The “Mona Lisa” just grabbed headlines after scientists used an x-ray to unlock one of her long-held secrets — what chemicals went into her creation.
Leonardo da Vinci used a distinct mix of oil paints in order to make his world-historic portrait of the woman with an ambiguous smile, a paper revealed after its publication Wednesday.
Da Vinci used the mixture as his base layer to prepare the panel of poplar wood — which he had never done before or after. It remains unique among his paintings, the team of scientists and art historians in France and Britain have suggested.
The research paper appears alongside a slew of papers unrelated to the art world, with titles like “Precise Equilibrium Structure of Benzene” and “Growth and Local Structures of Single Crystalline Flakes of Three-Dimensional Covalent Organic Frameworks in Water.” In other words, the new paper discovered this Mona Lisa fact only after a rare collaboration between chemistry labs and art museums.
Victor Gonzalez — the study’s lead author and a chemist at France’s top research body — has studied the chemical compositions of dozens of works by Leonardo, Rembrandt and other artists.
“He was someone who loved to experiment, and each of his paintings is completely different technically,” Victor Gonzalez said in an interview with the Associated Press. “In this case, it’s interesting to see that indeed there is a specific technique for the ground layer of ‘Mona Lisa.'”
Specifically, the researchers found a rare compound, plumbonacrite, in Leonardo’s first layer of paint. The discovery, Gonzalez said, confirmed for the first time what art historians had previously only hypothesized: that Leonardo most likely used lead oxide powder to thicken and help dry his paint as he began working on the portrait that now stares out from behind protective glass in the Louvre Museum in Paris.
Carmen Bambach, a specialist in Italian art and curator at New York’s Metropolitan Museum of Art, who was not involved in the study, called the research “very exciting” and said any scientifically proven new insights into Leonardo’s painting techniques are “extremely important news for the art world and our larger global society.”
Finding plumbonacrite in the “Mona Lisa” attests “to Leonardo’s spirit of passionate and constant experimentation as a painter – it is what renders him timeless and modern,” Bambach told the AP by email.
The paint fragment from the base layer of the “Mona Lisa” that was analyzed was barely visible to the naked eye, no larger than the diameter of a human hair, and came from the top right-hand edge of the painting.
The scientists peered into its atomic structure using X-rays in a synchrotron, a large machine that accelerates particles to almost the speed of light. That allowed them to unravel the speck’s chemical make-up. Plumbonacrite is a byproduct of lead oxide, allowing the researchers to say with more certainty that Leonardo likely used the powder in his paint recipe.
“Plumbonacrite is really a fingerprint of his recipe,” Gonzalez said. “It’s the first time we can actually chemically confirm it.”
The presence of plumbonacrite is “the most remarkable signature in the sample,” according to the paper’s abstract.
“Leonardo probably endeavored to prepare a thick paint suitable for covering the wooden panel of the Mona Lisa by treating the oil with a high load of lead II oxide, PbO,” the paper says.
“The analysis of fragments from the Last Supper confirms that not only PbO was part of Leonardo’s palette… but also plumbonacrite and shannonite (Pb2OCO3), the latter phase being detected for the first time in a historical painting.”
After Leonardo, Dutch master Rembrandt may have used a similar recipe when he was painting in the 17th century; Gonzalez and other researchers have previously found plumbonacrite in his work, too.
“It tells us also that those recipes were passed on for centuries,” Gonzalez said. “It was a very good recipe.”
Leonardo is thought to have dissolved lead oxide powder, which has an orange color, in linseed or walnut oil by heating the mixture to make a thicker, faster-drying paste.
“What you will obtain is an oil that has a very nice golden color,” Gonzalez said. “It flows more like honey.”
But the “Mona Lisa” — said by the Louvre to be a portrait of Lisa Gherardini, the wife of a Florentine silk merchant — and other works by Leonardo still have other secrets to tell.
“There are plenty, plenty more things to discover, for sure. We are barely scratching the surface,” Gonzalez said. “What we are saying is just a little brick more in the knowledge.”
The Horn editorial team and the Associated Press contributed to this article.