X-ray fluorescence spectrometry was done directly on the paintings in the Louvre Museum. Copyright: V.A. Solé/ESRF

How did Leonardo da Vinci manage to paint such perfect faces? For the first time a quantitative chemical analysis has been done on seven paintings by da Vinci directly in the rooms of the Louvre Museum without extracting any samples. The analysis identifies the composition and thickness of each layer of material laid down by the painter. The results reveal that, in the case of glazes, thin layers of 1 to 2 micrometers have been applied. The study, led by Philippe Walter and a team from the Laboratoire du Centre de Recherche et de Restauration des Musées de France, with the collaboration of the European Synchrotron Radiation Facility (ESRF) and the support of the Louvre Museum, was published July 15 in the journal Angewandte Chemie, International Edition.

X-ray fluorescence spectroscopy is a chemical analytical method. It consists of making lighter an area in a painting using X-rays. Later, the material emits other X-rays (fluorescence) which are characteristic of the studied area.

Leonardo da Vinci's paintings fascinate, partly due to a range of subtle optical effects that blur outlines, soften transitions, and  blend shadows like smoke. Known as sfumato, this technique is not only the result of the artist’s genius but also of technical innovations at the beginning of the 16th century. Minute observations, optical measurements, and reconstitutions have already described the sfumato, but new analysis can confirm the procedure of this technique, especially related to how the gradation is done.

The seven da Vinci paintings that have been analyzed are “Virgin of the Rocks,” “Mona Lisa,” “Saint John the Baptist,” “Annunciation,” “Bacchus,” “Belle Ferronnière,” “Saint Anne,” and the “Virgin and Child.” The scientists concentrated on the study of the faces, because they have the characteristics of the sfumato. They used a technique called X-ray fluorescence spectroscopy to determine the composition and thickness of each layer in nine faces (including Mona Lisa’s) painted by da Vinci throughout his 40-year career.

The scientists have also identified the pigments and additives of different recipes used by da Vinci as part of his glaze technique. A glaze is a fine, translucid layer, mainly organic, which contains a little bit of pigment. Superimposing glazes helps create an appearance of depth and volume. Thin layers of 1 to 2 micrometres were applied to obtain a total thickness of no more than 30 to 40 micrometres. The results obtained in this study help shed light on da Vinci's methods to make his art come alive.

Representation of the superimposition of paint layers on two sections of the Mona Lisa, a light zone near the nose and a darker shadow of the hair. After treating the data, the thickness and concentration of pigments in the different layers are shown. Copyright: C2RMF.