I recently learned (here on CoinTalk) that a US proof coin has an "S" mintmark. How do I know a British coin is a proof? Thanks!
The only information I could find was that some proof coins were struck with a plain edge to differentiate them from circulating coins that were struck with a milled edge. Hopefully, one of the foreign coin collectors can expand on this.
Proof coins? To be able to spot proof coins, you should know a lot of things: 1. The difference between a regular minting process and a proof minting process: A proof coin is always created with a different set of dies then the circulation dies. The main difference is that the proof dies are first sanded and then polished. Due to this treatment, the proof coin will have mirrorlike fields and will have frosted details (text, figures,...). Even the blanks are treated in another way: in the normal process, these blanks are automatically dropping together in a box (possibly causing slight bruises). In a proof process, the blanks are polished and afterwards all blanks are individually stored without getting in contact with the other blanks. The blanks will be checked 100% before the actual minting is done. In a circulation strike, all blanks and coins are handled automatically, resulting in a slightly bruised coin at the end of the process (also called bag marks). In a proof process however, all coins are handled individually by 1 person on a special low volume minting machine. And last but not least: during a circulation strike, each coin is struck only once, but in a proof process each coin is struck at least 2 times. This results is slighty improved details on the coins. 2. The packing For recent coins it is quite easy to spot proof coins. Often there is a even a clue on the actual coin like an added letter or another edge. In most cases, these proof coins are delivered by the different mint production houses in a special casing and in a special packing (folder, box,...) and some of them are even delivered with a numbered certificate of authenticity. In a rare occasion, this serial number is also present on the coin itself (mostly on the edge). 3. First strike coins and proof coins If you follow these guidelines, there only remains one problem which is called first strike coins. Due to the production process of the mint dies, the actual circulation dies also has leaves frosted details and shiny fields (note the difference in word choice). If you compare the fields of a first strike coin with that of a proof coin, you will notice the difference between mirrolike and shiny. With big magnifications under a microscope you will even notice the difference in the surface. The sharpness of the details should also be a lot better on proof coins then on a first strike circulation coin. 4. Dies mixup, mint errors,... It is a known procedure that the number of proof pieces struck is higher then the number reported. The reason of this overproduction is to replace any faulty coins or coins damaged in the minting process. After the actual minting figures are met, the leftover coins should be destructed (in a coin cruher or something). However, it is a known fact within the collectors world (and especially in the roll seraching part) that some of the mint production companies put the leftovers in the bin of the circulation coins. I warned you upfront that there was a lot to understand ;-)
