Who owns our past? For almost 1,000 years, the answer was simple. From the 12th century until 1997, anyone finding ancient gold or silver objects in the ground was obliged, under the common law of treasure trove, to report it. Ownership was determined by an archaic process. The system was flawed: it led to a guessing game about the intentions of someone hundreds of years into the past. It also failed to protect properly that great majority of artefacts not made from gold or silver.

Unfortunately, the statute law that replaced it, the 1996 Treasure Act, is no better. Its weaknesses have been exposed by the unhappy story of a Roman helmet found this year in Cumbria. The helmet is astonishing: a product of the greatest skill, a human face in bronze and tin of a quality far beyond most classical relics. It was made somewhere in the eastern Roman empire in the first or second century and brought to Britain around the time of the emperor Hadrian. Of a design described in the writings of Arrian of Nicomedia, it was probably worn in jousting contests. It is one of only three ever found in this country, and much the finest. It should be in a museum. Instead was sold last week at auction for £2.3m to an unknown buyer and may never be shown in public.

The story began in May, when the helmet was found by someone searching a field with a metal detector, near the Cumbrian village of Crosby Garrett. Because the helmet is not made of precious metal, and was not part of a bigger hoard, it did not fall under the scope of the Treasure Act, which would have required it to be handed to the crown in return for a reward – the process used recently to save the Staffordshire hoard from sale. Instead, it was rapidly restored and put up for sale by the auctioneers Christie's, almost before locals in Cumbria had heard of its discovery. There began a campaign to buy it for the Tullie House museum in Carlisle, near the western end of Hadrian's Wall – but last week it was forced to drop out of the bidding when the price passed £2m, far above the estimate.

No one is sure what will happen now. If the helmet was sold to a foreign buyer, an export ban could be imposed, giving the museum time to match the price. If – as rumoured – it has been sold to someone in Britain, the law cannot help. It is right that the finders of historical objects should get a reward, if only because without it artefacts would end up on the black market. But the Treasure Act is too restricted: it should be expanded to cover significant single objects not made of gold or silver. Millions were raised in 2004 to prevent the loss of Raphael's Madonna of the Pinks. The Crosby Garrett helmet – of equal beauty – should not be lost to Cumbria, where it has rested for almost 2,000 years.