If you were thinking of throwing out that tatty old globe in a pre-Christmas de-cluttering offensive, think again. Globes - even when in poor condition - can be worth impressive sums.

Mr Tom Newth, administrator at the Scientific Instruments Department at Christie's in London says dealers and others buy dilapidated globes to renovate them. At the company's most recent globes and planetaria auction in November, a 1770 French globe in poor condition fetched £2,300 sterling, far exceeding its estimate of £400 to £600. That price was achieved despite the poor condition of the globe.

According to Mr Newth, the case was cracked and it was missing a large amount of paper and plaster. "So much of it was actually missing - it wasn't a question of patching it up here and there." But it was an early globe, by a good name - Jean Fortin - two-thirds of it was "wonderful" and it was not beyond repair.

An 1845 Newton globe fetched £6,325, also far exceeding its estimate of £2,500 to £3,000. "And that globe - one could see from the catalogue - was not in particularly good shape," he says.

A pair of 1771 three-legged mahogany globes by George Adams fetched £36,700, while a pair of 1828 Newton globes fetched £29,900. The Newton globes had been the property of the late Sir Fitzroy Hew Maclean of Dunconnel, a well-known travel writer.

"I don't think they would have made so much for themselves if it weren't for the provenance. They weren't in very good condition," Mr Newth says.

It's little wonder that globes in poor condition go for high prices given sample values achieved by restored pieces. For instance, a 12-inch diameter 1807 restored globe in good condition on an elegant mahogany three-legged stand with a compass made £8,050. "It had been restored but restored extremely well. It was interesting from a geographical point of view - there were lots of things missing or wrong," he says.

Indeed, such mistakes or changes to the names of countries are the most obvious clue to dating a globe - which can help determine its value. In dating a globe, "the first thing to look for is Tasmania which, before 1846 was Van Diemen's Land. That's a good place to start. And obviously there are lots of places you can look to figure out the age of the globe. A lot of them have the tracks of explorers - Captain Cooke or whatever - on them," he says.

Modern reproductions of important scientific instruments can also achieve high values. A 1998 armillary sphere which was finished only eight months ago fetched £11,500. "It was a reproduction of an armillary sphere made by an important 18th century instrument maker who was mathematical instrument maker to HRH Frederick, Prince of Wales, the father of George III. The original of this is in the science museum. The chap came up to London one day, saw it and thought `I could make one of those' and did so," says Mr Newth. A 1963 lunar globe sold for £3,680. Lunar globes are "not that usual" and they can make their money on their rarity, he says.

But other globes tend to fetch humbler prices - in the region of £80 or a few hundred pounds. "There were quite a number made by Columbus Verlag, a Berlin company which started around the turn of the century. "They churned out lots of globes. They're all interesting but they're not worth a huge amount of money simply because there are so many of them."

Readers who want to have a globe valued can send a photograph of the object to Christie's, Scientific Instruments Department, 85 Old Brompton Road, South Kensington, London SW7 3LD.