Leonardo da Vinci used to experiment in each of his works, not only with composition techniques, but with the materials used. To paint the Mona Lisa, one of his masterpieces, it is likely that the artist used a unique mixture of oil and lead in the preliminary layer. A group of French scientists reached that conclusion when they found plumbonacrite in a mini-fragment of the famous canvas. The presence of this mineral, whose use in the pictorial technique was unusual at that time, suggests that the Renaissance genius tried to innovate once again by applying a thick mixture on the panel on which he represented The Gioconda.
The research, published this Wednesday in the Journal of the American Chemical Society, “It provides new information about Da Vinci’s palette,” explains Victor González, one of the authors of the study, a Frenchman of Asturian origin, in a telephone conversation. The findings may be “useful for the understanding and preservation of his paintings,” says the researcher from the National Center for Scientific Research (CNRS, for its acronym in French). The European Synchrotron Radiation Laboratory (ESRF), the Louvre Museum and the French Ministry of Culture also participated in the study.
Da Vinci, a great polymath who had the ability to combine art and science in the 16th century, left behind many manuscripts in which he developed his multiple sources of interest, such as engineering or architecture. But as his study highlights, he “left few clues about the materials used in his painting.” But scientists agree on his taste for experimenting. In each of his paintings, he adds research, “the composition of the layers is different, as are the materials used.”
A tiny sample, but with a lot of information
The tiny fragment analyzed, which corresponds to the first layer that the artist applied on the famous poplar wood board, provides key information. It measures less than 100 microns – a micron corresponds to one thousandth of a millimeter – and was on the upper right side of the painting that hundreds of people pass through every day at the Louvre Museum in Paris.
To analyze this small sample, protected between two sheets of glass, the team used the synchrotron located in the city of Grenoble, a kind of giant microscope. In the laboratory, they also used infrared analysis.
The results revealed the presence of a unique mixture of oil and lead, very different from that usually observed in oil paintings of the time. “We detected by surprise a compound called plumbonacrite,” says González. Da Vinci did not use it as a pigment and it was not part of his palette, the researcher insists.
On the contrary, the substance was formed after chemical reactions in the paint itself, which implies the presence of another compound. Researchers believe that other compound is lead oxide. In other words: to paint the first layer of the Mona LisaDa Vinci would have mixed his oil with lead oxide (litharge), a mixture with which a creamy, paste-like consistency is obtained.
There are several indications to reach this hypothesis, explains Marine Cotte, co-author of the study and who works at the ESRF in Grenoble. In 2019, plumbonacrite was detected in paintings by Rembrandt, the 17th-century Dutch master. In his case, he used it for impasto on his canvases, which accentuated the sensation of chiaroscuro. “Rembrandt put us on the trail,” notes the researcher. “We thought it was worth reanalyzing the paints we were working with to see if they didn’t also contain” the same compound, she explains.
‘The Last Supper’ and the manuscripts as clues
But what really provided more evidence were the results of a parallel study that was carried out using samples of The Last Supperthe mural made by the artist between 1495 and 1498 – before the Gioconda– in the refectory of the Santa María delle Grazie convent in Milan.
In addition to finding plumbonacrite in the samples, the researchers found undissolved particles of lead oxide. “It must be said that for the Gioconda We only analyzed a very small sample. We would still have found [óxido de plomo] in another sample”, clarifies Cotte.
To try to find more clues, researchers decided to delve into the artist’s manuscripts, available online. It wasn’t that easy. The words used by Da Vinci are not the same as those of today. And painting terms differ from chemistry terms.
After a long search, they found a page from the Arundel Codex where lead oxide was mentioned. [letargirio di piombo, en el manuscrito]. Only the context in which it was used was not that of painting, but that of pharmaceuticals. “We thought that if he used it for remedies, it is likely that he used it for painting as well,” explains Cotte.
In addition to having to confirm this hypothesis, many questions still remain. It is not known if there is plumbonacrite throughout the canvas. But the study, without a doubt, provides new elements to understand the recipes that Da Vinci used and the evolution of his paintings over time.
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