Rare compound detected in the Mona Lisa reveals a new secret, study says

Mona Lisa
A new study has found a rare compound in the base layer of the "Mona Lisa" that is formed from a lead oxide and oil combination, and provides a peek into one of Leonardo da Vinci's experimental painting techniques. Photo credit: Getty Images

By Taylor Nicioli of CNN

Leonardo da Vinci was a painter, inventor and anatomist, to name just a few of his talents - and now, you can add innovative chemist to the polymath's many gifts. It turns out the master artist was more experimental with his renowned Mona Lisa than previously thought - and was likely the creator of a technique seen in works created a century later, a new study suggests.

By using X-ray diffraction and infrared spectroscopy, a team of scientists in France and Britain has detected a rare mineral compound within the iconic piece. The finding provides fresh insight into how the work from the early 1500s was painted, according to the study recently published in the Journal of the American Chemical Society.

Along with lead white pigment and oil, the compound - known as plumbonacrite - was found in the base layer of the paint. A study published in 2019 had identified the mineral in several 17th-century works by Rembrandt, but researchers had not come across it in works from the Italian Renaissance until the new analysis. 

Plumbonacrite forms when lead oxides combine with oil. Mixing these two substances on a palette is a technique that later artists like Rembrandt used to help the paint dry, according to the study. Detecting the rare compound in the Mona Lisa suggested that Leonardo could have been the original precursor of this approach, said Gilles Wallez, an author of the latest study and a professor at Sorbonne University in Paris who also was a coauthor on the 2019 report.

"Everything which comes from Leonardo is very interesting, because he was an artist, of course, but he was also a chemist, a physicist - he had lots of ideas, and he was an experimenter… attempting to improve the knowledge of his time," Wallez said.

"Each time you discover something on his processes, you discovered that he was clearly ahead of his time," he said.

The Mona Lisa, like many other paintings from the 16th century, was created on a wood panel that required a thick base layer, Wallez said. The researchers believe that Leonardo had made his mixture of lead oxide powder with linseed oil to produce the thick coat of paint needed for the first layer, while unknowingly creating the rare compound. 

Analysing the Mona Lisa

Nowadays, researchers aren't allowed to take samples from the masterpiece, which resides at the Louvre in Paris and is protected behind glass, Wallez said. Using a 2007 microsample that had been taken from an area of the artwork just behind the frame, however, scientists were able to analyse the paint by using a high-tech machine called a synchrotron. The particle accelerator allowed the team to study the speck's composition on a molecular level.

"These samples have a very high cultural value," Wallez said. "You can't afford to take big samples on a painting, so a synchrotron is the best way to analyse them."

The base layer of Leonardo's mural The Last Supper was also found to have the same chemical makeup as the Mona Lisa even though the mural was painted on a wall, according to the study. The scientists had a much wider scope of samples from The Last Supper to look at, 17 in total, which came from the paint flaking off the wall over time, Wallez said. 

The Mona Lisa and The Last Supper are two out of fewer than 20 known paintings Leonardo made in his lifetime. Researchers hope they can discover more about the artist and his works with time.

"We have long known that Leonardo was an inveterate experimenter," said William Wallace, a distinguished professor and chair of art history and architecture at Washington University in St. Louis.

"Therefore, it is not all surprising that we see him experimenting in other media, especially given his dedicated search for the best painterly techniques (often untraditional) to create his 'living' works of art," said Wallace, an expert in Renaissance art and architecture who was not involved in the study.