Spectroscopy sheds a little light on Leonardo’s technique for ‘Mona Lisa’

French researchers studied seven Leonardo da Vinci paintings in the Louvre Museum, including the “Mona Lisa,’’ to analyze the master’s use of successive ultrathin layers of paint and glaze — a technique that gave his works their dreamy quality.

Specialists from the Center for Research and Restoration of the Museums of France found that Leonardo painted up to 30 layers on his works to meet his standards of subtlety. Added up, all the layers are less than 40 micrometers, or about half the thickness of a human hair, researcher Philippe Walter said yesterday.

The technique, called “sfumato,’’ allowed Leonardo 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 required samples from the paintings.

The French researchers used a noninvasive technique called X-ray fluorescence spectroscopy. They brought their high-tech tool into the museum when it was closed and studied the faces in the paintings, which are emblematic of sfumato.

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 said.

The analysis shows Leonardo was constantly trying out new methods, Walter said. In the “Mona Lisa,’’ Leonardo used manganese oxide in his shadings. In others, he used copper. Often he used glazes, but not always.