Magnetic properties of ancient coins

The almost ubiquitous presence of small amounts of iron in ancient coins gives them remanent magnetic properties which can be measured easily on commercial magnetometers. The stability of the remanence suggests that ancient coins might retain information about the geomagnetic field at the time and location of manufacture and so be valuable in setting up archaeomagnetic dating curves. However, measurements show that the remanence directions tend to be too scattered for this purpose, although it is quite possible to distinguish magnetically the obverse and reverse faces of struck coins.Saturation remanence studies of coins and coin analogues indicate that the main carrier of the remanence in struck coins is precipitated fine-grained iron (rather than the oxides of iron) which is produced during the minting process. The intensity of the remanence generated depends upon the thermo-mechanical history of the flan prior to striking, which suggests that the magnetic properties of such coins are not simply related to the provenance of the ores used.