Tracing the Origin of Ancient Coinage Exemplified on a Lydian Stater and a Roman Denarius

For more than 2,600 years, metal coins have been minted as the official currency of nations and states providing a standardized way for commercial exchange. Precious metals such as silver and gold were ideal metals for currency due to both their material properties and relative scarcity. The understanding of metal coin composition does not only provide hints on a society’s technological knowledge but also helps to unravel trading routes or even delivers clues on the economic situation of a state during the corresponding time.

Quantitative XRF analysis has proven an invaluable analytical technique in the field of Heritage and Conservation Science due to its non-invasive nature. Metals such as silver and gold usually have a better-defined chemical composition in analytical volume compared to other materials. Accordingly, quantitative results can contribute towards or directly help in answering research questions. This said, the non-invasive approach of quantitative XRF analysis used in Cultural Heritage studies still has its challenges. In addition, uncertainty may not only be related to the material analyzed but also the instrument or quantification algorithm used, as well as the measurement conditions applied.

Material heterogeneity at both millimeter and sub-millimeter scale especially in the depth represent one of the main challenges, since sample preparation is usually not an option. In addition, the quantification of metal alloys suffers from strong matrix effects due to the presence of high-density elements, as well as other artefacts including peaks from element line overlaps and diffraction. Despite these challenges, the analysis of metals is usually much easier than for example oxidic samples, therefore metals can still be considered an “ideal material”, suited for precise quantification. Nevertheless, these analytical challenges require a well-thought approach to ensure meaningful results.

In this article, quantitative XRF analysis will be exemplified on two case studies – the first being a Lydian gold stater, the first currency of the world, and second, a Roman silver denarius from the Roman Empire.

Fig. 1: The M4 TORNADO, benchtop micro-XRF spectrometer, being used for the analysis of ancient coins.

The Lydian Stater is considered the world’s first coin currency. The Stater was the official coin of the Lydian Empire, introduced before Lydia fell to the Persian Empire. The earliest Staters probably date from the second half of the 7^th century BC during the reign of King Alyattes (r. 619–560 BC). Beforehand, precious metals were already being used as measures of value, for example in the form of rings or ingots (bars). Coins with a standardized weight and legitimated by a governing authority, eliminated the problem of the unknown value of undefined trade currency [1].

For historians and numismatists, Lydian coinage holds significant importance due to its innovative concept: the striking or stamping of a piece of precious metal with a sort of "state seal" could grant it official recognition as currency. This indicated the state’s endorsement of the piece as acceptable payment. This key characteristic sets Lydian staters apart from earlier forms of money and establishes their connection to all later coins. 

The Lydian Stater is thought to have been composed of electrum, which likewise is a naturally occurring gold-silver alloy (Fig. 2). There has been a long discussion whether the coin alloy was of a natural origin or created on purpose. Currently, it is mostly accepted that the alloy was casted on purpose. Yet, fine gold nuggets could be panned from the rivers (Fig. 3) and further processed without purposely creating specific alloys.

Fig. 2: Lydia, Alyattes, 610–561 BC, 1/12^th Stater, ⌀ 8 mm, 1.16 g, Private collection.

Fig. 3: River gold nuggets, as can be found when panning for gold.

The actual composition of the gold could address the question of the alloys production. For this purpose, the following case study discusses the measurement campaign, results and their implementation of a 1/12th Stater from Alyattes, Lydia, dated c. 610–561 BC with a diameter of 8 mm and a weight of 1.16 g (Fig. 2).

The coin was measured on several positions using the Bruker ELIO at 50 kV, 80 µA and 120 s real time (Fig. 4). In the case of gold, no surface corrosion needs to be considered as an impeding factor. Quantitative XRF analysis is thus a relatively straightforward process. Quantitative analysis was performed using ESPRIT REVEAL Fundamental Parameter calibration (FP). For verification of the instrument's performance and for fine-tuning of the FP-based quantitative results, a set of gold reference materials was also analyzed.

Fig. 4: The Bruker ELIO measuring the Lydian Stater and a set of certified gold reference materials.

Measured values of the coin analyzed are in agreement with literature* at 52.5 ± 2 wt. % of gold, 45.6 wt.% silver and 1.69 wt.% copper. Interestingly, the composition of the coin is very close to the lowest possible gold-silver alloy with a yellow appearance (Fig. 5). However, when contextualizing the composition to the color appearance of the coin, it becomes very clear, that the gold, silver and copper content of the Lydian stater is exactly at the edge of still owning a “golden” hue. With only a few more wt% of silver, the color would significantly change to a silver tone, as can be seen in a ternary plot of the composition and color of gold, silver and copper alloys.The outcomes of the quantitative XRF analysis can thus provide a strong argument for purposely crafting specific compositions that consume the lowest amount of gold possible.

Fig. 5: Localization of the composition of the Lydian Stater in a ternary plot of gold, silver and copper. The golden coin is exactly at the edge of still owning a “golden” composition [2]. 

The denarius was the standard silver coin of ancient Rome, first introduced during the Second Punic War around 211 BC. It remained in circulation until the reign of Gordian III (AD 238–244), when it was gradually replaced. Though largely phased out, the denarius continued to be minted in very limited quantities, likely for ceremonial purposes, until the Tetrarchy period (AD 293–313). Over the era of the Roman Empire, the composition of silver coins varied significantly, depending on time and manufacturing location. Debasement of the Roman currency co-occurs with a reduction of the silver content, that was substituted by copper.

The main problem in the analysis of silver coins is the possibility of enrichment of silver on the surface either through intentional manufacturing or via cleaning processes. In the case of the Roman denarius measured, this characteristic becomes especially crucial. For this study the object of interest is a silver denarius showing Severus Alexander, who reigned 208 – 235 AC.

The coin was measured with the M4 TORNADO. The silver distribution image not only beautifully highlights the coin's surface topography, but it also shows a quite homogeneous silver intensity (Fig. 7). As the surface of silver alloys might not be representative of the actual cast, the left edge of the coin was polished and re-measured for quantitative XRF analysis.

Quantification was performed with the M4 TORNADO on basis of a FP algorithm. As in the case of the Lydian Stater, the concentration results were verified and fine-tuned using a set of certified reference materials (Fig. 8).

The mapping of the polished edge of the denarius reveals that the coin core has a higher content of copper while the exterior of the coin shows a higher silver intensity (Fig. 9, left). Quantitative results of three point measurements of each the core (Fig.8, red table) and the exterior (Fig. 9, green table) are strongly deviating. While the surface areas show a silver content of c. 95 wt% and a copper content of c. 5.7 wt%, the inside is composed of an alloy with significantly lower silver and higher copper content, being around 35 wt% and 64.7 wt%, respectively (Fig. 9). Interesting to note is also the enrichment of further traces such as gold in the exterior areas of the coin (Fig. 9, orange).

Fig. 8: Linear correlation between the measured and certified concentration of Ag in wt%, the deviation can be used for fine-tuning the FP-based calibration of the M4 TORNADO within the M4 software.

Fig. 9: Micro-XRF elemental mapping of a polished edge of the Roman denarius with the M4 TORNADO reveals a strong deviation between the core and the outer part of the coin.

The composition of the coin inside and exterior was determined with standard-supported FP-quantification.

Luckily, we know the time frame in which the coin was produced due to the displayed emperor. Severus Alexander reigned 208 – 235 AC, in the period of the Empire’s fall. As mentioned already, the silver content of Roman denarii directly correlated to the state of the Roman Empire.

Plotting the concentration results into a graph displays the decrease of the silver content on a time scale (Fig. 10), the coins core directly matches the dating expected when considering the emperor Severus Alexander (Fig. 9, coin interior), while the composition of the surface would indicate a completely false timeframe of production (Fig. 10, coin exterior).

Fig. 10: The silver content of the core at 35 ± 0.35 wt. % is in alignment with similar coins from the same period, while the exterior is not representative as it comparable to coins minted at the peak of the Roman Empire [3]. 

FP-based quantitative XRF analysis is a powerful tool for studying the composition of coinage. However, material-immanent characteristics such as surface depletion or enrichment, as well as heterogeneity, corrosion and patina must be considered. The non-invasive nature of quantitative XRF analysis is ideal for Cultural Heritage-relevant objects.

Supporting FP-based quantification with certified reference materials allows not only the verification of numerical results but also the fine-tuning of the outcome. The composition of metal alloys contains a sea of information that allows the provenance and the production of coinage to be traced.

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1. Millman, E. (2015, March 27). The Importance of the Lydian Stater as the World's First Coin. World History Encyclopedia. Retrieved from https://www.worldhistory. org/article/797/the-importance-of-the-lydian-stater-as-the-worlds/.
2. Edited version of image Ternary plot of approximate colours of Ag-Au-Cu alloys. Wikimedia Commons, 2024.
3. Edited version of image Fineness of early Roman Imperial silver coins. Wikimedia Commons, 2024.