► Mona Lisa reveals Leonardo’s pictorial techniques.

· Paris: 22/04/08

Two researchers, among which a physicist from the Institut des nanosciences de Paris (CNRS/Université Paris 6/ Université Paris 7) who teaches at the Université Evry Val-Essonne, have revealed for the first time the composition of Leonardo’s famous “sfumato”, a technique used for painting faces. After virtually unvarnishing Mona Lisa, they discovered the presence of a glaze, an oil painting technique invented by Flemish Primitives and which was not yet used in Italy at this time. They also identified the components of the first pictorial layer as well as the underlayer of the painting. This research was published on the website for the journal Applied Optics.

When we admire Mona Lisa’s smile, the light which reaches our eyes has crossed several layers of paint as well as a layer of varnish which has grown yellow over time. By analyzing the composition of this light at various points on the piece of art, the physicists were able to separate out the respective effects of the varnish, the surface coat, and the underlayer. The researchers first measured the composition of the light reflected at 100,000,000 different points on the painting. This allowed them to determine the effect of the varnish on each color, and to thus virtually unvarnish the painting, revealing the unaltered color pigments. The researchers then concentrated on the pictorial effect on Mona Lisa’s face (carnation). They found a single umber pigment in the surface layer. The measurements also revealed an exceptional level of color saturation. These two characteristics are the signature of the glaze, a technique invented by Flemish Primitives and which was not yet used in Italy at the time(2).

To achieve their means without needing to take samples from the painting, the physicists combined several especially developped techniques. Firstly, the photograph of the painting was taken thanks to a multi-spectral camera, which allowed the measurement of 100,000,000 light spectra at that many points on the painting. The same type of measurements were also taken on a number of pigments used in 16th century paintings, which were either covered or not by artificially aged varnish. The comparison of the light spectra from Mona Lisa with the control spectra allowed the researchers to digitally remove the varnish from Mona Lisa and to identify the pigments in the surface layer of her face.

Once the umber pigment was identified, there were two hypotheses to explain the facial coloring. The first was a mixture of white with different proportions of pigment. The second was a glazing technique, namely a superposing of a number of layers of the same, very diluted color. The difference between these two methods is that the color saturation is much higher in the case of the glaze. A comparison of the colors measured on the face with digital simulations(3) of the two hypotheses showed without doubt that a glazing technique had been used. This comparison also shows that the glaze was applied onto a mixture of 1% vermillion and 99% lead white, a mixture commonly used by Italian painters of this period, but only for the surface paint layer and not for the underlayer.

The researchers are considering making the identification of the components of stratified pictorial layers more systematic by using significant databases containing the optical properties of numerous reference pigments and bases. This will result in a non-destructive, portable method for analyzing works of art which will yield results in a matter of minutes.

Joconde1


© Pascal COTTE 2007 (this image is available from the CNRS photo library, phototheque@cnrs-bellevue.fr)

Image 1 – Mona Lisa as it is now, in an image obtained from spectra from a multi-spectral camera


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© Pascal COTTE 2007 (this image is available from the CNRS photo library, phototheque@cnrs-bellevue.fr)

Image 2 – Mona Lisa, virtually unvarnished


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© Mady Elias

Image 3 – This diagram represents the variations in clarity according to color saturation in Mona Lisa’s face. It shows maximum saturation. The experimental results are represented by crosses, those obtained through modelization are represented by the red curve. The prolongation of the curve crosses point U, which is characteristic of Umber. The other prolongation goes through point M, which is characteristic of a mixture of 1% vermillion and 99% lead white, making up the composition of the sfumato.


Notes:

1) INSP.
2) Another Italian painter, Antonello Da Messina, upon his return to Italy from a trip to Northern Europe, supposedly spread this technique to Leonardo among others. The Lady with the Ermine, another famous painting by Leonardo (which will be “unvarnished” by the same team soon), was painted earlier than the Mona Lisa and does not contain a glaze.
3) The researchers modeled the interaction of light and matter using the equation for radiative transfer, solved by a method recently developed at INSP.

References:

M. Elias, P. Cotte, Multispectral camera and radiative transfer equation used to depict Leonardo’s sfumato in Mona Lisa, Applied Optics, 1 June 2008 / Vol. 47, No. 16

Contact information:

Researcher
Mady Elias
T 01 44 27 63 73
elias@physique.univ-evry.fr

Public Information Officer
CNRS
Laetitia Louis
T 01 44 96 51 37
laetitia.louis@cnrs-dir.fr

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