In Defence of Lead Isotope Analysis

Article excerpt

The recent ANTIQUITY paper by Budd et al. (1996), 'Rethinking the quest for provenance', is the latest in a series of polemical papers on archaeometallurgy by the Ancient Metallurgy Research Group at Bradford. Having started with the appearance of arsenical copper in Britain in the Early Bronze Age (Budd et al. 1992), this series has now moved on to the application of lead isotope analysis to copper provenance studies. These papers contain very little, if any, new scientific data but instead attack established procedures and interpretations. In view of the very considerable number of journal pages that have recently been thus filled, it is time to ask what has been achieved.

The measurement of the lead isotope ratios of trace lead impurities in copper and bronze artefacts to identify the ore source for the copper was first investigated by Gale & Stos-Gale (1982). The principal advantage of this method for provenancing copper is that, on the basis of experimental tests (Barnes et al. 1978; Pernicka & Bachmann 1983), the lead isotopes can be assumed not to fractionate during the production of the copper. Therefore, the lead isotope ratios in the copper metal should be the same as those in the ore from which it was produced. This contrasts with the situation for trace element provenancing when the concentrations of the impurities carried through from the ore to the metal can vary depending on the production processes used. The different ore deposits used can be distinguished because their lead isotope ratios vary according to both the geological age of the deposit and the processes involved in its formation (Pollard & Heron 1996: 312).

A major application of the technique has been to investigate the Bronze Age trade in copper in the Mediterranean with a particular emphasis on the so-called 'oxhide' ingots. These ingots of copper, which normally weigh around 30 kg, date to the Late Bronze Age when Cyprus was a major producer of copper and are found on sites from Sardinia in the west as far as Syria in the east.

Prior to the ANTIQUITY paper (Budd et al. 1996), Budd et al. (1995a) raised again the possibility that the fractionation of lead isotopes occurred during ancient metallurgical processes and that lead isotope provenance studies were therefore invalidated. Strong reservations expressed regarding this hypothesis were subsequently proved correct by a report on a laboratory simulation of roasting which stated, but without providing numerical data, that lead isotope fractionation occurs to no measurable extent (Budd et al. 1995b). The Bradford results were to some extent anticipated by a paper at the International Archaeometry Symposium in Ankara in 1994 by Gale & Stos-Gale (1996), which demonstrated that lead isotope fractionation did not occur for silver subjected to roasting, smelting, cupellation and refining.

In the same paper, Budd et al. (1995b) suggest that tin isotopes fractionate during the melting and re-melting of bronze; so the tin isotope ratio observed in bronze could indicate the number of times that the metal has been re-melted and re-used in antiquity. Again, no prior experimental work was undertaken to provide a preliminary indication of the validity of the hypothesis.

The ANTIQUITY paper (Budd et al. 1996), in part summarizing an earlier paper in Journal of Mediterranean Archaeology (Budd et al. 1995c), expresses three areas of concern. First, many of the earlier lead isotope measurements are of insufficiently high precision; a programme of re-measurement is required, and lead isotope data-sets for ore deposits should be routinely published. Second, the geological histories of available ore sources are insufficiently unique for each ore source to have a lead isotope signature significantly different from those for all other ore sources; this problem cannot be overcome by statistical treatment. Third, the interpretation of the lead isotope data is more complex than originally conceived, particularly when possible mixing and recycling of metal from more than one source is considered. …