鈥淔INGERPRINTS鈥� based on impurities found in gold are the latest weapon against forgers of antique ingots. The fingerprint shows where the gold was mined, which should tally with the location on the ingot鈥檚 stamp.
Gold ingots dating from the 19th century gold rushes in the western US sell for up to 1000 times the market value of their weight. The most expensive to date, a brick from the California gold rush dubbed Eureka, sold for $8 million in 2001.
The provenance of Eureka was never in doubt because it was recovered from the wreck of the SS Central America, which sank on 8 September 1857 with a cargo of ingots from the California gold rush. But with such a lucrative mark-up, there is a huge incentive for crooks to manufacture fakes, says Fred Holabird of Holabird Mining, Environmental and Historical Consultants based in Reno, Nevada.
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Although a genuine ingot always has a set of markings detailing the manufacturer鈥檚 name, its purity, weight and dollar value, these can be hard to interpret. Many prospectors used non-standard weighing systems borrowed from the jewellery trade or drug dispensers. But even so, spotting a clumsy fake is usually fairly easy, says Holabird. For example, one such forgery 鈥� that he identified in the Smithsonian Museum鈥檚 Joseph Lilly Collection 鈥� has an 1857 tax stamp. But bullion taxes were not levied until 1864. 鈥淚f the guy鈥檚 a slob, I can bust him on the spot,鈥� says Holabird.
Now Holabird and his team have developed a technique for identifying more sophisticated forgeries. They use a laser to drill a hole in the ingot between 25 and 50 micrometres in diameter and depth, and analyse the vaporised gold for impurities using mass spectrometry. The resulting 鈥渇ingerprint鈥� is displayed as a series of peaks corresponding to the amounts of other metals mixed with the gold (see Graphic).
Although drilling even tiny holes in antiques is not popular with some collectors, the idea is to develop a database of ingots from known locations, the team told the American Academy of Forensic Sciences conference in Dallas, Texas last month. If the fingerprint of an ingot does not match the location shown on the stamp, it will be flagged up as a potential fake.
So far, the researchers have the details of around 50 ingots on their database, and Holabird says there is a long way to go. 鈥淲e need a ton more data,鈥� he says. Even so, they have made some useful discoveries. For example, high levels of tellurium are often found in Colorado gold, but rarely in Californian.