Chemical Analysis and Painted Colours: the Mystery of Leonardo's Sfumato

At the end of the 15th century, Italian painters explored the new effects made possible by the use of the oil medium. They created a sense of depth and relief by following the Flemish technique of glazes, which allowed the spreading of very thin and translucent layers, rich in medium and with low pigment content. A striking example is given by the realization of the shadows in the paintings of Leonardo da Vinci: the Master used the so-called ‘sfumato’ technique based on the use of glazes to obtain a ‘smoky’ aspect for the creation of flesh tones, with very subtle contours that seem to have no hard edges. Since the 16th century, his technique was famous due to the perfection of the works of art that glazes have allowed artists to reach. Analytical characterizations of painting materials have helped us to improve our knowledge about this technique. X-ray fluorescence measurements were carried out on seven paintings by Leonardo da Vinci preserved in the Louvre museum. This technique is widely used for qualitative determination of the pigments but it is very difficult to interpret the data quantitatively in the case of layered structures such as easel paintings. As well as the characterization of the palette, we obtain in-depth information on how Leonardo modelled his shadows. Comparisons between the different paintings of Leonardo highlight the specific features in the Leonardo technique.