Analysis of Roman Silver Coins, Augustus to Nero (27 BC - AD 69)

Silver coins formed the backbone of currency in the Roman Empire and are likely to have been the main media for long-distance monetary exchange. Imperial fiscal policies and financial problems can be detected through metallurgical analysis of imperial silver coinages. Roman emperors manipulated the silver content (fineness) of the coinage to solve short-term financial problems frequently caused by government overspending. For the most part, this manipulation involved the reduction of the silver content of the coinage – debasement - in conjunction with a drop in weight. 

In the 1970s an important study was published by D. R. Walker of Oxford, documenting the silver contents of Roman Imperial silver coins by X-ray fluorescence analysis (XRF) (Walker 1976-78). This appeared to be a definitive study of the subject, and until recently was the principal authority and reference for economic historians on the monetary policies of the Roman empire. However, during the late 1980s it was realised that there were serious problems with Walker’s data. These can be attributed to a faulty technique of analysis; Roman silver coins were produced from an alloy of silver and copper, which was deliberately treated in antiquity to remove some of the copper from the surface of the coin, giving impure coins the appearance of being pure. Walker had analyzed only the surfaces of coins, and assumed that this was representative of the entire objects, with the result that his figures for the silver content are far too high and very variable. 

In 1995 the silver coinage of the Flavian Emperors issued for the city of Caesarea in Cappadocia were the subjects of an initial enquiry where samples of metal were drilled out from the interior of a representative series of coins (Butcher and Ponting 1995). Several of the coins so analysed were the same specimens as had been analysed by Walker for the ‘Metrology’ and served to underline the problems with the use of ‘non-destructive’ analysis of silver coins. The analytical technique used for this project was atomic absorption spectroscopy (AAS), which, in addition to the silver and copper in the alloy, is also capable of measuring several minor and trace elements. These additional data enabled more detailed characterization of the alloys used for the coinage and this in turn has allowed the identification of changes in mint operation and location to be identified. 

Some small projects using this approach followed, investigating aspects of the silver coinage of Trajan and Septimius Severus (Butcher and Ponting 1997 and 1998). This further demonstrated the potential of the methodology for addressing many of the questions about how Roman silver coinages functioned and related to one another. 

In 2001 funding was obtained from the Leverhulme Trust (Grant No. RF&G/6/2002/0336) and the Faculty of Arts and Sciences Research Committee and the University Research Board of the American University of Beirut, Lebanon to undertake a one year project to apply this methodology to the coinage of the first Imperial dynasty, that of the Julio-Claudians. A more sensitive analytical technique was employed, inductively-coupled plasma atomic emission spectrometry (ICP-AES), that enabled much better quality data for important trace elements, such as arsenic, tin and bismuth, to be obtained. These data, together with scanning electron microscopy (SEM), micro-analysis (SEM-EDS), optical microscopy (metallography) and some limited lead isotope analysis by laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICP-MS) form the basis of a series of publications in print (Butcher and Ponting 2005a), in press (Butcher and Ponting 2005b) and in preparation.

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