Mona Lisa: the history of the world's most famous painting by Donald Sassoon

She presents an unresolved riddle after almost 500 years. Her identity remains uncertain; her smile is still an enigma. She is regarded by some as a femme fatale, and thought by others chaste. Even her beauty is debatable.

The Mona Lisa may be the world's most famous painting, the most analysed, romanticised, satirised and appropriated, but she is also unknowable. She returns our gaze, her secrets intact. "We feel perturbed in her presence," wrote the novelist and critic, Theophile Gautier, "by her aura of superiority."

Nowadays it is not easy to gain first-hand acquaintance. She hangs in the Louvre in a large gallery of Venetian paintings. Donald Sassoon points out that the unprepared visitor, seeing the crowd and cameras, might assume that their object is not a painting but a living celebrity. Like them, the Mona Lisa needs protection. Since 1974 she has been housed in a special container, set in concrete, behind two sheets of bullet-proof triple-laminated glass. In two years' time, when we reach the presumed 500th anniversary of her birth, she will be given a room of her own.

The first to mention her was Vasari. In his monumental account of Italian painters, sculptors and architects, he reported her physiognomy in minute detail. The errors are puzzling until you learn that he never saw the picture.

Much else in Vasari's account has been unpicked, including the identity of the sitter. He tells us that she was Lisa Gherardini, the wife of Francesco del Giocondo – Monna Lisa. ("Monna" is a contraction of Madonna – my lady – and "Mona" the erroneous spelling used in English.) But the notion that she was just a Florentine housewife does not content everyone. Various arguments have been put forward to give her a more aristocratic or idealised identity.

The problem of identification is compounded by Leonardo's decision to keep the portrait. When he left Italy in 1516 to become court decorator for François I of France, he took the Mona Lisa with him, possibly unfinished. Leonardo had no heirs, and this famous Italian painting remained in France.

It became part of the royal collection, and might have languished unknown had not history played an ace. The Revolution turned the Louvre, formerly a palace, into a public art gallery. Suddenly the Mona Lisa found herself owned by the French state and housed in arguably the most famous museum in the world. Writers began to circle around her, making a cult of Leonardo, trying to analyse the source of her power. She both enchanted and disturbed, becoming a devouring temptress.

George Sand tried to inject fresh realism; she listed Mona Lisa's drawbacks and detected an undertone of cold malice in her smile. But nothing could detract from the imaginative power of Walter Pater's famous evocation. According to Kenneth Clark, art historians and aesthetes in the 1920s knew it by heart.

Sassoon is alert to the complex historical process that turned a piece of high art into a global craze.

The theft of the painting in 1911 and its return in 1913 boosted this process, as did the decision, against the advice of conservationists, to send the painting to America in 1961 and Japan in 1974. But it was the deployment of advertising and the manipulation of the image by other artists that finally made the Mona Lisa pop, hip and cool.

My only disappointment is that Sassoon tells us nothing about her journeyings in the Second World War. He clearly has the confidence of the Louvre authorities and gained access to their files. Could he not have mentioned the rumour that the Mona Lisa fell into enemy hands and was seen in the Austrian salt mine, Alt Aussee, the largest of the Nazi repositories for cultural pillage? The suggestion that this was merely a copy cannot be accepted until we are certain about the Mona Lisa's whereabouts during the last two-and-a-half years of the war.

Frances Spalding