Pascal Cotte is an optical engineer with Lumiere Technology. He developed an extremely sensitive camera and has used it to scan artworks including the Mona Lisa, in order to reveal what lies beneath the surface paint, including marks and features that give clues toward the process of making the artwork.

Recently Pascal has published new findings of evidence of a spolvero, a series of pinholes used to transfer a sketch onto the wood plank canvas.

Source article: https://www.smithsonianmag.com/smart-news/theres-hidden-drawing-behind-mona-lisa

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ERIC: Science, history and art sometimes come together in the most fascinating ways. We got a great question relating to this, asking about a headline detailing a new technology, looking at layers of classic paintings and uncovering some of their origin stories. So we went straight to the engineer. My guest today is a good friend of the Museum of Science, Pascal Cotte, an optical engineer with Lumiere Technology, who has used his expertise to develop a camera that allowed him to unravel secrets surrounding perhaps the most famous painting of all time, the Mona Lisa. Pascal, thanks for calling into the podcast from across the Atlantic. How's the weather in France?

PASCAL: In France, it's raining now. But yes, it's very good. No problem.

ERIC: So I wanted to talk to you a little bit about your work with the Mona Lisa, how long have you been studying this piece of art?

PASCAL: Ah, the Mona Lisa, yes. I digitized the Mona Lisa October 2004, at the Louvre Museum, and I used a new camera. So I published last year a new discovery on Mona Lisa, using the same data that I collected in 2004. I collected 3 billion points of data on Mona Lisa. And it's taken years to analyze all of this data.

ERIC: That sounds like a whole lot of work. What does this data tell you about the painting?

PASCAL: For each pixel I have information of the interaction of the light with the materials and this interaction reveals the material inside the layer of paint. So I developed and I wrote software to extract information.

ERIC: And we were talking about your multispectral camera that you designed last time you visited the museum. It's no ordinary camera.

PASCAL: Yes, because I scanned all the wavelengths from the UV range to the infrared range and I cut small bands inside the visible range and infrared range. I increased all the technical possibilities, I go to the limit of the resolution. to the limit of the capabilities that how we can digitize, with the precision. This camera is very interesting because 20 years later, I reveal new information that today the Louvre Museum was not able to see the underdrawing of the Mona Lisa, today.

ERIC: And that's what I wanted to get to next this underdrawing or spolvero, what is it?

PASCAL: Ah, the spolvero. So, in the workshop of the master Verrocchio, where Leonardo learned the painting technique, they make a drawing on a paper and then to transfer the drawing onto the plank of poplar, on the wood - because at this time Leonardo painted only on wood - he made holes using a pin around all the line of the drawing. He put the drawing on the plank of poplar, and then he transferred with a black pigment. So the black pigment goes through the holes of the cartoon, then you remove the cartoon, and on the plank of poplar, you have only the small dots of black. Then you use a brush with very liquid pigment. And then you redraw a second time, all the outline of the drawing.

ERIC: So this discovery you've made of the existence of the spolvero - can you walk us through how it gives insight into the painting of the Mona Lisa?

PASCAL: Leonardo, of course, was a genius. No problem, but Leonardo changed his mind many times. And the spolvero reveals not only the underdrawing of Mona Lisa, but reveal that Leonardo changed his mind three times. So first, you make a drawing, probably not for Mona Lisa - probably for another project. This project could be a Virgin or Madonna. Then he changes his mind and makes the first version of Mona Lisa. But the real Mona Lisa, the real portrait of Lisa Gherardini, wife of Francesco del Giocondo. What we have published is an underdrawing of this version. And we clearly see that the head is turned slightly to the right. And of course, you have the final version today in the Louvre, with the face looking at you, the spectator. About the last version, we don't know who is this portrait. Some historians say, yes, this is the same Lisa Gherardini. Other art historians say no, no, it's a different project. So we don't know. I am an engineer, an optical engineer. The only things that I can say is: we are in front of a painting with at least three stages.

ERIC: So you are an engineer by background, how did the opportunity to scan the Mona Lisa come about?

PASCAL: Oh, it's just an opportunity one day, because I designed this camera. I am an engineer, I develop scanners. And one day I met someone of the Louvre Museum and - why can't we use your technique for painting? I said why not. Of course, scanning the Mona Lisa gave me a lot of interest. And I get a lot of demands from the art world. And so I decided, of course to invest more in this market.

ERIC: So your camera was not originally designed with classic artworks in mind?

PASCAL: No, the purpose was to obtain pictures with a very accurate color precision. When you take a photo with a usual camera, in fact, you have some colors that are quite impossible to reproduce. For the industry of fabrics, it was a big problem. Because if you have a fashion on this color, this is not the same. If you change a little, you have to be very precise when you digitize a color. And the only way to increase the precision of the color accuracy is to make it multispectral. There is no other way.

ERIC: Did that original camera look beyond visible light to infrared and ultraviolet?

PASCAL: No, no, just the visible range. But, when I worked for the Louvre Museum, I decided to increase the range covering not only the visible range, but to increase to the near infrared and the near ultraviolet, because the two are very important for works of art and to understand. Because we do not make a photo - we make a photo of course, but we also make a measurement, we make scientific measurements. And we do not measure a picture but we measure the interaction of the light and the materials. Because the layer of paint, for example for Mona Lisa, the thickness is about one millimeter. But the light can penetrate inside this layer of paint. And interact not only on the surface of the painting but interact also on the background of the painting.

ERIC: And this is where you can take the measurements from your camera and use them to learn more about the techniques of an artist. Can you give an example?

PASCAL: For Leonardo it was very important. This was very important because he paint only on a white surface, because the white reflects the light. So he had in his mind that in the final version of the painting, the light has to penetrate, reflect on the white surface, and go back to the spectator. Sometimes, when he had to make, for example, some shadows, he will extinguish the white by putting a layer of brown or black pigment, just a layer, transparent, but just to attenuate the light, because he always thinks that will make it more easy for him to paint where the shadows are already extinguished.

ERIC: And finally, what are you working on now? And is your research on the Mona Lisa is still ongoing?

PASCAL: I work on many painters in the laboratory of the University of Bologna. We work on Guercino, the famous Italian painter. But of course I continue to work on the Mona Lisa because there are a lot of things to discover.

ERIC: Well, we look forward to hearing about what else you'll find out. Pascal, Thanks so much for talking with me.

PASCAL: Thank you, and everybody. Hi Boston!

ERIC: For more on the intersection of art and science. Visit our website for our Sparks of Science video series, featuring a look at a visitor favorite at the Museum of Science our audio kinetic sculpture called Archimidean Excogitation. Until next time, keep asking questions.

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