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Viewers of the Mona Lisa have been intrigued not only by her smile, but also by the level of perfection Leonardo da Vinci achieved in the portrait. Among the mysteries is his sfumato technique (meaning to "evaporate like smoke," the term comes from fumo, the Italian for "smoke") applied to contours and shadows, to enhance the face and provide smooth transitions. This was done with glazes, a series of extremely thin translucent layers. The oil glaze technique was widely used by early Flemish painters, but Leonardo brought it to an unprecedented level of perfection.
One reason preserving the mystery of the sfumato technique is that scientific analysis is particularly challenging, because samples, however microscopic, cannot be removed from the Gioconda or any other painting by Leonardo; their integrity must be preserved. For insight into the conservation and restoration of the paintings, the Center for Research and Restoration of the Museums of France (Centre de recherche et de restauration des musées de France, C2RMF) decided to investigate. It was under this project that Laurence de Viguerie conducted the research for her doctoral degree conferred by UPMC Paris-VI.
First stage: discovering the binder recipe
Laurence de Viguerie began by focusing on the binder, i.e., the fluid that holds the solid particles, or pigments, ensuring their consistency. In addition, it hardens over time, creating a permanent, translucent protective film. What makes the formulation of Leonado’s binder even more remarkable is that craquelure (fine cracking) is less pronounced in the shadows.
Second stage: analyzing the underlying paint layers
Laurence worked with Aglae, the Louvre’s elemental analysis particle accelerator, a sophisticated nondestructive analytical tool. An innovative methodology based on ion beam analysis was used to characterize samples of paint used by Leonardo’s contemporaries.
Third stage: X-ray fluorescence spectroscopy
The works were then examined using X-ray techniques. While X-ray investigation is now conventional, a portable setup and software developed at the European Synchrotron Radiation Facility enabled the use of X-ray fluorescence spectroscopy to identify the composition and thickness of each layer of paint.
Findings
An analysis of no fewer than 13 portraits resulted in a detailed description of how the faces were painted. The layers of paint with relatively little pigment used to produce the sfumato effect are so thin that twenty to thirty were required in order to render certain shadows. This is proof of Leonardo’s technical mastery, and also explains the considerable time he devoted to finishing each painting, as each layer had to be allowed to dry. Laurence de Viguerie’s thesis provides new perspectives for the restoration of Leonardo’s priceless works.
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