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

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.