The electrotyping process was invented sometime around 1850, almost immediately after electrical batteries good enough to manage the process were invented. It was at first an industrial process, and the first copies of coins were probably made this way some time in the mid 1850's. They can vary in quality, and the best ones would be very dangerous fakes, except for one flaw all electrotypes have in common and is fairly easy to spot.
The process is rather simple, at least in theory. One side of a genuine ancient coin is impressed into a soft substance that captures the details of the coin. Normally a fine clay is used, but almost any substance that will take the impression can be used. The impression is dusted with a very fine conductive powder (usually graphite), and then metal (usually copper) is electroplated onto the surface, forming a thin metal shell that can have a remarkably accurate image of the original coin.
The flaw with this process is each side of the coin must be copied separately as it's own hollow metal shell, as illustrated by the electrotype of the obverse of a British medieval Cnut penny above, and with the obverse and reverse halves below.
To turn these two halved into a convincing fake, the edges have to be trimmed and the two halved joined. While for thin coins it may be possible to connect the two shells directly, thicker coins require the shells to be filled to give them strength and weight, prior to joining. Generally the shells are filled with lead, but even clay could be used. The choice of filling will significantly affect the weight of the finished fake.
This presents the forger with three problems :
1) The two halves have to be joined with a fairly strong binding agent, which usually means soldering. This joint is going to be very difficult to fit perfectly, and virtually always one can see evidence of it on the coin's edge. The example above shows the edge of the Julia Soaemias illustrated at the top, and while more poorly joined than most, it illustrates the problem. Even on good joints, traces of the line will almost always be visible.
2) Having to fill the halves makes controlling weight difficult, so electrotypes will seldom be of correct weight.
3) Electrotyping normally involves electroplating of copper, so to create fake gold or silver coins means having to then plate the finished copper fake with gold or silver, which seldom gives a convincing look.
The quality of electrotypes can vary considerably, with the limiting factor being how accurately the clay captured the image as the coin was impressed. As with casting, air bubbles can become trapped between the coin and the clay, resulting in depressions in the clay that show up as rounded lumps on the coin. Within the circled area above, the small very round bump just under her nose is almost certainly the result of such a trapped air bubble. Another problem can occure as the coin is pulled from the clay, as bits of clay may pull away with it leaving ragged depressons in the clay, which show up as irregular bumps on the coin. The slightly irregular bump to the lower right of the nose may have been caused by this, although it may also be a captured impression of a minor encrustation on the original coin.
In spite of these potential problems, if done very carefully electrotypes can capture amazingly accurate images of the original coins.
Electrotyping is no longer commonly used to make copies of ancient coins. Forgers seldom use it any more because of the difficulty in hiding the lines on the edges, but a few museums still make them for study purposes (for which they can be very useful). Because of the problems electrotypes have caused in the past, all responsible people making such copies for study purposes will provide them only as two parts, without joining. This prevents any possible confusion with originals. However, many thousands of electotypes were made fully joined in the late 19th and early 20th century, and they show up fairly often. One should take the time to learn how to recognize them, and be on the watch for them.
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