|Forgery, An ancient deceptive forgery
Chinese Kai Yuan Tong Bao, 開元通寶.
This forgery of this Chinese coin demonstrates the important point that forgeries can be very old. The coin is a Kai Yuan Tong Bao from the Chinese Tang dynasty about 620 AD and this one is unusual because it's double-sided with the obverse legend on both sides. Such coins do exist but they are very rare and most examples seen are forgeries.
Such double-sided forgeries have been made from coins from almost all Chinese dynasties. The most common method of making them is to cut down two coins and bond the two obverse faces together. The best way to spot these forgeries is to carefully examine the outer rim, which must have a joint. Double-sided coins made this way often sound odd when dropped onto a hard surface. This is because the usual vibrations from a single piece of metal are dampened causing the coin to sound like it is cracked, which technically it is.
Although this coin looked convincing, it was suspicious because of its rarity and because it didn't sound right when dropped on a table. However, no joint could be seen anywhere along the rims, even with a microscope. There was also a significant amount of genuine hard green patina covering parts of the rim where the joint should be. Such dark green patina is most commonly seen on coins that are at least a few hundred years old. The patina covering the rim was continuous from the front to the back of the coin so the coin must be a single piece of metal. It was also confirmed that the patina had grown on the coin and had not been attached artificially. The coin must be real.
Suspicion remained and, possibly out of frustration, the coin was finally thrown to the floor breaking it in into two halves, revealing the truth. Indeed, the coin was a forgery made from two Kai Yuans and examination of the inside showed why it had been hard to detect. The inside surfaces where the two halves were joined were covered with crystals of a hard purple-red patina made of an oxide of copper known as Cuprite, CuO2. This type of crystalline patina cannot be faked currently, and is often seen on coins that also have the dark green patina. The purple patina usually grows in enclosed areas that are exposed to less moisture and oxygen than areas where the dark green patina grows, and these crystals grow slowly.
This forgery was obviously several hundred years old, which explains how the dark green patina was able to grow across the joint along the rim and thus hide it. The age and condition of the coin suggest it must have been buried for a long time, and most Kai Yuans were buried around the time of the Tang dynasty, over 1000 years ago. Although coin forgeries have been made for hundreds of years, this might be one of the oldest and demonstrates that just because a coin is old, it doesn't mean it's real.