Non-Destructive Provenancing of Bluestone Axe-Heads in Britain
Williams-Thorpe, Olwen, Potts, P. J., Jones, M. C., Antiquity
Introduction to non-destructive characterisation of axes
Scientific provenancing of stone axe-heads has become one of the key elements in assessing trade and contact in prehistoric Britain (e.g. Bradley & Edmonds 1993). However, stone axe studies using the traditional method of petrographic thin sectioning (mainly work by the British Implement Petrology Committee; e.g. Clough & Cummins 1988) have suffered both from the necessity of removing a slice or core from the implements, and also from a growing recognition char petrography alone does not always provide an unambiguous answer to questions of provenance (e.g. Fell & Davis 1988; Williams-Thorpe et al 2003).
In order to address these problems, we have adapted and characterised at the Open University several methods of non-destructive analysis for archaeological stone artefacts. The two techniques that are most appropriate for stone axe studies are geochemical analysis using portable X-ray fluorescence (PXRF; Williams-Thorpe et al. 1999); and magnetic susceptibility measurements using a hand-held kappameter (Williams-Thorpe et al. 2003). The required equipment is readily portable and therefore analyses can be made in situ at museums or in the field. The facility with which the measurements can be made and, above all, the entirely non-destructive nature of the work, has resulted in a considerable increase in the number of artefacts that are made available to us for study (we estimate an increase of typically around ten-fold over our previous studies that relied on destructive sampling of artefacts). The geochemical and geophysical data that are obtained can be compared with analogous data on potential outcrop sources, and considered in combination with petrographic information derived from hand specimens, and from existing thin sections where these are available.
An important factor in our interpretation of magnetic susceptibility and PXRF data is the use of a number of statistical techniques to support the comparison of artefact and source data. In particular, an atypicality index has been developed to quantify the plausibility of an artefact originating at a particular source (Jones & Williams-Thorpe 2001). This index is especially useful for our application, because it allows robust comparison of data that have different analytical and sampling uncertainties (PXRF data may have larger uncertainties than the source data with which they are compared). While statistics cannot always provide unambiguous answers to questions of provenance, its use in this context introduces an important element of objectivity in assessing, and comparing, artefact outcrop sources.
Notwithstanding the usefulness of these non-destructive methods, it is important to recognise that there are limitations in their application. The surface characteristics of unprepared rock or artefact surfaces--variations in relief and mineralogy, and weathering layers--are of especial importance in PXRF, where most of the analytical signal originates within the 1-2mm or so of material that is nearest to the analyser (magnetic susceptibility is rather less affected by such factors, because the method assesses a greater depth of material, around 20mm).
The presence of weathering layers may cause particular problems, because weathering processes can alter the mineralogical and chemical composition of rocks from the original (bulk rock) composition that is frequently required for provenancing purposes. We are currently investigating ways of quantifying, and compensating for, the analytical effects of weathering in certain rock types (Bernardini et al. in prep.).
The vexed question of whether petrographic or non-destructive (chemical and magnetic) characterisation is more 'successful' for British stone axe provenancing has no easy answer. Both methods rely heavily on the availability of a comparative database for potential sources, so a comparison of robustness of technique is only meaningful for cases where such a database exists for both petrography, and for chemical and magnetic characteristics. …