Detecting Fakes and Forgeries
The majority of ancient coins on the market today are genuine, but occasionally forgeries of ancient coins turn up. Interestingly, one also will sometimes find ancient counterfeits - coins that were created in the same times as those that they mimic, but which were not genuine state-issue coins. These latter are collectors' items in and of themselves. The former, though, are our concern.
There are different reasons for forging coins. Some are truly created with the intent of fooling collectors and cheating them out of money, but more often, the "forgery" is a replica that was never intended to be taken for the real thing. Such replicas might be sold in a museum gift shop or similar venue.
There are only a few methods that modern forgers use to make imitations of ancient coins. Each of them has problems and leaves telltale signs on the coin which can be easily detected by an expert. Most, however, will fool a novice. If you suspect a coin is a fake, your best course of action is to have it authenticated by an expert appraiser. Below are the most common methods of forging coins and the telltale signs that they leave.
The forger will use sand or wax to create a mould from a genuine coin, then pour molten metal into the mould and allow it to harden. Casting coins nearly always leaves a casting seam around the edge of the coin. Attempts to file away the casting seam also leave behind telltale signs along the coin's edge. More sophisticated methods use metal moulds, lost wax casting, and plastic casting.
The electrotyping process was invented in the 1850s but is seldom used today. It involves using clay or other soft material that takes an impression. A coin is pressed into the clay. The resulting copy of the coin is dusted with conductive powder and then copper. When electricity is passed through it, a thin copper shell forms. That shell will be one side of the new coin. Each side has to be electro-typed separately, and that is the method's major flaw. Like cast coins, the electrotyping method tends to leave visible seams that must be buffed away, and even the buffing leaves evidence behind. A second problem with this method is that once the two sides are joined, the coin nearly always has to be filled with some substance to make it solid. And it's nearly always impossible to get the weight of the coin right.
Tooling involves using tools and other methods to improve the appearance of a usually genuine coin, sometimes going as far as carving an entirely new coin face on one that has been worn away. This may include something as simple as buffing down the material that makes up the patina to smooth a coin, or as elaborate as engraving details that have been lost to wear and time.
The most difficult forgeries to detect are those that use the old methods of striking coins, but they're the least likely forgery for most collectors to encounter. Because the dies to strike the coins must be carefully created, it's not worth going to all that effort unless it's to strike only a few coins of high quality. Occasionally, a forger might attempt to reproduce a hoard of a type of coin. In either case, an experienced numismatist will often be able to spot the fakes almost immediately.