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Mints not Mines: a macroscale investigation of Roman silver coinageOpen Data

Jonathan R. Wood, Matthew Ponting and Kevin Butcher

Cite this as: Wood, J.R., Ponting, M. and Butcher, K. 2023 Mints not Mines: a macroscale investigation of Roman silver coinage, Internet Archaeology 61. https://doi.org/10.11141/ia.61.10

Summary

Matthew Ponting looking through a magnifying glass drilling into a Roman coin
Matthew Ponting drilling into a Roman coin

Although silver coins have been investigated through the lens of geological provenance to locate argentiferous ore deposits exploited in their production, we consider that this avenue of research may be a cul-de-sac, especially for studies that rely heavily on deciphering lead and silver isotope signatures that may have been altered by the addition of lead and copper (and their associated impurities) during silver refining and debasement, and by ancient recycling of coinage. Instead, we focus our attention on mints, by analysing the compositions of over 1000 silver coins from the early 1st century BC to AD 100. We propose that lead from the west Mediterranean was used exclusively to refine silver at mints in the West, and that an unknown lead supply (possibly from Macedonia), used in the East by the Late Seleucid ruler Philip I Philadelphus and later Mark Antony, was mixed with western lead. Extensive mixing of lead and/or silver coins is particularly evident under Nero and Vespasian, aligning with historically attested periods of recycling following currency reform. We further propose that coins minted in the kingdom of Mauretania used different lead and silver sources from the majority of coins minted in the western Mediterranean, and that silver coins minted at Tyre are derived from silver refined in the west Mediterranean. Coinage minted at Alexandria is consistent with debasement of recycled Roman denarii, thereby suggesting that denarii were deliberately removed from circulation to mint tetradrachms during the early Imperial Roman period.

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  • Keywords: recycling, silver, denarius, mints, bismuth, logratios, isotopes
  • Accepted: 7 July 2023. Published: 19 Sept 2023
  • Funding: This article has been funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 835180).
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  • Associated archive: Analysis of Roman Silver coins, Augustus to the reform of Trajan (27 BC - AD 100) [data-set]. York: Archaeology Data Service [distributor] https://doi.org/10.5284/1035238

Corresponding author: Jonathan R. WoodORCID logo
Jonathan.Wood2@liverpool.ac.uk
University of Liverpool

Matthew PontingORCID logo
University of Liverpool

Kevin Butcher
University of Warwick

Full text

Figure 1: Distributions of a continuous trait: a) the variance within groups is greater than the variance between groups; b) the variance between groups is greater than the variance within groups. Mixing batches (e.g. silver coins): c) a stable mean, and fluctuating variance (i.e. the mean of the mixture is the same as those of the individual batches); d) a fluctuating mean, and stable variance (i.e. the means of the individual batches converge in the mixture). Adapted from Perreault 2019, fig. 3.12.

Figure 2: Lead isotope plot for 208Pb/204Pb vs. 206Pb/204Pb for the coins from the Denarii and Provincial datasets, delineated by the geographical regions of the mints: East, Rome and West.

Figure 3: Histograms of the ages of the lead in millions of year (Ma) associated with the coins, calculated from the LIA values for the coins from the Denarii and Provincial datasets using the model and parameters of Albarède and colleagues. The data have been delineated by the geographical regions of the mints: Rome (upper left), East (upper right), West (lower left) and East and West combined (lower right).

Figure 4: Histograms of the silver concentrations (%) in the coins from the Denarii and Provincial datasets for emperors (i.e. leaders) delineated by the geographical regions of the mints: East (purple); Rome (brown) and West (green). Emperors with few coins associated with their names were not plotted as there were insufficient coins to build a histogram. Coins from the Civil War (AD 69) are anonymous. The coins from Antony include 11 legionary denarii and one cistophorus. All the Hellenistic coins were minted at Tyre. Note the different scales on the y-axis, reflecting the number of coins available for testing.

Figure 5: Boxplots of log (Au/Ag) for the coins from the Denarii and Provincial datasets, delineated by the geographical regions of the mints. The boxes indicate the medians and interquartile ranges (IQR). The whiskers indicate points that fall within 1.5 × IQR. Jitter has been applied to the points to prevent obscuring values due to overlaps.

Figure 6: Boxplots of log (Bi/Pb) for the coins from the Denarii and Provincial datasets, delineated by the geographical regions of the mints. The boxes indicate the medians and interquartile ranges (IQR). The whiskers indicate points that fall within 1.5 × IQR. Jitter has been applied to the points to prevent obscuring values due to overlaps.

Figure 7: Histograms of the ages of the lead in millions of year (Ma) associated with Roman lead ingots from Cartagena in south-east Iberia (Trincherini et al. 2009) and the ages of galena in metalliferous regions in Iberia (Milot et al. 2021). Ages calculated from the LIA values using the model and parameters of Albarède and colleagues

Figure 8: Lead isotope plot for (a) 208Pb/204Pb vs. 206Pb/204Pb and (b) 208Pb/206Pb vs. 207Pb/206Pb for the coins from the Denarii and Provincial datasets from the East geographical region. The coins plotted are those with low crustal ages (<200Ma) (see Figure 3 - upper right). The grey triangles denote the LIA values of lead ingots from Cartagena in south-east Iberia (Trincherini et al. 2009).

Figure 9: Logratio analysis (LRA) of the compositional data for the coins from the Denarii and Provincial datasets with 95% confidence ellipses around the group means of the twelve groups (large symbols: Alexandria (Al: 103), Antioch (An: 144), Antony (Ao: 11), Caesarea (Ca: 38), Cyprus (Cy:10), Ephesus (E: 20), Gaul (G: 19), Lyon (L: 60), Spanish (S: 20), Tyre (T: 9), Mauretania (M: 12), Rome (R: 577)). 61.6% of the variance is explained by the two principal dimensions.

Figure 10: a) PCA of the six logratios selected, which account for 94.1% of the total logratio variance. Since 26.0 + 51.3 = 77.3% of the variance is explained by these two dimensions, this means that 0.773 x 94.1 = 72.7% of the total logratio variance is explained by this two-dimensional solution. The main separation of groups is along dimension 1. However, the overlapping logratios of Sb/Zn, Au/Sn and Sb/Sn suggest that groups can be differentiated along dimension 2 (e.g. Lyon, Spanish and Gaul). b) Boxplots of log(Sb/Zn) for silver coins from Gaul, Lyon and Spanish mints. The boxes indicate the medians and interquartile ranges (IQR). The whiskers indicate points that fall within 1.5 × IQR. Jitter has been applied to the points to prevent obscuring values due to overlaps.

Figure 11: Scatterplot of the two logratios, log(Cu/Bi) and log(Au/Cu), for the coins from the Denarii and Provincial datasets delineated by the mints.

Figure 12: Density plots showing regions of varying densities of coins on the bivariate plot of the logratios of (Cu/Bi) and (Au/Cu) delineated by geographical region (a). There appear to be three groups (b) of high density for the East (East Groups 1, 2 and 3) and (c) Rome (Rome Groups 1, 2 and 3) and (d) one for the West (West Group 1) (although a second appears to be emerging). Coins were extracted from inside contours that enclosed a region of high density (lowest density = 0.7; highest density = 0.1): East (0.5), Rome (0.25) and West (0.25).

Figure 13: (a) Plots of Au/Ag × 100, (b) Plots of log(Bi/Pb) and (c) Plots of the Pb crustal age (Ma) for the clusters identified from the LRA and PCA delineated by regions: East, Rome and West. Note that there appears to be a bimodal signature for log(Bi/Pb) in the East and Rome data and that an approximate value of log(Bi/Pb) ≈ -5 is used to delineate the two peaks; that is low-bismuth lead <-5, and high-bismuth lead >-5. This empirical limit is applied to investigate silver coinage in Figures 14, 15 and 16.

Figure 14: Histograms of log(Bi/Pb) for the coins from the West mints for the rulers under whom the coins in the dataset were minted. Civil War coins are anonymous. The red dashed line is an empirically determined limit between lead with high and low levels of bismuth determined from Figure 13.

Figure 15: Histograms of log(Bi/Pb) for the coins from the East mints for the emperors/rulers under whom the coins in the dataset were minted. Emperors/Rulers with few coins under their name have not been plotted, unless specifically referred to in the text. Hellenistic coins are all attributed to the mint at Tyre. The coins of Philip (Philip I Philadelphus, of the Seleucid kingdom) are those minted in his lifetime (c. 94-83 BC). Posthumous Philip are coins struck with his likeness in the late Republican period. Coins of Antony are denarii from the Denarii dataset, apart from the single cistophorus (log(Bi/Pb) ≈ -3) which is from the Provincial dataset. The red dashed line is an empirically determined limit between lead with high and low levels of bismuth determined from Figure 13.

Figure 16: Histograms of log(Bi/Pb) for the coins from the Rome mint for the Emperors under whom the coins in the dataset were minted. The red dashed line is an empirically determined limit between lead with high and low levels of bismuth determined from Figure 13.

Figure 17: Lead isotope plots for (a) 208Pb/204Pb vs 206Pb/204Pb and (b) 207Pb/204Pb vs 206Pb/204Pb for the coins from the Denarii and Provincial datasets, delineated by the geographical regions of the mints: East, Rome and West. The black squares denote the LIA values of lead ingots recovered in Germania (Bode 2008; Bode et al. 2009). The red diamonds are from the LIA analyses conducted on lead pipes at Pompeii (Boni et al. 2000).

Figure 18: Lead isotope plot for (a) 208Pb/204Pb vs 206Pb/204Pb and (b) 207Pb/204Pb vs 206Pb/204Pb for the coins from the Denarii and Provincial datasets with values of log(Bi/Pb) <-5, delineated by the geographical regions of the mints: East and Rome. The black squares denote the LIA values of lead ingots recovered in Germania (Bode 2008; Bode et al. 2009). The red diamonds are from the LIA analyses conducted on lead pipes at Pompeii (Boni et al. 2000). The LIA values for British lead and silver is from Ponting (2018). The grey inverted triangles are from the Augustan shipwreck of Comacchio (Ferrara, Italy), provenanced by Bode et al. (2021) to Macedonia. The coins of Philip (minted at Antioch) and Antony highlight that coins minted in the East during the late Republican period appear to separate from those minted in the Imperial period.

Table 1: Groups of coins identified from the density plots in Figure 12. The number of coins and the mints and emperors/rulers associated with the coins in each group are presented.

Table 2: Mean, standard deviation and median of the compositions of the coins in each group identified from the density plot in Figure 12. The log(Au/Ag) and log(Bi/Pb) ratios and the Pb crustal age values determined form the LIA are also presented. n denotes the number of coins in each group. NLIA denotes the number of coins in each group with LIA measurements.

Table 3: Comparison of the compositional data of overlapping clusters (see Tables 1 and 2 for details).

Table 4: Comparison of the compositional data of East Group 1 and Rome Group 1 (see Tables 1 and 2 for details) with the calculated dilution of Rome Group 1 in a 1:3 silver to copper debasement ratio.

Table 5: Elemental and lead isotope data for seven denarii that lie below the main LIA data in Figure 2. Elemental values are in parts per million (ppm), apart from silver and copper which are in wt%.

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