Mona Lisa, the icon of Western painting, painted at the very beginning of the 16th century by Leonardo da Vinci, needs no introduction.
In Master Verrocchio’s workshop, Leonardo learned a method of preparing paintings which included making a drawing on the paper and then transferring it onto the wood board by spolvero. In this technique, already described by Cennini in the 15th century, the drawing is first perforated with a needle, producing holes along the contours, and then placed on the plank; the subsequent application of a powdered black pigment leaves a series of black dots.
Differently from other portraits painted by Leonardo, for instance, The Lady with an Ermine, Ginevra de’ Benci and La Belle Ferronnière, where the spolvero had been detected, no traces of this technique were discovered in Mona Lisa.
The traditional means of observing spolvero under paintings is to use IR photography and IR reflectography. Despite the advancement in this techniques, also recent studies did not evidence the presence of this technique in Mona Lisa.

In 2019, on the request of the Louvre Museum, Mona Lisa’s portrait was digitized with Lumiere Technology’s high-resolution multispectral camera. In the paper Mona Lisa’s spolvero revealed by P. Cotte and L. Simonot, published in the Journal of Cultural Heritage, Vol. 45, 2020, Pages 1-9,, the authors report a methodology which involved the use of a multispectral camera producing high-resolution images of 13 filters, including 3 IR filters with a high signal-to-noise ratio and optimal spatial resolution. Subsequently the images were processed with the layer amplification method (L.A.M.), which amplifies weak signals and thus reveals new details. To validate the approach, a test board was made to compare the L.A.M. with traditional IR reflectography.
This research paper demonstrated that a drawing underlying a thick layer of lead white can be highlighted by using a multispectral camera and the L.A.M. technique. IR alone is insufficient; it is necessary to combine spectral bands, especially in the visible range.
As with IR reflectometry, a limitation of this method is when the underlying drawing is located under a very dark pictorial layer. Notably, the more absorbent (dark) the pictorial layer, the more difficult it is to detect spolvero. This was found also in Mona Lisa, where the presented results correspond only to light areas containing lead white. The comparison with the prepared test board confirmed this hypothesis. The L.A.M. technique demonstrated to be more efficient than IR to observe the spolvero or the preparatory drawings. It is conceivable that multiple scans combined with HDR software could achieve the necessary dynamics to overcome the limits of this technique.

Through this process, the authors were able to reveal, for the first time, the spolvero in Mona Lisa, demonstrating that a preparatory drawing was employed by Leonardo also in this painting. This discovery is important to understand the creation of this masterpiece but also to contextualize the work in a period during which this technique was widely used. It also proves the existence of a cartone (drawing on paper) which can be copied indefinitely; perhaps other versions were made from this drawing.

The results of this paper, overall, enlarge the understanding on Leonardo’s technique, demonstrating once more the importance of heritage science in contributing to the knowledge of cultural heritage and thus to the art history.