Academic journal article Antiquity

Exploring Morphological Bias in Metal-Detected Finds

Academic journal article Antiquity

Exploring Morphological Bias in Metal-Detected Finds

Article excerpt

Introduction

The growing availability of large-scale geo-referenced datasets within archaeology is an opportunity that is starting to be exploited enthusiastically. In England and Wales, where metal-detecting is a legal pursuit, the Portable Antiquities Scheme (PAS) is a very important source of such data. It started in 1997 as a pilot project in a small number of areas, and can now be regarded as a mature organisation with good links to the metal-detecting community. It has recorded over a million objects, which are all publicly available via the internet on their database (PAS 2016). These data are naturally attracting scholarly attention, having, for example, the potential to help characterise landscapes (see, for example, Brindle 2014: 11-14). They also add considerably to the range and numbers of objects known, and to patterns of regional use (see, for example, Mcintosh 2014), allowing issues of trade, production and identity to be explored in ways that were not possible before.

The recovery bias in the data that relates to where people are allowed to detect, and where they choose to do so, is naturally an issue. The PAS itself has carried out research on this and has publicised the results (Brindle 2014: 18-21; Robbins 2014: 39-47). Others, such as Bevan (2012), have developed methodologies to help explore and understand the patterns, and to allow the data to be used more confidently. What has not been considered in any detail is the impact that morphological bias in the types of artefacts recovered may have according to their shape. That a metal-detected and a conventionally excavated assemblage from the same site will normally differ is recognised within the specialist literature, and the implications for dating are sometimes noted in passing (Brickstock et al. 2007: 98-99, tabs 5 & 6; Cool 2008: 158-59). More extensive doctoral surveys that have exploited PAS data have sought to explore the phenomenon (e.g. Chester-Kadwell 2009: 69-73). What has not been done is to inspect the data rigorously and quantify the impact. This has probably been due to the difficulties of acquiring the large dataset that would be needed. Recent work that we have conducted on modelling brooch use in Roman Britain (Cool & Baxter 2016) has, however, provided a dataset that can be interrogated to discover the impact of morphological bias. The modelling work reported elsewhere required the data to be inspected for a range of biases. The initial results of the ones connected with metal-detecting were of sufficient interest to warrant further work to see whether similar bias was observable in more recent PAS data, and it is the result of this that is reported here. The implications are important not only for people who seek to use PAS data for the study of Roman Britain, but also for potentially explaining the unusual patterns in material recovered from other periods.

Roman brooches--why are they important?

From the first century BC to the end of the second century AD, brooches were made and used in England and Wales in their tens, if not hundreds, of thousands. Other than coinage, they are the most commonly encountered type of Roman metal small find. Most excavations of early--to mid-Roman sites recover them, and they are regularly found by metal-detecting, with over 18 000 recorded by the PAS in the first decade of the scheme's national coverage (Worrell & Pierce 2014: 399). The PAS total now (February 2016) stands at just under 23 000. Brooches were made in an almost bewildering number of types and sub-types, many of which are very regional. Their changing fashions make them a useful dating tool, with great potential for addressing questions of regionalism and identity.

Possibly because of the sheer volume and variety, nobody, until recently, had ever produced a detailed, dated typological system for these brooches. This position changed when Mackreth's magisterial study was published in 2011. …

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