1. Introduction

Characterisation studies of materials both organic and inorganic (clays, rocks, minerals, human remains, etc.) are now almost commonplace, and techniques are becoming increasingly refined. In particular the characterisation of rocks used for tool manufacture has a long history and that of obsidian is no exception (see, for example, Cann and Renfrew 1964; Renfrew et al. 1966; 1968; Taylor 1976; Shackley 1998a; 1998b; Cauvin et al. 1998; Poupeau et al. 2007). Many geologic sources have been located and geochemically characterised and artefacts, often found hundreds of kilometres from any known source area, have been analysed and related back to an individual source. Hence, almost de facto, obsidian has come to signify widespread contacts and networks of exchange. But behind this apparent 'success story' (Williams Thorpe 1995) lie a number of unanswered questions concerning the way in which we can use provenance data to make archaeological interpretations from the representation of obsidian sources within a given assemblage.

In the Near East obsidian is found regularly and in an increasing number of assemblages from the Epi-palaeolithic onwards. The quantity (numbers and weight) and its proportion in relation to flint and other materials used for tool manufacture varies from assemblage to assemblage. Distance from source plays some role in this, but is by no means the whole story (Barge and Chataigner 2003). Where obsidian has been geochemically characterised, the obsidian used in a particular area often comes from more than one source, though the overall distributions appear to respect some sort of 'boundary' or limit. Different source areas seem to have 'served' particular regions: obsidian from the central Anatolian sources is predominant in the western half of Turkey whereas obsidians from the eastern sources are found only in the east. These distributions overlap (at least from ASPRO period 4 to period 6) from the Euphrates to the Levantine coast (Cauvin and Chataigner 1998, 336-43).

Behind this overarching picture, however, there is a much less satisfactory situation. Firstly, from the majority of sites only a few artefacts have been geochemically analysed and the basis of the selection of such samples is often not clear, nor are they related to techno-typological characteristics, so that it is not possible to determine in what way they are representative of a particular assemblage. Secondly, the characterisation of sources is not as simple or complete as it might appear. In each of the source areas there are a number of separate volcanoes producing obsidian, sometimes with multiple flows, not all of which have a distinct geochemistry (Poidevin 1998; Poupeau et al. 2005). Not only that, intensive study both of the geology and the archaeology of source areas such as Göllüdağ and Nenezi Dağ has demonstrated that at some outcrops there were workshops, some making specific items and/or having specific distribution targets (Balkan-Atli et al. 1999; Binder 2002). The need for more detailed survey and sampling of geological source areas is therefore very real, although it does not form the focus of this article. The final, but perhaps most significant, complicating factor concerns the type of questions we might prioritise as archaeologists.

Most of the key archaeological questions, whether they relate to economy, symbolism or social meaning, concern the way in which material was used within specific contexts. The estimation of the proportions of obsidian from different sources for the site as a whole is, therefore, of limited value because it conflates chronological and spatial variation, making it almost impossible to investigate functional and symbolic aspects of use. To do this we need to carry out analysis at a contextual level, for example comparing individual buildings or midden deposits. For each context, as well as needing to quantify the proportion of obsidian from each source, we need to establish the form in which it reached the site; the technological and typological intricacies of the reduction techniques associated with that would enable us to compare the way obsidian was worked, used and discarded in different activities and locations so that we can begin to understand in what forms different households had access to obsidian and how its use and meaning varied through time. In essence, this requires us to sample at the level of individual contexts within the site rather than at the level of the site as a whole. In turn, this requires an extremely large number of analyses because each context is essentially a separate assemblage.

However, practical problems/difficulties arise where the analysis of substantial numbers of artefacts is required. The cost of analysis of large assemblages can rapidly become prohibitive, even though the cost per sample has recently declined significantly. Also analytical facilities are available in only a few laboratories and largely outside the countries in which the obsidian is found, and there are often difficulties in obtaining permissions to export large numbers of artefacts or the more special pieces. Until recently the destructive nature of such analysis has compounded the problem. New, non-destructive, methods such as PXRF, which enable analysis to be done in the field, promise to address some of these issues (see, for example, Craig et al. 2007), although its feasibility for assemblages of several thousand pieces remains to be assessed. Also, since not all sources have been characterised by XRF, it can be difficult to relate some analytical groups of artefacts to source (cf Maeda 2003) unless samples located to specific flows at source are simultaneously analysed as controls; such samples are not generally available, particularly for the eastern Anatolian sources.

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