The adoption of silver currency for use in trade revolutionized the medieval economy across northwest Europe beginning in 660–670 CE. Researchers have sought to determine the origins of the coins' silver to help illuminate long-distance trade routes; periods of economic expansion and contraction; and how, where, and when the region began minting silver. A group of researchers from Cambridge University in England has recently used new, minimally invasive techniques to shed light on the geographical origins of the precious metal. The tests showed that Byzantine silver made up the earliest coins as their popularity grew. Only a century later, however, this source was overtaken by silver from a mine in Francia (roughly corresponding to modern-day France), under Charlemagne's control. See also: Archeology; Mining; Silver

A selection of coins analyzed for gleaning the origins of medieval silver coins, with Early Period coins (a, b, and c) above and Late Period coins (d, e, and f) below. [Credit: The Fitzwilliam Museum, Cambridge (CC-BY-NC-ND)]

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The scientists used geochemical testing procedures involving a combination of lead-isotope and trace-element analysis. A sample of 49 silver pennies and denarii (plural of denarius, a type of ancient Roman silver coin) from museum collections were selected for examination. The coins served as a representative sample from England, the Netherlands, and eastern Francia during two key periods of interest—namely pre- and post-750 CE. The ability to accurately date artifacts using lead isotope ratios, in tandem with a breakdown of exactly which trace elements exist in the silver, provided a framework for determining where the coins' metal originated. Elements could "contaminate" the silver, either from naturally existing in the source vein or by coming into contact with the silver during the mining and processing of the ore, or via the tooling of the coins. Some inaccuracies may inevitably have been introduced by the practice of recycling materials in the relevant time period—for instance, melting down of existing silver artifacts to mint more coins can homogenize silver samples, stripping the samples of their more unique indicators. Additionally, incorporation of lead during the silver’s refining process increases the chance of foreign lead contamination, as the lead and silver could originate from different locations. See also: Dating methods; Radiocarbon dating

With these caveats in mind, the coins were analyzed using in-situ ("in place" sampling without destroying the sample) laser ablation, removing very small amounts of material from the coins' surface using radiation. The ablated material was then examined using plasma mass spectrometry, an analytical technique that can be used to measure trace amounts of different elements. The spectrometer separates ions by their mass-to-charge ratio, enabling the determination and quantification of each compositing element. Researchers subsequently compared the analytical data with existing elemental datasets from coins of the same era and from the same locations, as well as analyses of ore from sources in Britain, the Frankish Empire, and the Mediterranean region to determine the most likely origin of each coin’s silver. The coins were ultimately divided according to which period of interest they were produced: 29 coins from the Early Period (660–750 CE) and 20 from the Late Period (750–820 CE). See also: Accelerator mass spectrometry; Mass spectroscope

The first group of coins were evidently produced in England, Frisia, and to a lesser extent, Francia. Despite being somewhat imprecise in geography and chronology, all of these early coins share high gold values (0.6–2%) and a consistent lead isotopic range. These coins showed strong correlation in isotopic and chemical features with available data for Eastern Roman/Byzantine silver, and the lowest gold concentrations in these coins fell to around 0.6–0.7% by weight. In contrast, the second group of coins contained Melle silver, which is characterized by very low gold content (below 0.1% by weight). New coins had begun to be minted from finer silver, decorated with the names of kings. The prevalence of these finer coins initially varied by region but became much more widespread and consistent in use as spurred by coinage reforms by Charlemagne in 793 CE. The data suggest that the spread of Melle silver was the result of an intentional administrative decision, which can help in understanding the political and trade atmosphere of the region at that time. Byzantine silver for the earlier coins was likely imported and accumulated around the sixth and early seventh centuries, but the existence of surviving artifacts and lack of coins produced after a certain point suggests that the shift in source was not due to scarcity. Existing stockpiles were utilized early on, but the mining of a new silver source for coinage would suggest a significant economic shift. As time went on, the composition of the coins became and remained more consistent, further supporting the idea that the shift was intentional and coordinated. See also: Alloy; Silver alloys; Silver metallurgy

Overall, the study demonstrated the usefulness of minimally invasive sampling techniques for collecting data from museum artifacts, enabling archeologists to date and geologically track the creation and movement of these items without destroying them. The researchers hope to inspire similar research into historical patterns of resource acquisition and movement. See also: Archeological chemistry; Archeological chronology