Reading Roman History through Geochemistry

Reading Roman History through Geochemistry

It might seem an unlikely link – Roman History and Geochemistry. However Oxford Earth Sciences have recently been assisting Ashmolean Museum and University of Warwick CDP researcher George Green, in his quest to understand as much as possible about Roman gold coinage.

As George explains, when it comes to Roman coins,’ a lot has been achieved from numismatic and art history perspectives. We have a pretty good idea of the various typologies, including when they were minted and at what target weight. Yet significantly less scholarship focuses on exactly how these economically important objects fit into the Roman world: how were they handled and hoarded, and where did the metal to make them come from? This project (https://www.ashmolean.org/gold-coinage-roman-world) aims to change that.

The Coin Hoards of the Roman Empire web app (http://chre.ashmus.ox.ac.uk/) provides an excellent database of Roman hoards that contain gold coins. By tracing back their publication history, you can (sometimes) find the original catalogues that list the types and, importantly, the weights of the gold coins that were deposited. The weight of a gold coin from a hoard is a useful measurement. It can tell us whether the hoarder trusted the face value of the coin, regardless of its intrinsic, metallic value (as we do today), or whether he or she stored only the heaviest coins – those which contained the most gold. A hoard of Roman gold coins is thus actually a series of decisions, frozen in time. By collecting the data from multiple hoards we can turn the stories from individual hoarders into an economic narrative for the whole empire across time.’

So how does geochemistry help with this? George has been using the coins in the Ashmolean’s collection to gain an insight into how these aurei and solidi, to give them their Latin names, were made. The Department of Earth Sciences has given the project access to the Laser Ablation Inductively Coupled Plasma Mass Spectrometer LA-ICPMS, with the help of Phil Holdship and Duane Symthe. Despite its complicated name, the technique is quite simple. The coins are placed in an ablation chamber where a laser atomises an almost imperceptible amount of gold from the edge of the coin; this is then passed through a mass spectrometer. Atoms of elements have unique masses, which we can use to identify them. By identifying the trace elements present in the gold coins we can ‘chemically fingerprint’ these objects, and so attempt to determine the global provenance of the gold, which in turn can reveal key information relating to the socio-economic and political conditions at the time they were produced.

In the lab: George preparing to use the Earth Sciences LA-ICPMS to analyse the chemical composition of the coins.

Roman treasure! Some of the gold coins being analysed for their trace element compositions.

 

 

 

 

 

 

 

 

 

 

 

Before being analysed, the coins are catalogued, photographed and loaded into a specially designed plastic and silicone rubber sample holder. The holder had to allow for the edges of the coins to be ablated, while keeping them all almost perfectly level, without any damage to the objects. The effective, yet simple solution was to cut slits into a block of silicone rubber, enabling the slits to pinch the upright coins firmly in place.

Gold coins in the ablation chamber, about to be atomised by the laser. Photo courtesy of George Green.

The analyses have produced some tantalising early results. Some coins of Claudius seem to have the same chemical markers as Celtic gold pieces, for example – could this be gold brought back from Britain? Furthermore, coins minted shortly after Trajan’s invasion of Dacia (present day Romania) in the second century have elevated levels of a variety of trace elements. We know that the Romans exploited the gold mines in Dacia, but does this provide chemical proof that this gold was used to strike coinage? The analyses are still ongoing, but George hopes they will continue to answer interesting questions about the provenance and recycling of gold in the Roman world.

For more information about George’s research and the Gold Coinage in the Roman World Project, click here.