Numismatics and Linked Open Data

The Opportunity

As a type of evidence for the ancient world, coinage is unique. Coins are monetary objects, and thus a key element in modern attempts to reconstruct the workings of the ancient economy. For example, through the process of “die-study” it is possible to determine with some degree of accuracy how many dies were used to strike a given coinage. This provides us with a way to quantify ancient monetary production. Since coins can also be attributed to particular rulers or cities with some degree of certainty, this makes it possible to ascertain the monetary output of different cities, kingdoms and empires, and to compare them with one another. There now exists a substantial body of scholarship devoted to the estimation of size of production, but comparatively little as yet to its broad analysis or representation in interactive media such as timelines or maps.²⬈#p1

Coins are also archaeological objects in that they have findspots. Coins within archaeological contexts have much to tell excavators about the contexts they are digging, but also more broadly about the monetary profile of the site they are excavating compared to others of similar or different types; from multiple sites a regional history may emerge (see, e.g., Reece, 1982). But findspots also give coins a trajectory. If we know where a coin was made and where it was found, we have evidence for movement, connectivity, and economic circulation (see Map 1).⬈#p2

Few archaeological objects from antiquity can be mapped from source to deposition with such certainty as coins, and yet again we are only beginning to exploit the possibilities of this evidence in analytical and representational tools. Moreover, with the advent of the metal detector, individual coin finds and their recording are no longer confined to excavation material. The Portable Antiquities Scheme (PAS) in the United Kingdom, for example, now has recorded findspots for some 412,000 coins (Pett 2014; see Map 2). The PAS has led to new works of synthesis (e.g., Leins, 2012; Walton, 2012).⬈#p3

Other similar databases have seen online publication in recent years. Finds databases for Roman coins found outside the Roman Empire exist for both Germany and Poland. The University of Oxford is creating a large-scale database of Coin Hoards of the Roman Empire (CHRE), to join with the American Numismatic Society-led initiatives Coin Hoards of the Roman Republic (CHRR) and the Inventory of Greek Coin Hoards (IGCH) to paint a broader picture of coin hoarding spanning a millennium from Archaic Greece to the end of the Roman Empire.³ These projects are at varying stages of integration into the wider numismatic research ecosystem. Only when it becomes possible to compare data sets across multiple modern source countries will we be able to write the larger monetary history of ancient imperial spaces. ⬈#p4

Unlike most other forms of archaeological evidence, coins are official objects: their designs and inscriptions can tell us about the intentions of their issuers and, perhaps, the preconceptions of their users (See e.g., Fig. 1). The iconographic and epigraphic repertoire of ancient coinage is a huge, and substantially un-mined resource for examining areas from local religion to imperial economic policy; from individual political ambition to communal statements of identity. And there is scope here, as recent work has shown (Kemmers, 2006; von Kaenel and Kemmers, 2009), to marry the evidence from findspots to that of the iconography of the objects, and thereby expose patterns of administration invisible from other sources.⬈#p5

Finally, there is the sheer quantity of material that survives. As we have already noted, after 20 years of recording, the PAS contains information on 412,000 coins, and the rate of discovery, and thus growth of material is not yet showing signs of abatement. This is just one country. Initiatives exist to record individual finds as well as hoards in numerous other European countries, most recently, the Netherlands with the enactment of the Portable Antiquities of the Netherlands (PAN).⁴ Moreover, the collections of Museums—both local and national—raise the numbers of coins available for study literally into the millions (Callataÿ 1997b). To these sources we must add also those coins that appear in commerce every year, most of which are publicized online through various auction houses (e.g., Fig. 2).⬈#p6

Numismatic material thus presents an exciting set of opportunities to address questions unanswerable through other source material, and to do so with substantial quantities of data, a significant amount of which is already available online in a variety of forms.⬈#p7

The Problems

However, two of the the principal barriers to the exploitation of this data lie in its sheer volume and its location in a variety of institutional and non-institutional settings. Many, many coins are described online, but they are described in different languages, are hosted in different systems and in different formats, according to different standards, and with different aims (compare Figs. 3a-3d).⬈#p8

To add to the complexity, numismatics has its own way of describing numismatic objects, necessitated by and tailored to features specific to coinage. Coins, for example, have two sides, both of which must be described. They are both pictorial and textual. Physical characteristics such as material, weight, diameter, and the relationship of the heads to tails (obverse to reverse) directions (axis) can be recorded. Coins have denominational systems that vary with time and place, and those times and places may require systems of chronology and geography that vary from standard, modern formats (e.g., “154/3-153/2 BC”, “first quarter of the second century”, “Byzantium”, “Constantinople.”) There is also a vast array of information about individuals involved in the production of coinage whose names and titles may vary over time, and find no easy analogue in any other discipline: “Augustus”, “Octavian”, “Tresvir”, “moneyer”, “die-engraver”. In a further twist, it is often necessary to record not just the details of an individual coin, but also its relationship to others: the context of its circumstances of discovery, a taxonomic arrangement, a commercial transaction, or its current physical disposition.⬈#p9

Solutions: Creating a Linked Discipline

Coinage is thus a rich source for the study of the ancient world, and the study of Roman Imperial coinage in particular is well established in the print medium. Roman numismatists have, over the past century, divided their discipline into four discrete areas of study. The basic structure of Imperial coinage has been the focus of a type corpus known as Roman Imperial Coinage (RIC), printed in 10 volumes, providing a basic description of each of the 40,000+ recorded varieties of the coinage. It is the standard reference work for all who catalogue and publish Roman coins in any context. These contexts may be divided into three separate types, which have formed the subject of the other three areas of focus of numismatic study: collections, hoards, and individual finds. Roman coins exist in the hundreds of thousands in the major public collections across the world. Hoards (coins buried together in antiquity) are found today in astonishing numbers across the former territory of the Empire. Single finds are similarly common, both within archaeologically excavated contexts, where their scientific value is enhanced, or as chance or metal-detector finds. As Fig. 4 describes, all four of these areas of study are fundamentally interlinked since collections may contain hoards and single finds, and any coin from any context must be described by RIC type for it to be properly published and usable in historical synthesis.⬈#p10

In a very obvious sense, Roman numismatics was a prime candidate for the introduction of a Linked Data approach to the entire range of publications required by the discipline. In 2011 the American Numismatic Society began, in collaboration with a number of strategic partners, the process of developing the necessary infrastructure for the creation of linked Roman numismatic data. These technological principles have been expanded to include a significant portion of Hellenistic numismatics at the date of this publication, and collaborators in England, France, and Spain are working on pre-Roman Gallic and Iberian coinage.⁵⬈#p11