Mona Lisa gives up the secret of her smile.

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Mona Lisa's mystical smile still puts viewers under a spell. Leonardo da Vinci attained the precision and finesse of his paintings with a technique he himself perfected--sfumato (from the Italian for "foggy"). In this method, several layers of color are applied over each other. The colors meld together and lend the face a mysterious glow. Philippe Walter and his team at the Louvre in Paris have examined the faces of seven paintings signed by the master with a new noninvasive X-ray fluorescence spectroscopy technique. As the scientists report in the journal Angewandte Chemie, Mona Lisa's secret lies in many whisper-thin layers of a transparent glaze.

Da Vinci's technique is fascinating, as the gradation of color from light to dark barely is perceptible and looks natural. "Neither brushstroke nor contour is visible: lights and shades are blended in the manner of smoke," marvels Walter. The details of how the sfumato technique worked had not been determined before. The paintings were irradiated with X-rays. Every chemical element then gives off a characteristic fluorescent light, which allows the element to be quantified. "Until now, the analysis had remained qualitative, because all the pigment layers were considered simultaneously New technical advances and software have now allowed us to resolve cross-sections of the layers and to quantitatively analyze the composition...