Other than ‘Not tonight’, the thing that is most remembered about the Empress Josephine is her extravagance. Her time, like ours, was an age devoted to shopping, and the tradesmen supplying luxury goods to the rich often had much more difficulty in extracting payment. When, in 1799, Josephine bought her Château de Malmaison Napoleon being engaged at the siege of Jaffa she actually borrowed FFr15,000 from the vendor in order to make up the first instalment of the payment. When she died in 1814, she left unpaid bills going back years, and a FFr3 million mountain of debt to her children, Queen Hortense and Prince Eugène de Beauharnais.

For a decade and more, Josephine was the leading celebrity of Europe, and she was very much nouveau riche ‘I was not born for such grandeur,’ she said. The great difference between her and so many later celebrities is that she had excellent taste; she became one of the foremost collectors and patrons of the empire.

The heyday of Malmaison was brief, from the beginning of 1800, when Napoleon was already First Consul, to the autumn of 1802, 18 months before he proclaimed himself emperor. In those months, not only was the little château a principal seat of government, but it was also a happy and remarkably informal family home, despite the procession of architects, decorators and furnishers. After 1802, Napoleon spent less and less time there, and when he and Josephine divorced in 1809, Malmaison was settled on her. Among her building works was a top lit gallery.

Her collection of more than 350 paintings, as well as sculpture and works of art, was not only made up of purchases and commissions:a major element was loot from the imperial campaigns. At its heart was the collection taken from the Electors of Hesse-Cassel after the Battle of Jena in 1806, including Claude’s Tobias and the Angel and Metsu’s Breakfast. Unlike the treasures Napoleon had amassed from all Europe, Josephine’s looted works were not restituted on the fall of the empire. To the fury of the Germans, they were bought by Tsar Alexander, an admirer of Josephine and friend to her children, and spirited away to Russia. This explains the presence of much of Josephine’s collection in the State Hermit-age at St Petersburg.

Now, some of the finest of these treasures will be on view in Somerset House. After her divorce, Josephine paid visits to Italy, where Prince Eugène was Viceroy, and the collection shows an unusual balance between Italian and Dutch paintings. She was on excellent terms with Canova, the leading sculptor of the day, and here is his famous Dancer, commissioned by the Empress in 1806 and delivered in 1812. She also patronised French artists, as well as porcelain manufacturers, and she possessed some of the earliest examples of the style troubadour, which is usually associated with the Restoration. A notable example is Valentina of Milan, by François Fleury Richard, bought in 1802. The background to the subject is the 1407 assassination of the duc d’Orlèans, so it would not have been possible for her to have bought the picture two years later, after Napoleon’s judicial murder of the duc d’Ehghien.

Josephine was a true leader of fashion, and despite the wars, her Empire lines were faithfully imitated even across the Channel. It would be hard to imagine a more perfect blend of sumptuous simplicity and sheer stylishness than her silver lamé tulle court dress, shown with a number of personal effects, letters and relics. A more subtle diffusion of her influence is seen in the 1801 portrait by François Gérard. She is seated at ease on a comfortable canapé (or sofa). This is thought to have been an invention of the artist, but rapidly became the latest thing.

One of the greatest treasures of the collection, the 3rd-century bc cameo portrait of Ptolemy II Philadelphus and Arsinoë, known as the Gonzaga Cameo, has a personal as well as an artistic value. It had belonged to Queen Christina of Sweden, as well as the Dukes of Mantua, and in token of the remarkable friendship that sprang up so suddenly between them, Josephine presented it to Tsar Alexander in 1814.

‘France in Russia: Empress Josephine’s Malmaison Collection’ is at the Hermitage Rooms, Somerset House, London WC2, July 25 to November 4 (020?7845 4630; www.hermitagerooms.org.uk)