London:
For nearly 500 years, people have been gazing at Leonardo da Vinci's portrait of Mona Lisa with a sense of bafflement - first she is smiling; then the smile fades; a moment later it returns only to disappear again.
Now, scientists claim to have uncovered the secret of how da Vinci produced the optical effects that created Mona Lisa's enigmatic smile.
A team in Europe has found that the artist managed to achieve his trademark smoky effect, known as sfumato, on the painting by applying up to 40 layers of extremely thin glaze thought to have been smeared on with his fingers.
The glaze, mixed with subtly different pigments, creates the slight blurring and shadows around the mouth that give the Mona Lisa her barely noticeable smile that seems to disappear when looked at directly.
Using X-rays to study the painting, the scientists were able to see how the layers of glaze and paint had been built up to varying levels on different areas of the face, 'The Sunday Telegraph' reported.
With the drying times for the glaze taking months, such effects would have taken years to achieve.
The scientists at the Laboratoire du Centre de Recherche et de Restauration des Musees de France and European Synchrotron Radiation Facility also claim that the artist used his fingers to apply the glaze to his paintings as there're no brush marks or contours visible on paintings.
Writing in the scientific journal 'Angewandle Chemie', Dr Philippe Walter, who led the team, said: "The perfection of Leonardo da Vinci's painting technique has always been fascinating.
"The gradation of tones or colours from light to dark is barely perceptible. Above all, the way the flesh is rendered gives rise to many comments because of its crucial role in the fascination exerted by Leonardo's portraits.
"The thinness of the glaze layers must be underlined - it confirms the dexterity of the painter to apply such thin layers. Moreover, the measured slow and regular evolution of the thickness of the glaze layers implies that numerous layers... have to be applied to obtain the darkest shadows.
"Even today, Leonardo's realisation of such thin layers still remains an amazing feat."
The scientists believe that Leonardo experimented by creating different types of glaze and with different pigments to perfect the sfumato effect.
Francis Ames-Lewis of the Leonardo da Vinci Society said: "Leonardo da Vinci was concerned with producing smooth tonal gradients from light to dark without any perceptible change like we see in real life and sfumato was essential to this. In the Mona Lisa, he captures a complex and ambiguous personality and conveys it with the help of sfumato."