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The collection of German-born arms maker Emil Buehrle hit the headlines in February 2008 when masked robbers made off with major works by Cezanne, Degas, Monet and Van Gogh in Switzerland's biggest-ever art theft.

The show will give the public a sneak preview of the collection before it moves residence in 2015 to a new wing of the Kunsthaus designed by British architect David Chipperfield.

The magnate's collection, which contains around 180 paintings and sculptures including some of the world's finest works of Impressionism and post-Impressionism, is usually housed in a villa adjoining Buehrle's former home by Lake Zurich.

The special event, which runs until May 16, is one of the highlights in a series to mark the centenary of the Kunsthaus. The series culminates in a show revisiting Picasso's first museum retrospective, held in the Jugendstil building in 1932.

"The presentation here can be considered as a dress rehearsal of this potential move," Lukas Gloor, the collection's director and show's co-curator, said in an interview with Reuters, adding the new location was not just being considered on security grounds.

"The uniting of the Buehrle collection with the Kunsthaus collection is aimed at addressing a larger audience than the one we can draw to the current premises and placing it in the central location it rightfully belongs."

The new wing of the Kunsthaus promises to turn Zurich into Europe's leading center for French Impressionism outside Paris.

Two of the Buehrle collection's four works, including its trademark "Boy in the Red Vest" by Cezanne, are still missing and the collection has been closed to viewers ever since the robbery, except for those arranged by special appointment.

"The immediate consequence for our collection's museum was the fact that we had pretty much to close it down," Gloor said. "We were no longer a publicly accessible museum."

Gloor was moderately optimistic about the chances of retrieving the missing masterpieces.

"A great many of the pictures of that level that are stolen are recovered in the two to five years after a robbery. If not, then statistics, I'm afraid, turn against us."

NAZI LOOT

Buehrle studied literature and art history and served in the German army in World War I before amassing a fortune from the Oerlikon tool and munitions works in Zurich. The firm sold weapons to Germany after World War One, helping the country to re-arm covertly while it was banned from making its own arms under the Treaty of Versailles.

Gloor discounted any notion of using the Kunsthaus's backing to add to Buehrle's collection.

"We see the collection as a witness of a certain period in history and of a particular individual's tastes. For us the collection is complete," he said.

In the late 1940s, Buehrle was forced to return 13 works bought during World War II and subsequently found to have been looted by the Nazis from Jewish families in France, though a Swiss court ruled he had bought them in good faith.

"Buehrle's collecting was from a very early stage onwards overshadowed by questions of stolen art," Gloor said, adding Buehrle offered to buy back all 13 works and was successful in nine cases.

Oerlikon sold arms to both sides during World War II and Buehrle was able to splash out large sums on a number of major works in the Cold War era as orders rolled in from the United States and other NATO members.

Though much of Buehrle's art was collected after the war and before his death in 1956, controversy returned in the 1990s amid fresh questions surrounding the provenance of some works.

"We are today in the very fortunate position that the collector very actively addressed these issues," Gloor said of Buehrle's careful documentation of further purchases.

The exhibition addresses this theme by highlighting the incomplete records of ownership of Manet's "Sultane," a mysterious oriental beauty in a diaphanous shift and veil once owned by a Jewish industrialist who died in Auschwitz.

"We have come to the conclusion that there are too many missing links in this case for us to feel pressed to restitution," said Gloor.

(Editing by Stephanie Nebehay and Paul Casciato)