X-ray and Infrared Microanalyses of Mona Lisa ’s Ground Layer and Significance Regarding Leonardo da Vinci’s Palette

An exceptional microsample from the ground layer of Leonardo da Vinci's Mona Lisa was analyzed by high-angular resolution synchrotron X-ray diffraction and micro Fourier transform infrared spectroscopy, revealing a singular mixture of strongly saponified oil with high lead content and a cerussite (PbCO 3)-depleted lead white pigment. The most remarkable signature in the sample is the presence of plumbonacrite (Pb 5 (CO 3) 3 O(OH) 2), a rare compound that is stable only in an alkaline environment. 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 review of Leonardo's manuscripts (original and latter translation) to track the mention of PbO gives ambiguous information. Conversely, the analysis of fragments from the Last Supper confirms that not only PbO was part of Leonardo's palette, through the detection of both litharge (α-PbO) and massicot (β-PbO) but also plumbonacrite and shannonite (Pb 2 OCO 3) phases were detected for the first time in a historical painting. Ferronniere (c. 1495−1497, Museé du Louvre), he applied an 50 orange oil-based ground layer, made of white and red lead 51 (Pb 3 O 4) directly on the wooden panel. A hypothesis proposed 52 by Ravaud et al. to explain these different ground layers is 53 related to the size of the different wooden panels: 2 a large 54 panel like the one used for the St. Anne would have been 55 difficult for Leonardo to build by himself and might have thus 56 been ordered in a carpenter studio where the wooden support 57 was frequently also covered by gesso (or gypsum) as was 58 common in Italy. Conversely, smaller panels such as the ones 59 used in the Belle Ferronniere or the Mona Lisa did not require 60 carpentry skills, so the wooden planks could have been 61 purchased unpainted, leaving Leonardo more freedom to 62 experiment with mixtures of his own for the ground layers. 63 From 1485 to 1490, each known easel painting of Leonardo 64 presents a different type of ground layer. Their only common 65 features are that they are oil-based and that they contain the