Surface elemental and structural analysis of ancient Indian punch-marked coins (600 to 200 BCE): insights into metallurgy and economic practices

www.nature.com/scientificreports OPEN Surface elemental and structural analysis of ancient Indian punchmarked coins (600 to 200 BCE): insights into metallurgy and economic practices Amit Kumar Upadhyay1, Neeraj Kumar Giri2, Rajiv Prakash3 & Hirdyesh Mishra4 Punch-marked coins (PMCs) are the oldest coins in India and among the most widely circulated globally, often found in hoards that highlight their extensive use. This study utilizes X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to analyze the surface elemental composition and chemical properties of nine series (S-0 to S-VIII) of Janapada (S-0) and imperial PMCs (S-1 to S-VIII) dating from 600 to 200 BCE, housed in the Numismatic Society of India at BHU, Varanasi, based on the Gupta-Hardakar classification related to the PMCs. XRD results reveal four prominent diffraction peaks corresponding to metallic silver (Ag) in the face-centred cubic (fcc) phase, with a slight variation in d-spacing (∼ 0.05 Å), suggesting subtle changes in the lattice structure due to smaller atomic radius elements. XPS analysis shows the non-uniform distribution of different elements, with Ag being predominant, alongside copper (Cu), lead (Pb), and trace elements, including gold (Au), only in Janapada PMCs (S-0). The binding energy curves indicate that Ag and Cu are in pure metallic forms, while Pb exists as Pb₂O₃. Importantly, no silver or copper oxide peaks were detected, indicating the metals’ purity throughout the coinage process. The variations in d-spacing observed in these historical samples offer a microscopic perspective into the broader contexts of ancient economies, technologies, and cultural practices. The silver content in these PMCs decreases as Cu and Pb increase across the series up to S-V, followed by a sudden rise in S-VI, which lacks Pb. The presence of Pb induces brittleness and may serve as an indicator of the coins’ age. Additionally, the carbon detected by XPS could result from smelting, surface contamination, or environmental deposition. These findings reflect a high level of metallurgical knowledge and alloying techniques developed between the sixth and third centuries BCE. The varying Ag content raises questions about the economy and demand for coins. At the same time, the structural variations identified through XRD and XPS can aid archaeometallurgists in estimating these coins’ chronological and geographical origins, serving as valuable tools for authentication. Keywords Punch marked coins, XRD, XPS, Classification, 600 to 200 BCE Recently, with the advancement in various spectroscopic and microscopic techniques, the study of historically important punched marked coins become very fascinating for getting various chemical compositions at the microscopic scale1–10. Analyzing the chemical composition of ancient coins provides valuable insights to archaeologists and numismatists, revealing details about manufacturing techniques, age, economic status, minting locations, and authenticity. These coins, made from various metal alloys across different periods in Indian history, reflect the evolution of metallurgy. They also offer facts about ore origins and production methods11,12. Focusing on Indian Punch-Marked Coins (PMCs), particularly silver PMCs abundant in Asia and often found in hoards, presents complexities in classification, such as distinguishing between local and imperial 1Department of A.I.H.C. and Archaeology, Banaras Hindu University, Varanasi 221005, India. 2Department of Physics, Indian Institute of Information Technology, Bhopal 462003, India. 3School of Materials Sciences, Indian Institute of Technology, BHU, Varanasi 221005, India. 4Physics Section MMV, Department of Physics, Banaras Hindu University, Varanasi 221005, India. email: Scientific Reports | (2024) 14:30190 | https://doi.org/10.1038/s41598-024-76356-3 1 www.nature.com/scientificreports/ varieties13. The transition from local to imperial PMCs coincided with Magadha’s rise as a dominant imperial power around 600 BCE, leading to significant changes in the region’s currency system. New coins merged old punch-marked styles with new imperial symbols, minted using a multiple-die striking method. This evolution can be traced through distinct phases identified by Gupta and Hardaker14, reflecting advancements in design and size, and shaping a uniform monetary system under imperial rule. Studies on PMCs delve into their metal compositions, reflecting ancient economic theories like those in Kautilya’s Arthashastra15 from the third century BCE. Kautilya proposed combining metals like copper and silver to create different values of coins. P. L. Gupta’s 196316 research aimed to ascertain copper-silver ratios in coins, finding copper content typically at 20–30% but occasionally up to 30–50%. Ahmad and Prasad17 expanded on this by exploring additional alloying elements like tin, iron, lead, and antimony as hardening elements. Various studies of the analysis of the chemical composition on the surface of the ancient coins by nondestructive X-ray fluorescence (XRF) techniques were reported in the literature18–22. Based on XRF techniques, Vijayan et al.3 compare the Indian PMCs and Alexander’s coins revealing differences in silver element dominance in Alexander’s coins. Meenakshi et al.23 also used XRF to distinguish between genuine and fake medieval Indian silver coins. Detection of Ag and Cu on the surface by µXRF techniques shows that emphasizing the peak intensity ratios of Ag Kα/Lα, Cu Kα/Ag Kα, and Cu Lα/Ag Lα between coins suggests that employing a multi-standard approach offers the most dependable identification of Ag enrichment and Cu depletion on the surface24,25, The analysis of PMCs by particle-induced µX-ray emission (PIXE) techniques identifying a range of trace and minor elements including K, Ca, Ti, V, Cr, Mn, Co, Ni, and Rb, alongside major ones like Ag, Cu, Au, Pb and Fe enhancing our grasp of ancient coin metallurgy and economic practices. Mamania’s26 team used XRD and SEM-EDX analysis to determine that eight silver coins were made of a silver-copper alloy, extracted from argentiferous galena through cupellation. Marussi et al.27 examined 160 denarii and antoniniani using µ-EDXRF and SEM-EDX to assess composition changes and currency devaluation in Roman coins. Volpi et al.1 explored medieval denarii characteristics using p-XRF, SEMEDX, and FTIR, highlighting the complexities of analyzing diverse data from archaeological artefacts like coins, which exhibit significant heterogeneity1. These investigations illuminate ancient minting practices and the use of metal alloys to meet coinage demands. In this study, we analyze variations in copper and silver across nine-coin specimens, minted between 600 and 200 BCE, selected based on Gupta and Hardaker’s classification14. Unlike previous research, which often randomly selected coins1–24, our approach is more structured, considering both the historical and economic contex (...truncated)