The enigmatic smile remains a mystery, but French scientists say they have cracked a few secrets of the “Mona Lisa.” French researchers studied seven of the Louvre Museum’s Leonardo da Vinci paintings, 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 da Vinci painted up to 30 layers of paint 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 Friday.

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.