Forgery is an age-old problem that affects just about all objects of value and one that is inherently unavoidable. A collector friend and expert on primitive art was once asked by a reporter if there were any forgeries in his collection, to which he answered yes, about 5%. When asked which ones, he responded "Well, I don't know. If I knew they wouldn't be there."
The study of forgeries can be complex but like anything else complexity can lead to interest and enjoyment. Because coins vary so much, there are often no simple detection methods and certainly no infalible experts. The idea that dealers and commercial grading houses can automatically detect forgeries is probably misleading. This is not to diminish their expertise but only to encourage one's own natural ability to look at and learn about forgeries. What is essential though is experience. Some of the best people at detecting forgeries are those that have specialized in one area for a long period of time.
Although there are now many good online resources for forgeries (some are listed here), forgeries are often posted with little or no analysis. Rather than posting more here, it might be interesting to examine some case studies. The first 10 short articles are reproduced from the book, The Historical Cash Coins of Vietnam, and thus discuss Asian forgeries. Other studies include ancient coins and some struck coins. I will continue to add more as I have time to write them and I would invite anyone with interesting examples to drop me a note and I will be happy to post them here.
After making the point there are not always simple rules for spotting forgeries, there are in fact a couple of golden rules. The first is not about coins though, it is about us.
1. Always be happy to discover your new $1000 coin is worthless.
2. A very rare $1000 coin should look just like a cheap $1 coin.
The first of course means that any emotional involvement can be like wearing blinders.
The last is obvious but the tendency not to see this difference can be surprising.
Asian Coin Forgeries
Ancient and Struck Coin Forgeries
Sometimes, the task is to prove that a coin is authentic. In some cases, authentic coins can look suspicious and in other cases a coin may not look suspicious but forgeries of the coin are known to be common, such as the old Spanish Pillar Dollars or pieces of eight.
Some Forgery Websites
Forgery Network, A large database of coin images, not much information on the coins.
Athenian Owl Studies, excellently written article by author Reid Goldsborough