31 January 2019
A team of scientists have been using a newly-developed technique at ISIS Neutron and Muon Source to investigate a selection of Roman coins minted at different times during the Empire’s history. Their goal is to see what the coin’s silver content can tell us about the state of the Roman Empire at the time each coin was minted.
You can learn a lot about a civilisation from its coins. On a superficial level, the designs that embellish a coin’s two faces can tell us who was sitting on the throne and what their religious and cultural proclivities were at the time. But, if you are able to look deeper – into the metals that make up the coin itself – you can learn a great deal about the economic and social conditions within the issuing state, kingdom, or empire, at the time that the coin was minted. For example: a politically and economically stable state will mint coins that contain a high level of valuable metals, such as silver and gold; while a state undergoing political upheaval and economic difficulties might be tempted to mix the high-value metals with low-value metals to make the money supply stretch further – a process known as debasement.
The Roman Empire was particularly guilty of this. Until AD 64, the Empire, under a succession of long-reigning and effective Emperors, had been stable and undergoing rapid expansion. However, under the reign of Nero, they started to debase their coinage by adding copper to the silver – reducing the silver content from 100 per cent to 90 per cent. Over the next 150 years, under a succession of Emperors, their coin’s silver content was reduced to just 50 per cent. By AD 270, when the Empire itself was in decline, the silver content had been reduced to just five per cent.
When a currency is debased in this way, sooner or later the citizenry catch on to the con and start to demand higher prices for the goods they are selling and higher wages for their work – resulting in inflation. In the case of the Roman Empire, debasement eventually produced annual inflation that, according to some estimates, may have been as high as 1,000 per cent… and you don’t have to be an economist to know that isn’t good for the economy!
In an attempt to prevent the citizenry from catching on, those minting the coins did have a few tricks up their sleeves. Because a silver/copper alloy looks dull and discoloured when compared to a pure silver coin, they would employ a bit of metallurgical wizardry to give the coins the appearance of pure silver. The blank coins (coins before they have been struck) would be heated so that the copper near the surface oxidised. They would then be soaked in an acid that stripped the copper out of the alloy – leaving a surface layer of silver that gave the coins the appearance of being pure silver. This could be done in coins made up from as little as 20 per cent silver.
Fast forward a couple of millennia and the same tricks that fooled the Roman citizenry, can make it very difficult for scientists to identify whether a coin has been debased – without damaging the coin in some way, it can be difficult to identify what lies beneath the superficial layer of sliver. This is where ISIS Neutron and Muon Source comes in.
Scientists working at ISIS, together with researchers from the University of Warwick, the Ashmolean Museum, and RIKEN Nishina Center, Japan, have a developed a technique that uses muons to identify the metallurgical make-up of artefacts, such as coins, without even having to scratch the surface. They have used the technique to study a selection of coins minted during different periods of the Roman Empire in a totally non-destructive way.
The experiment was carried out at RIKEN-RAL facility at ISIS, where muons are produced by accelerating protons and smashing them in a carbon target. The muons created by these collisions are implanted into the sample under study – in this case a Roman coin. Muons are the short-lived, heavy-weight cousins of the humble electron – which orbit around the nucleus of atoms. When a muon is captured by an atom, it displaces an electron and causes the atom to emit energy in the form of X-rays. Because they have a mass 207 times greater than an electron, muons can generate powerful X-rays that can pass back out of the coin and be detected. The X-rays are unique to the atom that emitted them – making it possible to identify the elements inside the sample.
In this way, the team were able to create chemical profiles of the coins and identify their metallic composition. Their findings, that the surface composition ranged between 100 and 66 per cent silver and the interior ranged between 51 and 49 per cent silver, shows that debasement was taking place and that the technique works. Future work will apply the method to wider range of coins and be used to provide valuable detail about Roman Empire in the year that coins were minted.
Last updated: 25 February 2019