Rethinking the Quest for Provenance

Article excerpt

One of the larger - and more expensive - present programmes of study in archaeological science explores the provenance of prehistoric bronzes from the Mediterranean. What are the bases of research? What will the findings tell us about the real place of metal as it moved in the ancient world?

The provenance postulate

The availability of spectrographic methods in the 1930s made it possible to analyse large numbers of ancient metal artefacts with a view determining their provenance. Some of these studies (e.g. Pittioni 1957) followed from a 19th-century tradition, pioneered by the Austrian scholar Wocel (see Caley 1951; 1967), in which it was proposed that the impurities in ancient copper artefacts would directly reflect those in the ores from which they were smelted. Some scientifically informed commentators (Thompson 1958) expressed concern as to whether this naive insistence was justified, but the work continued in the absence of any detailed understanding either of the geology of metal ores or of the chemistry of their smelting. Although most researchers have come to recognize the complexity and limitations of compositional data (see for example Pernicka 1995), Pittioni-style provenance studies continued for decades. Despite Thompson's warning, tens of thousands of prehistoric metal artefacts were subsequently analysed. The Studien zu den Anfangen der Metallurgie (SAM) analytical programme (Junghans et al. 1960; 1968; 1974) and the huge analytical programme in the former Soviet Union (Chernykh 1994) stand as the two largest monuments to this endeavour. Interpretation of the data in these compilations has generally used statistical procedures to extract groups of artefacts having similar composition. On the broadest level, such groupings are undoubtedly significant. In much of Eurasia, the use of unalloyed copper pre-dates the use of arsenic-rich material, and this is later succeeded by tin-bronze. On a more detailed level attempts to link compositionally similar artefacts to common geographical sources have almost always proved inconclusive. Artefacts grouped on the basis of composition have often come from unrelated contexts, sometimes widely scattered across vast areas, as Butler & Van der Waals (1964) noticed in commenting on the SAM programme.

When the assumptions which underpin these approaches are examined, it is easy to see the pitfalls. For trace element provenancing to succeed it is necessary to assume that the artefacts under consideration were fabricated using similar manufacturing processes, derived from a strictly limited number of sources and smelted in such a way as to produce a metallic product with a limited range of impurities. Furthermore, one is obliged to make further fundamental assumptions about alloying and recycling. Whereas some copper deposits may contain distinctive traces of particular trace or minor elements, very few have been studied mineralogically to the point where quantitative estimates of different mineral species' contributions to ore 'as-mined in prehistory' can be made. Similarly, there is little detailed understanding of the behaviour of impurities in primitive smelting processes. Simple thermodynamic models are problematic and many experimental studies have been little more than imaginative reconstructions of hypothetical processes, sometimes using inappropriate materials and poor control and monitoring.

Under these circumstances modern researchers are rightly circumspect in their use of impurity data for provenance studies, although useful studies have been undertaken for small regions; attempting for example to outline compositional variation within artefacts already grouped archaeologically by virtue of typology or context (e.g. Begemann et al. 1995). It is now clear that different deposits may share closely similar geochemical characteristics so that particular copper deposits simply will not yield metal of unique minor or trace element composition. …