Fake art and hidden gems that fooled experts at London show

A top London art gallery will Wednesday unveil an exhibition of works that had tricked its own experts, until scientific advances revealed cleverly produced fakes or unexpected treasures.

The new show at the National Gallery - called "Close examination: fakes, mistakes & discoveries" - displays some 40 pictures aimed at illustrating the challenges faced by leading experts in trying to identify art works.

"This exhibition is about looking closely at paintings as physical objects," said Betsy Wieseman, cocurator of the exhibition, which runs until September 12.

"It examines the relationship between scientific research and art historical research."

The gallery's scientific department, which was created in 1934, has used various techniques (infrared, X-rays, electronic miscroscopes) to produce a string of artistic revelations, some welcome but others less so.

One piece on display is "Group Portrait," a painting on wood acquired in 1923 and originally thought to be a work from the 15th century.

But clues indicated it had been produced much more recently - there were anachronistic details in the painting and some of its pigments were only used from the 19th century.

"Nothing in this painting is coherent with a painting of the 15th century," said Ashok Roy, the gallery's director of scientific research, adding the fake was produced in the 20th century.

The scientists' task is even more taxing when several artists have left their brushstrokes on the same canvas in their efforts to produce copies of celebrated paintings by old masters, said Wieseman.

In June 1874, the museum acquired two paintings supposedly by Italian master Sandro Botticelli: "Venus and Mars" and "An Allegory."

At the time the latter was considered the better of the pair and was deemed to be worth more. But experts eventually worked out that it was a pastiche cobbled together by one of the Renaissance painter's disciples.

"We can't ever imagine how it could have been mistaken as a Botticelli," said Wieseman

Also part of the exhibition are works which were deliberately modified for commercial reasons, such as "Portrait of Alexander Mornauer", or because of concerns over morality, such as "Woman at a Window."

The first, painted between 1464-1488 by an unidentified artist, was transformed in the 18th century so it appeared to have been the work of Hans Holbein, whose pictures were in great demand.

To achieve this, the background was painted blue and the subject's hat was changed, details which were first picked up in X-rays and then confirmed by analysing the layers of paint that make up the picture.

"We bought it knowing it was not a Holbein, and we could say it is now more valuable as it's a rare example of German portraiture of the 15th century," said Roy.

In "Woman at a Window," an Italian painting thought to have been produced in the first half of the 16th century, the alterations were made in the Victorian era in a bid to satisfy the more restrained tastes of the time.

The image of a provocative Renaissance woman had her hair changed from its original blonde to brunette, a suggestive glance was made to appear more innocent, and her bodice was rendered less revealing.