Counterfeiting
To modern eyes the gold sovereign appears quite a small coin, but with a face value of £1 it had, in 1895, the same purchasing power as £150 in 2007. Gold is also a highly dense metal, so a small coin like a sovereign can contain nearly a quarter of an ounce of metal. Another reason for limiting the production of double sovereigns (£2) and quintuple sovereigns (£5) was the relative ease of removing gold from these larger coins — chemically, by filing, or using other techniques; for example, the drilling of small holes into the coin followed by hammering to conceal the holes, or "sweating": shaking a leather bag full of coins for a long period and collecting the gold dust that was knocked off.
Detection of counterfeit coins can be done either visually by comparison with known genuine coins, by using a coin gauge, or by precise weighing and measurement against standard dimensions (see Specifications above).
There are many recorded fake or counterfeit gold sovereigns in circulation, although they are still relatively scarce in comparison to the numbers of genuine coins due to the difficulty of accurately replicating such a small coin economically. With numismatic fakes, the counterfeiter might use the correct proportions of gold, but try to replicate an older coin with special rarity value. Such fakes only present a potential problem to the numismatic collector as they still contain standard bullion content.
Occasionally one comes across fake sovereign coins where the gold is replaced or alloyed with a substitute metal to look like gold. For example, a limited number of fake sovereigns did appear in circulation which were produced with 9ct gold instead of the correct 22 carat composition. Such fakes can be relatively easily detected by measurement and weighing using jewellers scales. Experienced coin dealers will generally detect such fakes immediately as they are obviously underweight or have incorrect size or thickness dimensions. It is more common to come across counterfeit copies of larger gold coins such as the Krugerrand which are easier and more economical to copy.
Gold is, however, a difficult material to counterfeit without detection due to its unique density and colour. Gold is one of the densest metals and therefore much heavier (for equivalent size or volume) than the common metals such as lead, brass, copper and steel that are used to make fake bullion coins. Fake gold coins are either oversize or underweight, or both. A fake made from lead to exactly the same thickness and diameter as a genuine sovereign would be 35% lighter than the genuine coin. If made the correct weight and diameter, it would be 54% too thick.
The sovereign is a "protected coin" for the purposes of Part II of the Forgery and Counterfeiting Act 1981.
Read more about this topic: Sovereign (British Coin)