University of Rome La Sapienza researchers examined a silver coin from the third century BC only to find out that it was actually a piece of lead coated with thin silver--antique counterfeit money. From News@Nature:
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...Link (Thanks, Mark Pescovitz!)
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.
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.