A unique collection of German art accumulated by a wealthy family will be auctioned at Sotheby's in London on Tuesday.

The Beck collection was assembled by a collector and his son who ran a central heating firm in inter-war Stuttgart.

Capturing the spirit of one of modern art's most dangerous and turbulent periods, the collection features works by Kandinsky and Klee as well as noted German artists Oskar Schlemmer, Willi Baumeister, August Macke and Max Beckmann.

One work was used to plug a hole in a ceiling

"The collection provides a revelatory insight into art in Germany in the first half of the 20th century," said Sotheby's director Helena Newman.

"It is unique because of the extraordinary range of works," she said.

The 500 pieces in the collection are expected to fetch in excess of �13m.

Among the works collected by Paul Beck and his son Helmut, who died aged 91 last year, is Halbakt (half nude) painted in 1912 by Alexej von Jawlensky with a pre-sale price of �1.2m.

"This is the most stunning picture in the sale. It is very sensual," Ms Newman said.

She said the painting was once found folded in half and being used to stuff a hole in the ceiling of the studio of German artist Josef Eberz, who was a friend of von Jawlensky.

Artist friends

But Ms Newman said the painting had survived this unorthodox use and was in perfect condition.

Another section of the collection features a host of works by Stuttgart artist Adolf Holzel who offered them as payment for having a central heating system fitted in his house.

"What makes this sale so exciting is that the two generations of Becks personally knew many of the artists," Ms Newman said.

The Becks sold central heating

"Paul met Kandinsky in Paris in the 1940s and they met regularly after that to take tea together."

The Becks also patronised artists banned and harassed under Nazi rule.

"They bought a lot of their works, often at not inconsiderable risk to themselves."

The sale has been forced as the seven heirs of Helmut Beck could not agree on how to divide the collection, she said.

"This is a very personal collection. The family lived in a fairly modest Stuttgart home.

"Some pictures were on the walls, but others were simply kept in drawers. That is why they are so beautifully maintained," Ms Newman added.