Some say Leonardo's masterpiece has a twin - another portrait of the Florentine merchant's wife, painted when she was younger. Alastair Sooke investigates.
One of the surprising things about Leonardo da Vinci is that despite his colossal reputation as an artist he didn't actually produce many paintings. Over the course of a long career lasting almost half a century, he began probably no more than 20 pictures. Only 15 have survived that scholars agree are wholly his.
So the discovery of a new painting by Leonardo would be a very big deal indeed. According to recent reports, a picture by the 19th Century French artist Paul Gauguin has been sold privately for a record-breaking $300 million (£197 million). Imagine how much an authentic Leonardo could make if one ever came to the market.
The thing is, attributing artworks to Leonardo is notoriously difficult. One painting currently touring cities across Asia exemplifies what I mean. It used to be known as the "Isleworth Mona Lisa", but its current owners have rebranded it the "Earlier Mona Lisa" because they believe that Leonardo himself painted parts of it a decade or so before beginning arguably the most famous picture in the world: the Mona Lisa in the Louvre.
In December, the Isleworth painting went on show to the public in an exhibition at the Arts House at the Old Parliament building in Singapore. In March, it will travel to Hong Kong before arriving in China and then visiting other destinations in Asia.
At first glance, it is remarkably similar to the Louvre's Mona Lisa. A woman with dark hair and an enigmatic smile sits at a slight angle to the viewer on a loggia opening onto a panoramic landscape. Except this woman is obviously much younger than the subject of the Louvre painting. If Mona Lisa had been painted a decade earlier, then this is how she would appear.
So what is this unusual picture? A curious, sexed-up copy of Leonardo's masterpiece? Not according to the Mona Lisa Foundation, a Swiss non-profit organisation leading research into the painting on behalf of the anonymous international consortium that owns it. Three years ago, the foundation convened a press conference in Geneva at which they presented "the results of 35 years of research and convincing arguments" suggesting that the painting was in fact an earlier portrait of Mona Lisa, which Leonardo had left unfinished.
The event sparked headlines around the world. Here, it seemed, was a newly discovered, authentic Leonardo. How exciting! But the day after the press conference, Leonardo expert Martin Kemp, emeritus professor of art history at the University of Oxford, published a blog post rubbishing the foundation's claims and explaining in detail why he considered them to be erroneous. Perhaps the idea of a second Mona Lisa was too good to be true.

Masterpiece or copy?
In fact, art historians had known about the Isleworth Mona Lisa for some time. Shortly before World War One, the maverick English connoisseur Hugh Blaker spotted it in an old manor house in Somerset, where it had hung for more than 100 years, having been bought in Italy as an original masterpiece by Leonardo.
Sensing something special beyond the covering of dirt and varnish, Blaker acquired it and brought it to his studio in Isleworth in west London (hence the painting's moniker). Not long afterwards, his stepfather John R Eyre published a monograph proposing that Leonardo had worked on two versions of his portrait of Mona Lisa, and that the Isleworth picture was the first one.
The painting was subsequently bought by the American collector Henry F Pulitzer, who in turn published a self-serving book arguing that the Isleworth picture was in fact Leonardo's only real portrait of Mona (short for Madonna) Lisa Gherardini, wife of the Florentine merchant Francesco del Giocondo. (Pulitzer concluded that the Louvre painting was an idealised portrait of someone else.)
Yet despite all this theorising, the idea that the Isleworth Mona Lisa was actually by Leonardo never gained traction with important scholars. In 1979, following Pulitzer's death, the picture disappeared inside a Swiss bank vault, where it remained until 2008, when it was acquired by the consortium that now owns it.
At the same time, the Mona Lisa Foundation was established to investigate the question of the picture's attribution to Leonardo. According to its vice president, the stamp dealer David Feldman, the foundation "does not have any stake in the painting" and endeavours "to examine facts in the most objective light possible".
However, he would not comment directly when asked if there was any overlap between the owners of the painting and the foundation's board. And to the question of whether the consortium's ultimate goal was to sell the painting as a real Leonardo, he replied: "I cannot disclose any information relating to the consortium." ?BBC