The secrets behind some of the world's most popular paintings are being explored in a new exhibition at The National Gallery.
Close Examination - Fakes, Mistakes and Discoveries demonstrates the results of investigation by scientists, art historians and conservators.
Portraits include Woman at a Window, a 16th century painting acquired by the gallery in the mid-19th century of what appeared to be an innocent, demure Victorian beauty.
It was later discovered that the woman had originally been painted as a sultry seductress - her hair was originally blonde and her bodice was more revealing before she was altered to suit more restrained Victorian tastes.
Other paintings on display include Raphael's The Madonna of the Pinks (1506-7).
The work had long perplexed art historians, with arguments about whether it was genuinely a Raphael. But infrared reflectograms revealed subtle differences between the underdrawing and the finished painting.
No copyist intending to pass off his painting as a fake would have made changes from an original drawing.
The gallery's own errors are on show at the exhibition.
It bought what it thought were two Botticellis from the collection of Alexander Barker, the son of a fashionable bootmaker, at a Christie's auction in 1874.
It was later discovered that the painting the gallery paid most for - An Allegory - was not a Botticelli.
It paid £1,050 for Venus and Mars - one of the best known works in the collection - and £1,627 for An Allegory.
Experts have also been able to solve a long-standing conundrum over authorship of The Virgin and Child with Two Angels (1476-8).
The painting had been attributed to the workshop of Andrea del Verrocchio.
After removing layers of old retouching work and examining the underdrawing with infrared reflectography, it became clear that Verrocchio painted the Virgin and the angel on the left, while his assistant Lorenzo di Credi painted the angel on the right and the infant Christ.
A Man with a Skull was thought to be by Hans Holbein but tree ring dating has since shown that the painting, "snapped up" by the National Gallery in the mid-19th century, post-dates Holbein's death in 1543.
Despite modern scientific methods, including infrared imaging and X-ray images, fakery is still a big business. Exhibition co-curator Marjorie Wieseman said it was now "extremely difficult" to sell a fake.
She said: "I like to think that we're at least one step ahead of any potential forgers. But art is more and more expensive so the rewards (for forgers) are greater and greater."
The exhibition runs at The National Gallery till September 12.