International. New analyses published in the Journal of the American Chemical Society show that Leonardo da Vinci's taste for experimentation extended even to the base layers beneath his paintings.
Remarkably, samples from both the "Mona Lisa" and the "Last Supper" suggest that he experimented with lead(II) oxide, which caused a rare compound called plumbonacrite to form underneath his artwork.
An aura of mystery has surrounded the paintings and pigments in da Vinci's studio, prompting scientists to explore his writings and artwork for clues. Many early 16th-century paintings, including the "Mona Lisa," were painted on wood panels that required a thick "base coat" of paint to be placed before the artwork was added.
Scientists have found that, while other artists typically used plaster, da Vinci experimented by laying down thick layers of white lead pigment and infusing his oil with lead(II) oxide, an orange pigment that conferred specific drying properties to the painting above.
He used a similar technique on the wall below the "Last Supper," a departure from the traditional fresco technique used at the time. To further investigate these unique layers, Victor Gonzalez and his colleagues wanted to apply up-to-date, high-resolution analytical techniques to small samples of these two paintings.
The team conducted their analyses on a small "microsample" previously obtained from a hidden corner of the Mona Lisa, as well as 17 microsamples obtained from the entire surface of the "Last Supper."
Using X-ray diffraction and infrared spectroscopy techniques, they determined that the soil layers of these artworks contained not only oil and lead target, but also a much rarer lead compound: plumbonacrite (Pb5(CO3)O(OH)2. This material had not previously been detected in Italian Renaissance paintings, although it was found in later paintings by Rembrandt in the 17th century.
Plumbonacrite is only stable under alkaline conditions, suggesting that it formed from a reaction between oil and lead(II) oxide (PbO). Intact grains of PbO were also found in most of the samples taken from the "Last Supper."
While painters were known to add lead oxides to pigments to help them dry, the technique has not been experimentally tested on paintings from da Vinci's time.
In fact, when the researchers searched his writings, the only evidence they found about PbO was in reference to skin and hair remedies, although it is now known to be quite toxic. Although you may not have written it down, these results demonstrate that lead oxides must have had a place on the old master's palette and could have helped create the masterpieces we know today.
The authors acknowledge the funding of the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie actions.
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