In the 19th century, however, the picture was sold for pounds 10,605 - then the highest price recorded for any painting at auction and worth pounds 630,000 at today's prices.
'It's a very charming picture with a wonderful history,' the Duke said yesterday. 'She will be restored and go on view at Chatsworth next spring, probably in the big dining room.'
Georgiana was the daughter of the first Earl Spencer and married the 5th Duke when she was only 17. Her gaming debts and an illegitimate child by Lord Grey gave the Duke the excuse to send her into exile - he had pursued an affair with Lady Elisabeth Foster from an early point in the marriage, and expected Georgiana to bring up Lady Elisabeth's children. It may be that the portrait was got rid of by the fifth Duke after Georgiana's disgrace.
Its known history begins with Anne Maginnis, a schoolmistress, who bought it in 1839 for pounds 50 and cut off the Duchess's feet in order to fit the full-length picture over her mantelpiece. She sold it to a picture restorer who reduced the painting to a half- length and sold it to a collector, Wynn Ellis, for pounds 63.
At Christie's in 1876, the Duchess was fought over by the Earl of Dudley, Baron Ferdinand de Rothschild and Agnew's, the Bond Street dealers, who emerged victorious having paid pounds 10,605.
Three days later she was stolen from Agnew's by Adam Worth, a notorious criminal of his day and the inspiration for Conan Doyle's Moriarty. Worth demanded a ransom from Agnew's but they turned him down. Worth had cut her out of the frame and then snipped small strips off the canvas and sent them by post to prove he had her - but Agnew's remained adamant, and they still have the snippets.
In 1901, shortly before his death, Worth handed her back. With the Pinkerton detective agency acting as go-between, the rolled canvas was delivered to Morland Agnew's hotel room in Chicago. New strips of canvas were added round the sides and Gainsborough's impasto was flattened by relining.
Agnew's sold her to the American millionaire Pierpont Morgan for pounds 32,000 and she was sent for sale yesterday from the estate of Morgan's granddaughter, Mabel Satterlee Ingalls.
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