Paris's Louvre Museum warned that the wood on which the Mona Lisa was painted is bending.

Her unknown identity has plagued art historians for centuries. Her enigmatic smile has seduced millions of art lovers. Now the mystery of the Mona Lisa is deepening.

Earlier this week the Louvre Museum in Paris, where the Renaissance masterpiece by Leonardo da Vinci is housed, warned that "the thin panel of poplar wood, on which this mythical image is painted, is more warped than it was previously." Its deterioration, they said, has aroused "some worry."

Repairing the world's most famous artwork is no easy task, especially since da Vinci has an uncanny way of making life difficult for conservationists. Experts are unsure of the materials the Italian artist used and their current chemical state.

"Basically nobody wants to touch it, because nobody wants to mess it up," said Henri Zerner, a French art history professor at Harvard University in Cambridge, Massachussetts. "What if you—oops!—lost the ear of the Mona Lisa?"

Sfumato

Da Vinci painted the portrait in Italy over a long period beginning in 1505. The painting was immediately celebrated as a great work of art, and da Vinci himself loved it so much that he always carried it with him, until it was eventually sold to France's King François I.

The identity of the subject has long been fiercely debated. The most likely candidate is Lisa Gherardini, the wife of a Florentine silk merchant, Francesco del Giocondo.

Another, many say outlandish, theory—revisited in novelist Dan Brown's hugely popular bestseller The Da Vinci Code—is that the painting was an androgynous self-portrait. Some see similarities between the facial features of the Mona Lisa and those of a da Vinci self-portrait painted years later.

Da Vinci used a technique known as sfumato—the blurring of sharp edges by blending colors—to leave the corners of the eyes and the mouth in shadow.

"It's an extraordinarily rich portrait. That is what portraiture is about: an effort to stop time," said David Rosand, an art history professor at Columbia University in New York. "The beauty of that face will last forever, but in life it will not."

Mocking Quality

According to Margaret Livingstone, a professor of neurobiology at Harvard University, the Mona Lisa's smile is so elusive that it disappears when looked at directly.
That's because direct vision is excellent at picking up detail, but less suited to picking up shadows. Peripheral vision, on the other hand, picks up low spatial frequencies like the blurry smile of the Mona Lisa.

"She has a mocking quality," said Livingstone, who is the author of the book Vision and Art. "When you're not looking at her, she seems to be smiling behind your back, and then you look at her and she stops."

The turbulent history of the Mona Lisa has added to its universal fame. In 1911 the painting was stolen from the Louvre by a former employee who believed it belonged in Italy. The thief walked out of the gallery with the picture under his painter's smock. He was apprehended in Florence, Italy, two years later, and the painting was safely returned.

Since then, the Mona Lisa has been frequently caricatured, dissected by psychologists like Sigmund Freud, and portrayed as a femme fatale in advertising campaigns.

"The story around the Mona Lisa [is] more famous than the painting itself," Zerner said.

Bending Wood

To preserve the fragile work, curators many years ago enclosed the Mona Lisa behind a thick pane of glass. The barrier guards against climatic changes and camera flashes from the six million people who visit the Louvre every year.

But experts say the true Mona Lisa is difficult to see, because it has been buried under thick layers of various varnishes. Over the years, the painting has gained a dull brown-and-yellow tint from chemical changes in the varnish.

Now the wood on which it's painted is also changing.

Wood is particularly difficult to repair, because it easily absorbs and releases water, changing its dimensions and shape. There is always a chance of doing more harm than good.

To make matters worse, the experts don't really know what materials da Vinci worked with.

The Center for Research and Restoration of Museums of France will now conduct a technical study to determine what materials the painting is made of and evaluate its vulnerability to temperature changes.

Said Rosand: "I would be surprised if that painting wasn't in fairly desperate need of some sort of help."