By LISA ABEND / MADRID | Time.com 
Although the copy, which depicts La Gioconda with a narrower face, redder dress and significantly more pronounced eyebrows than the original, has been in the Prado's collection for centuries, no one thought much of it, and it was generally attributed to an unknown Flemish artist. But when the Prado's conservators began to study it in preparation for an upcoming show in Paris, they realized there might be more to the work than previously recognized. Using infrared technology, they detected a lush Tuscan landscape -- the same as in Leonardo's original -- hiding beneath the coat of black varnish that had been added probably in the 18th century and obscured the original background. (See whether a male model inspired Leonardo da Vinci.)
That wasn't all they found. Infrared reflectography can reveal the sketches -- called underdrawings -- and changes that a painter makes in the course of composing a work. By comparing reflectography images taken of the Mona Lisa in 2004 with the copy (they matched), Prado conservators determined that the replica was painted while Leonardo was himself still at work on the original. "There is textual evidence from contemporary observers that Leonardo had assistants in his workshop making copies," says Miguel Falomir, the Prado's curator of Italian Renaissance art. "This is the first time we've found technical evidence of it as well."
The painting is still being cleaned and restored, but the findings were presented two weeks ago at a technical conference for specialists held in conjunction with the blockbuster Leonardo exposition currently on show at the National Gallery in London. The reaction from experts in the field has been unanimously positive. "So far, I haven't heard one discordant voice," says Falomir. (See photos of theMona Lisa imitations.)
Which isn't to say that the discovery hasn't raised questions, including who painted it and when. "It had to have been a pupil, and someone very close to Leonardo at the time," says Matthew Landrus, art historian at Oxford University and the Rhode Island School of Design. "And no one was closer than Andrea Salai and Francesco Melzi." Still, he points out, that there is no definite evidence of their work. Salai, who some historians believe became Leonardo's lover, entered the workshop in 1490; Melzi, who the Prado believes to be the likelier author, joined around 1506.
But the bigger question is why Leonardo would have had his students replicating his work in the first place. Certainly the practice of making copies was not unique to him; many Renaissance artists had their students attempt to recreate their work. The reason may have been pedagogical, but more likely, says Falomir, it was financial. "When people think about these great geniuses creating, they forget that even these artists had to eat. Selling copies was a way of earning money." (Read "How a 'New' da Vinci Was Discovered.")
Landrus suggests it may have been more than that. After all, the Mona Lisa was still in Leonardo's possession when he died. It was never turned over to Francesco del Giocondo, the man who, according to 16th century biographer Giorgio Vasari, had commissioned the portrait of his wife, Lisa Gherardini, in celebration of the birth of their second son. "It's possible that Leonardo realized, 'Hey, I've got a pretty good painting here,' and had the copy made so he could keep something for himself," says Landrus. "Only later did he recognize that he didn't have to give the original away."
But Alison Wright, a specialist in Italian art at University College London who attended the meeting where the Prado discovery was presented, sees the copying as contemporary recognition of Leonardo's importance. "It's just conceivable that there was a copy made to sell, but it's an odd painting, and a commissioned portrait, so it's hard to imagine what the market would be," she says. "It's more likely that it was a matter of Leonardo's students recording his every movement, even while they were still falling from his brush." (See how experts authenticate art.)
The discovery is already causing art historians to re-examine their understanding of how Leonardo's studio functioned, and to revise the picture they have of how the most famous painting in Western art captured a singular moment between sitter and artist. "Once again," says Wright, "we see that technical analysis can shed light on a case we thought was shut."
But if the Prado copy raises new mysteries, it also clarifies some things. The newly restored copy, with its gleaming landscape in the background and sharp lines defining the spindle of the chair and the ruffle of the bodice, fills in details obscured by the yellowing varnish on the real Mona Lisa. "The original hasn't been restored in a long time," says Prado curator Falomir. "The copy invites you to see it with new eyes."
