Recent research has highlighted Leonardo da Vinci's experimental use of lead(II) oxide in the base layers of his works of art, notably the Mona Lisa and The Last Supper, which may have played a role in the formation of his iconic masterpieces.
Leonardo da Vinci is world-famous for his innovations in the arts and sciences. Now, new analysis published in the Journal of the American Chemical Society shows that his taste for experimentation even extended to the underlying layers beneath his paintings. Surprisingly, samples taken from the Mona Lisa and the Last Supper show that Leonardo da Vinci experimented with lead(II) oxide, resulting in the formation of a rare compound called "plumbonacrite" underneath his artwork.
The paints and pigments in Leonardo's studio have always been shrouded in an aura of mystery, with scientists looking for clues in his writings and artwork. Many paintings from the early 15th century, including the Mona Lisa, were painted on wooden boards, requiring a thick layer of "primer" before the artwork was added. Scientists discovered that while other artists typically used gesso, Leonardo da Vinci experimented with laying down thick layers of lead white paint and adding lead(II) oxide, an orange pigment that gives the paint above it special drying properties.
This tiny speck of paint from the Mona Lisa reveals previously unknown aspects of the artist's creative process. Image source: Adapted from Journal of the American Chemical Society, 2023, DOI: 10.1021/jacs.3c07000
He used a similar technique on the wall beneath the Last Supper - a departure from the traditional fresco techniques used at the time. To further study these unique layers, Victor-Gonzalez and colleagues wanted to apply the latest high-resolution analysis techniques to small samples of the two paintings.
The team analyzed microscopic "samples" previously obtained from a hidden corner of the Mona Lisa, as well as 17 microscopic samples obtained from the surface of the Last Supper. Using X-ray diffraction and infrared spectroscopy techniques, they determined that the artwork's strata contained not only oil and white lead, but also an even rarer lead compound: plumbonacrite (Pb5(CO3)O(OH)2).
The substance has never been detected in Italian Renaissance paintings before, although it has been found in Rembrandt's late paintings from the 1600s. Sodium leadate is only stable under alkaline conditions, suggesting that it is formed from the reaction of oil and lead oxide (PbO). Intact lead oxide particles were also found in most samples from the Last Supper.
Painters are known to add lead oxide to their paints to help them dry, but this technique has not been experimentally demonstrated for paintings from Leonardo's time. In fact, when researchers looked through Leonardo da Vinci's writings, the only evidence they found for lead oxide was in regards to skin and hair treatments, even though it is now known to be highly toxic. While he may not have written it down, these results suggest that lead oxide must have had a place in the Old Master's palette and may have helped create the masterpieces we know today.