In a back room of the British Museum that resembles the floor of a furniture warehouse, large crates are being wheeled in and carefully prised open. They look undistinguished from the outside, like a consignment of bananas headed for the supermarket shelves. But their contents, as is testified by the delicate movements of their handlers, are precious - almost unimaginably so. They hold the 80-odd works that are the centrepiece of the museum's next show, opening on Friday, called Forgotten Empire: The World of Ancient Persia. John Curtis, the museum's keeper of the ancient Near East department, invites me to peer inside one of the crates. There is a superb statue of a black dog, sitting on its haunches with a sphinx-like sense of dignified calm. "It is a very impressive piece, slightly larger than life-size, found in Persepolis," says Curtis. He looks as if he would like to stroke the dog on its shiny head, but museum practice would force him to put on a pair of plastic gloves to handle the piece. And besides, there are other treasures to admire.

Curtis does not give the impression of easy excitability, but there is no mistaking his pleasure as he gives me a quick guided tour of the pieces that have already been unpacked: a silver plate found buried in the palace of Persepolis, inscribed by Darius I, one of the great rulers of Persia's Achaemenid period, from 600BC to 400BC; a Greek statue of Penelope, booty from one of the many wars with the old enemy; some exquisitely crafted jewellery. The pieces have arrived from Tehran, where most are housed in the city's National Museum. But Curtis's relief at their safe passage has an added level of satisfaction; for until they arrived just three days ago, there were grave doubts that they would be able to take their place in the exhibition at all.