An ingenious counterfeit-coin scam has been rumbled by scientists in Italy. But no one is going to jail, because the forgers lived more than 2,000 years ago.

Giuseppe Giovannelli of the University of Rome 'La Sapienza' and his colleagues took a close look at what seemed to be a silver coin minted in southern Italy in the third century BC. It turned out to be a lump of lead with a thin silver coating.

This is not the first example of counterfeiting in the ancient world, but the researchers say that in this case the silver coating seems to have been created by a sophisticated chemical process.

"We are not yet aware of any other counterfeit coins like this one," says Giovannelli.

Tainted trove

The coin is part of a hoard that was stashed in a pottery vessel and uncovered in 1948 near the town of Parabita, which is near Gallipoli. For years it was thought to be a regular silver coin. But in 2003 an investigation by researchers at the University of Lecce, near Parabita, revealed that under the remaining patches of silver the piece was made of lead.

Giovannelli and colleagues think that the coin was minted with deception in mind. The Italian researchers have used modern techniques to study the structure and composition of the metals, and have tried to work out how the fakery was done.

A couple of simple counterfeiting methods have been spotted before. Old forgers could cover a metal lump with thin silver foil and heat it to fuse the foil on to the surface. They could also fake the look of a coin by chemically treating the surface of an alloy (which may or may not have contained precious metals) to give it a silvery or golden sheen.

But the microscopic structure of the silver layer in this case differs from that produced by either of these methods. Instead it looks like something generated by a much more modern electroplating process, say researchers. Metallurgists of the time are not thought to have known about this technique.

Copy of a copy

To solve the mystery, the Italian researchers devised a treatment that produces an effect similar to electroplating, using only materials known to be available in the third century BC.

They immersed a lead object first in copper acetate (a compound made by letting vinegar corrode copper), and then in a silver solution created by dissolving silver chloride in dilute ammonia (which ancient chemists made from urine). This gave a film with "a cauliflower-like microtexture, with close similarities to that of the residual metal coating of the coin", says Giovannelli.

Would the fakery have fooled merchants of the time? Metalsmiths knew how to detect forgeries of precious metals by measuring their density, using Archimedes' method of submersion in water to measure volume. But few would have gone to such extremes for coins handed over in a busy market, says Giovannelli.