Yahoo: "The technique, called "sfumato," allowed da Vinci to give outlines and contours a hazy quality and create an illusion of depth and shadow. His use of the technique is well-known, but scientific study on it has been limited because tests often required samples from the paintings.
The French researchers used a noninvasive technique called X-ray fluorescence spectroscopy to study the paint layers and their chemical composition.
They brought their specially developed high-tech tool into the museum when it was closed and studied the portraits' faces, which are emblematic of sfumato. The project was developed in collaboration with the European Synchrotron Radiation Facility in Grenoble.
The tool is so precise that "now we can find out the mix of pigments used by the artist for each coat of paint," Walter told The Associated Press. "And that's very, very important for understanding the technique."