At first glance this looks like a fairly common Legion XXI denarius of Mark Antony, just highly worn and somewhat corroded. On closer inspection there is more too it than that as it has some fairly unusual features.
For those not familiar with the term fourree, it is a French word meaning filled, which in numismatics describes a coin with a higher purity silver (and occasionally gold) foil over a baser metal core of usually a copper alloy. Most fourree are ancient counterfeits by counterfeiters who's intent was to pass them as solid metal coins. There is debate on possible officially minted fourree coins, but other than the emergency tetradrachma of Athens at the end of the Peloponnesian war and some Claudius fourree found in England which are questionable, I personally believe all fourree coins are ancient counterfeits.
Most fourree have silver foil wrapped around a copper or bronze core, usually with a layer of solder holding it on. There are a few of ways of wrapping the foil over the core, sometimes with the joint on the edge, and sometimes as with this example the joint is on one of the surfaces with two over lapping sheets of foil. There will nearly always be a joint where the foil sheets meet but on many examples it can be very difficult to see. Fortunately on the coin discussed here, the joint is very clear and we can see that the fourree maker started with two sheets, one larger than the other. The smaller sheet was placed on the reverse, just covering it. The larger sheet was placed over the obverse and wrapped around so that the edges are fully covered and this sheet then overlaps the reverse sheet. The joint is a sightly irregular circular line seen only on the reverse. Either during striking, or possibly by hitting the flan with a hammer prior to striking, the joint was flattened and while clearly visible on this specimen now, I suspect prior to the wear or weathering this coin has received, it was not so obvious when freshly minted. Here are two close ups of that joint :
This joint leaves no doubt that we are looking at a ancient fourree, not just a deteriorated and corroded official coin.
Wear and deterioration of the coating has exposed large areas of the core where we can see the most unusual feature of this example :
Unlike most fourree with copper or bronze cores, this coin has a billon (baser silver) core with the look of either badly mixed silver and copper, an alloy where the silver and copper have separated over time due to crystallization, or possibly a little of both. Note the fairly large blob of oxidized copper to the upper right which is now the entire core would look if this were just a copper core.
One might ask why a counterfeiter would reduce his profit by using a base silver core rather than just copper. A possible answer could be he was working in a time and place where banker's stamps were commonly applied to these coins to check for copper cores. Such Banker's stamps are commonly seen on Mark Antony denarii, so that may have been a concern to this counterfeiter. While a banker's stamp could easily expose a the copper, with this silvery billon core no copper would be exposed dramatically reducing risk of detection, so reducing the forgers risk as he passed his coins into circulation.
The coins specifications are :
Weight: 2.78 grams, although the coin appears very worn with enough of the plate missing that we can assume it was heavier, probably between 3.0 and 3.1 grams, when minted.
Size: 15.5 x 17.3 mm.
Thickness: 1.7 mm
Purity: While impossible to determine the exact purity of the core without better testing than I can do here, the visual appearance of the core and the separation of the silver and copper suggest an alloy of somewhere between 20 and 25% silver.
Official Mark Antony Legionary denarii are about 82% silver and range from about 3.1 to 3.85 grams, with most examples between 3.5 to 3.7 grams. A counterfeiter picking 100 official coins averaging 3.75 grams out of circulation could melt them down and add enough copper to the alloy to many about 397 new flans of 25% silver at 3.1 grams, a roughly 400% profit margin. Those coins would have the following characteristics:
1) The style of these dies is pretty good and would be difficult to detect as false when mixed with a larger group of official coins.
2) At 3.1 grams when minted they are lighter weight than normal but still within the lowest weight possible for official coins.
3) The foil coating provides for the look and feel of the higher purity official coins.
4) The billon core dramatically reduces the risk of detection via test marking.
This coin is rather worn, and that would not have happened if it had not passed successfully as official at that time.