Scientists reveal the secret of how Leonardo painted the masterpiece "Mona Lisa"

The Mona Lisa has revealed another secret: Using X-rays to peer into the chemical structure of a tiny sliver of the celebrated work of art, scientists have gained new insight into the techniques Leonardo da Vinci used to paint his groundbreaking portrait of the woman with the enigmatic smile.

The research, published Wednesday in the Journal of the American Chemical Society, suggests that the famous, learned and inventive Italian Renaissance master may have been in a particularly experimental mood when he began work on the Mona Lisa in the early 16th century.

The oil paint recipe used by Leonardo as a base layer to prepare the poplar wood panel appears to have been different for the "Mona Lisa", with a special characteristic chemical composition, a team of scientists and art historians from France and Great Britain has discovered.

"He loved to experiment, and each of his paintings is technically completely different," said Victor Gonzalez, lead author of the study and a chemist at France's top research body, the CNRS. Gonzalez studied the chemical composition of dozens of works by Leonardo, Rembrandt and other artists.

"In this case, it's interesting to see that there really is a specific technique for the base layer of the 'Mona Lisa,'" he told The Associated Press.

Researchers have found the 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 assumed: that Leonardo most likely used lead oxide powder to thicken the base paint mixture and help it dry as he began work on the portrait of the woman now looking behind protective glass to the Louvre Museum in Paris.

Carmen Bambach, an Italian art specialist and curator at New York's Metropolitan Museum of Art, who was not involved in the study, called the research "very exciting" and said that any scientifically proven new insights into Leonardo's painting techniques are "extremely important news for the art world and our global society".

Finding plumbonacrite in the Mona Lisa testifies to "Leonardo's spirit of passionate and constant experimentation as a painter – that's what makes him timeless and modern," Bambach said.

The fragment of paint 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 was taken from the upper right edge of the painting.

Scientists peered into its atomic structure using X-rays in a "synchrotron," a large machine that accelerates particles to nearly the speed of light. This allowed them to discover the chemical composition. "Plumbonacrite" is a byproduct of lead oxide, which allows researchers to say with more certainty that Leonardo probably used the powder in his paint recipe.

"Plumbonacrit is really an imprint of his recipe," Gonzalez said. "This is the first time we can confirm this chemically."

After Leonardo, the Dutch master Rembrandt may have used a similar recipe when painting in the 17th century. Gonzalez and other researchers have previously found "plumbonacrite" in his work.

"It also tells us that these recipes have been passed down over the 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're going to get is an oil that has a very nice golden color," Gonzalez said. "Flows like honey".

But the "Mona Lisa," which the Louvre said was a portrait of Lisa Gerardini del Giocondo, the wife of a Florentine silk merchant, and other works by Leonardo still have secrets to uncover.

"There are many, many more things to discover, for sure. We've barely scratched the surface," Gonzalez said, and "what we're saying now is just a little bit more knowledge."