French researchers have completed new studies of paintings by Leonardo da Vinci, including his "Mona Lisa." The enigmatic smile remains a mystery, but the French scientists say they have cracked a few secrets.

Specialists from the Center for Research and Restoration of the Museums of France found that da Vinci used up to 30 layers of paint on his works to meet his standards of perfection.

Added up, all the layers are less than 40 micrometers, or about half the thickness of a human hair.

The technique, called "sfumato", allowed da Vinci to give outlines and contours a hazy quality and create an illusion of depth and shadow.

Philippe Walter, CNSR Senior Researcher, said, "We have developed a new software to interpret the data, and obtain new information concerning the superimposition of the layers of paintings. In the case of "Mona Lisa", Leonardo Da Vinci used a mixture of maybe oil and resins, a binder with a very low amount of pigments. And with this mixture it was possible to create a very impressive aspect of the painting - a realistic, like a 3-D painting."

Da Vinci's technique is well-known, but scientific study on it has been limited because tests often require samples from the paintings.

The researchers used a non-invasive technique called X-ray fluorescence spectroscopy to study the paint layers and their chemical composition.

The researchers were allowed to be "face to face" with the Mona Lisa for four hours to take a whole series of measurements.