The world record price, 10 times what the auctioneers Christie's had estimated, was paid by a dealer who acts for a massively wealthy Japanese religious sect.
Last night, the school was still trying to come to terms with its huge windfall. John Lever, the headmaster, said: 'I am fairly up in the clouds. We are going to have a very pleasant job sitting down and wondering how to spend the money.'
The school, near Wimborne, which has 500 pupils, will probably spend almost half the pounds 6.5m it will get from the sale on a sports hall and a theatre. Its governors will meet tomorrow to decide what to do with the money.
Yesterday's extraordinary price reflects the fact that no Assyrian relief of this quality is ever likely to come on the market again. They are all in museums.
The six-foot gypsum bas-relief from the palace of the Assyrian king Ashurnasirpal II was excavated in the mid-19th century in what is now Iraq by a British archeologist. It was given to Sir John Guest, then owner of Canford Manor, who helped to pay for bringing the finds to Britain.
He incorporated it into a wall in a special building which, when the house became Canford School in 1923, was converted into the tuck shop. With a dart board next to it, covered by whitewash and thought to be a plaster copy, it remained undiscovered until recently.
The auction room was packed for the most important antiquities sale of the summer. The Japanese dealer N. Horiuchi was leaning over the back of an 18th-century sofa almost obscured by the crowd.
At the other side of the room was one of Christie's staff, with a telephone to her ear, relaying the competitor's bids in Italian. Mr Horiuchi capped every bid from his rival with a curt nod. At pounds 6.5m he thought he had it and stood up to leave but the Italian tried one more time without success.
Later the Japanese dealer came up with a succession of different explanations for his purchase. He said he intended sending the relief for exhibition at the Metropolitan Museum, New York, had bought it for a European friend and also explained that he acts as a consultant to a museum fund and that, if he recommends strongly, they buy.
The last is thought to be the most likely explanation. Mr Horiuchi is the most distinguished antiquities dealer in Japan and he is known to be forming a museum collection for a rich Japanese religious sect.
The relief depicts a eunuch carrying a mace, bow and quiver. Behind him a winged and bearded divinity is anointing his back from a cone-shaped container. It comes from the palace of the king, who ruled from 883 to 859 BC. Carved reliefs adorned the palace at Nimrud, depicting him as spiritual, military and political leader.
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