experts from the National Center of French scientific research (Cnrs) thanks to the European super microscope Esrf (European Synchrotron Radiation Facility), the synchrotron light structure in Grenoble. The results are published on Journal of the American Chemical Society.
Plumbonacrite also on Vinci’s Last Supper
By analyzing a microscopic sample of the Mona Lisa’s preparatory layer under synchrotron X-rays, “we found a relatively high amount of plumbonacrite, a compound that we think is due to a specific mixture of oil and lead oxide,” explains researcher Victor Gonzalez. The presence of plumbonacrite was also found on fragments of Leonardo’s Last Supper, confirming Leonardo’s desire to innovate, through the preparation of thick and opaque backgrounds treated with large quantities of lead oxide. Researchers have examined Leonardo’s manuscripts, looking for some clues about this unusual painting technique. “It was incredibly difficult – admits Marine Cotte of the ESRF – because the words used by Leonardo are very different from current terminology and because of the gap between the terms used in paintings and in chemistry”. They eventually found a reference to one chemical compound in particular in the context of a pharmaceutical practice, but scientists believe he may have used it in paintings as well.