Revealed, sort of, at last

Richard Gray has the story in the Telegraph:

Scientists have discovered how Leonardo de Vinci managed to achieve his trademark smoky effect, known as sfumato, on the painting known as the Mona Lisa; by applying up to forty layers of extremely thin glaze thought to have been smeared on with his fingers. The glaze, mixed with subtly different pigments, creates the slight blurring and shadows around the mouth that give the Mona Lisa the barely noticeable smile that seems to disappear when looked at directly.
Using X-rays to study the painting, the researchers were able to see how the layers of glaze and paint had been built up to varying levels on different areas of the face. With the drying times for the glaze taking months, such effects would have taken years to achieve. The scientists also suspect that he used his fingers to apply the glaze to his paintings as there are no brush marks or contours visible on the paintings.
Leonardo is known to have employed the sfumato effect to seamlessly blend shading together and to blur outlines. But the exact techniques used to achieve this have long fascinated and intrigued art experts.
The new discoveries have been made by scientists at Laboratoire du Centre de Recherche et de Restauration des Musees de France and the European Synchrotron Radiation Facility.
Writing in the scientific journal Angewandle Chemie, Dr. Philippe Walter, who led the study, said: "The perfection of Leonardo da Vinci's painting technique has always been fascinating. The gradation of tones or colours from light to dark is barely perceptible. Above all, the way the flesh is rendered gives rise to many comments because of its crucial role in the fascination exerted by Leonardo's portraits. The thinness of the glaze layers must be underlined: it confirms the dexterity of the painter to apply such thin layers. Moreover, the measured slow and regular evolution of the thickness of the glaze layers implies that numerous layers... have to be applied to obtain the darkest shadows. Even today, Leonardo's realisation of such thin layers still remains an amazing feat."
The scientists used a technique known as x-ray fluorescence spectroscopy to study the painting, as it allowed them to examine the layers of glaze and paint in detail without having to take samples that would damage the masterpiece. A beam of high energy X-rays were focused on the painting which allowed the researchers to determine how the layers of glaze and paint had been built up while also giving them information about their composition.
As well as the Mona Lisa, the team also studied skin tones in six other of Leonardo's most famous paintings, including Virgin of the Rocks, Madonna of the Carnation, Saint John the Baptist, and the Virgin and the Child.
They found that each layer of glaze was around just two micrometers, around fifty times thinner than a human hair. In the lightest areas of the skin, the glaze was found to be very thin, but in the darkest areas it had been built up layer upon layer to be up to 55 micrometers thick.
Grains of black and red pigment were also found in the glaze, but were so small that they would be impossible to detect using conventional analysis techniques.
Dr. Walter and his team believe that Leonardo experimented by creating different types of glaze and with different pigments to perfect the sfumato effect.
Professor Francis Ames-Lewis, a distinguished art historian and vice-president of the Leonardo da Vinci Society, an organisation devoted to the scholarly study of the Italian painter's work, said: "Leonardo da Vinci was concerned with producing smooth tonal gradients from light to dark without any perceptible change, like we see in real life, and sfumato was essential to this. What is extraordinary is the meticulous way he carried this out and the precision of his technique. In the Mona Lisa, he captures a complex and ambiguous personality and conveys it with the help of sfumato."