Important discovery on Leonardo da Vinci: found traces of dusting on Mona Lisa

A study of the Mona Lisa, the most famous masterpiece by Leonardo da Vinci (Vinci, 1452 - Amboise, 1519), has revealed for the first time, through noninvasive analysis conducted by digital means, that the artist created the celebrated portrait of Mona Lisa using the spolvero technique: the study will be published in the Journal of Cultural Heritage and was conducted by Pascal Cotte, an engineer at Lumière Technology laboratories, and Lionel Simonot, a specialist in optical properties of materials at the University of Poitiers. Cotte and Simonot’s discovery thus confirms that the Mona Lisa was not painted freehand: Leonardo first made a preparatory drawing and then transferred it to the final support using the spolvero technique. This was a technique, widespread among Tuscan painters, used to bring back a drawing: a cardboard with the drawing was used, which was pierced along the contours, then placed on the support and dabbed with a bag filled with charcoal. The painter would then remove the cardboard and find, on the support, a dot trace of the drawing.

The analysis was conducted with Lumière Technology at the request of the Louvre. The two scholars started from the available documentation: previous analyses, especially the latest reflectographs conducted between 2004 and 2006, had revealed the presence of pentimenti, but had not mentioned the presence of traces of the dusting. The painting was first digitized with a multispectral camera developed by Lumière Technology with the aim of maximizing optical and digital performance: the camera is capable of producing high-resolution images on 13 wavelengths (ten in the visible band, and three in the NIR band, close to the infrared band).

“The problem in finding preparatory drawings,” the study says, “lies in the fact that the charcoal pigment used for dusting does not have a spectral signature. The goal is therefore to detect very small spectral differences related to the charcoal, obviously knowing a priori the area where the preparatory drawing is located. [...] The pictorial layers of ancient paintings are usually thick, approximately, 1 millimeter, and their optical properties (absorption and scattering) allow light to penetrate according to its wavelength. However, the extreme complexity of the interaction between light and material affects all components of the painting surface: Varnishes, the variety of pigments, pigment grain size and shape, binders (e.g., oil, mastic, wax, egg), seccatives (lead), additives (glues, varnishes, putty, glass), preparation methods, overlays, thickness of each layer, preparatory drawings and surface preparation (calcium carbonate, calcite), and aging factors of materials. Without precise knowledge of the structure of the layers, it is impossible to establish a predictive model to obtain an image at the exact depth desired.” To overcome this obstacle, Cotte and Simonot’s analysis combined the images obtained from the multispectral camera with an innovative method dubbed L.A.M. (“layer amplification method”), which involves the use of different computational coefficients to be applied to the images obtained with the multispectral camera in order to derive additional images located between different wavelengths. Cotte and Simonot revealed that this method resulted in 1,650 different images.

The method, the two scholars explain, can also give false positives: the images must therefore be validated by an expert who verifies the acquired information (“it must be a painting specialist,” the two engineers say, “so that he can correctly interpret the detected signals”). To prove the goodness of the method, the two also prepared a test board, also made with the dusting technique, covering the traces of the drawing with layers of color spread with increasing thickness: the L.A.M. technique applied to the test painting allowed them to detect the dusting much better than reflectographs.

And in the Mona Lisa, traces of dusting were found in two places in the painting: on thehairline at the forehead, and on the lower edge of the right hand. These are two points in the painting where the paint surface absorbs little light, and it was therefore possible to obtain pinpoint results. Moreover, the traces on the forehead are arranged in a different position than in the finished painting, a sign that Leonardo changed his mind in the process. Finally, there is a third detail that was detected by the multispectral analysis, a small drawing near the head, whose shape almost resembles that of a hairpin, for which Cotte and Simonot have no certain explanation (“it seems to belong,” they write in the study, “to an earlier project that Leonardo later abandoned”: perhaps, they speculate, a decoration in the hair of the Mona Lisa that the artist later did not include in the final drafting).

“We have shown,” the two researchers explain in their conclusions, “that a preparatory drawing under a thick layer of white lead can emerge using a multispectral camera and the L.A.M. technique. Infrared reflectographs alone are not sufficient: it is necessary to combine spectral bands, especially in the visible zone.” And again, the research demonstrates for the first time the use of dusting on the Mona Lisa, and consequently proves the existence of a cartoon from which the work would have been taken. Moreover, this discovery, argue Cotte and Simonot, “allows us to estimate the degree of freedom Leonardo allowed himself in the execution of a painting.”

Interestingly, this finding is entirely incidental, since the purpose of the investigation was to know the spectral signature of the pigments used by Leonardo, which had been requested of Cotte and Simonot by the Centre de recherche et de restauration des musées de France. “It was a great emotion,” Cotte told the Journal of the CNRS, the Centre National de la Recherche Scientifique (French counterpart of our CNR). “I had to show the images to so many people to convince myself that I wasn’t dreaming.” Finally, Cotte and Simonot thank the Louvre for authorizing the digitization of the painting and thus making the research possible.

In the photograph below: the Mona Lisa with images obtained from Cotte and Simonot’s study highlighting where traces of the dusting were found.

Important discovery on Leonardo da Vinci: found traces of dusting on Mona Lisa

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