Metals in Antiquity

Martha Goodway, Smithsonian Center for Materials Research and Education

This international symposium was held at Harvard University in Cambridge, Massachusetts, 10-13 September 1997. It was also sponsored by the University of Bradford and drew more foreign participants than Americans. I wish I could say this was entirely due to the quality of the symposium, but I am afraid it had more to do with the relatively small numbers working in this field in the United States compared with other parts of the world, especially in Europe. So many participated that speaking time was completely filled and there were too many papers to mention all of them; many had to be given as posters. The symposium opened with reports of current research, followed by a day devoted to ore deposits and extractive metallurgy, and another given over to the social context of metal production and use, both theoretical and ethnographic. The symposium closed with a workshop on metals analysis.

Concerning current research, Chase et al. mentioned analyses done at MIT of a single galena crystal that had a linear distribution of lead isotope ratios rather than a single point, the galena having been deposited over a period of time, and concluded that at least some isotope data will have to be looked at linearly. Knapp reported on the Sydney Cyprus Survey Project (SCSP), which is mapping human modification of the landscape, relating industrial sites to agricultural villages and urban centers. He said that though the heaps of ancient slag, which amounts to more than four million tonnes, are protected in Cyprus, spoil heaps are not. A copper smelting workshop has been located by this survey at Politico Phorades and according to Kassianidou plano-convex slags are present with ceramics giving a 16th century BC date that is awaiting confirmation by AMS. Nikolaas van der Merwe observed iron smelting in Malawi, where natural draft furnaces a meter and a half high produced high carbon steel. Baboula and Northover tackled the distribution of metal grave goods in Late Minoan Crete and concluded that the Late Minoan metal shortage occurred earlier in the period than had been thought. They had done this by comparing richer assemblages with poorer ones rather than simply examining elite materials alone. Seventeenth century brass from villages in Canada analysed by Hancock showed a pattern of higher tin in red brass than in yellow brass, and rivets whose compositions were not always the same as the sheet they fastened.

In a discussion of ore geology and provenance Ixer mentioned the usefulness of poorly processed metal slag in determining which ore was chosen for smelting, since most deposits are the result of more than one geological process, hence offer a choice of ores. He warned against the ‘magpie principle of sampling,’ collecting just the pretty bits. I have seen this happen, brightly colored ore minerals collected (as if they could have been overlooked!) from an area that was, as it turned out later, inaccessible in ancient times. Regarding provenance, Thomas asserted that compositional matches don’t matter; that because of overlaps only negative evidence can be used in sourcing. That led me to wonder about all the materials we have not yet characterized, and provoked much discussion in the midst of which someone, I don’t recall whom, remarked that ‘to hoard is human.’ Luckily for us.

The Bochum group presented some early results from their project identifying the sources of third millennium copper in Mesopotamia as Bahrain (the ancient Dilmun) and Oman (part of Magan.) These coppers contain nickel and arsenic, in Omani copper about a percent of each. The Gales presented more lead isotope data bearing on recent discussions of Cyprus and Sardinian ore sources, which they find are more complex than was shown in their earlier work. There is more than one field in Cyprus, but these still can be discriminated from Laurion ores. The use of ellipses to indicate error limits, rather than the usual error bars, was recommended. Ingot forms were also discussed; Gale claimed that oxhide ingots were from Cyprus even when found on Sardinia, later discussed in the paper by Tykot, and there was an interesting speculation that LBA bun ingots were metal that was remelted.

Macfarlane related Mexican and Andean lead isotope ratios to subduction along the Middle American trench and the Peru-Chile trench, in both cases lead ore becoming more radiogenic (206Pb/204Pb ratio increasing) with distance from the trench, though there are a few deposits that do not fit the pattern. Isotopic fractionation of lead during processing was dismissed since there is no effect unless more than 40-70% of the lead evaporates. In a discussion of South Indian copper alloys Srinivasan reported a 4th century ingot of zinc, dated from an inscription on the ingot. Her analyses suggest local sources for the alloys of South Indian images. The Prehistoric Gold Research Group (PGRG) of the British Isles gave a kind of tag-team paper in which they identified the manufacture of new objects from the scrap gold of a specific source, the two hoards from Downpatrick in County Down, Ireland. They feel they can link local gold to its hard rock source, and that the earliest Irish gold comes from the north, not from Wicklow.

Shimada et al. reported on the extensive work done at Batan Grande on the north coast of Peru. They have located what may be a workshop for precious metals. Some alloys in the objects from the elite tombs dating to ca. 1000AD contain amounts of copper and arsenic that together are roughly equivalent to the silver content. They have found prills with arsenic contents as high as 28-32% but most objects contain only 1-2% arsenic. It is not known what properties arsenic confers on precious metals. They have observed that cleaning these metals, even the mildest sort of cleaning, affects their color. O’Brien, the excavator of the EBA (2500-1900BC) mining site at Ross Island, County Kerry, presented the evidence for smelting at this site. He takes the absence of slags as pointing to the use of sulphur containing ore (fahlerz) rather than secondary deposits. A droplet of metal was analysed and contained about 7% arsenic. "Sheet’ seems to be formed of several of these droplets hammered together. Arsenic was only gradually displaced by tin. Northover commented that older analyses of bronzes, due to the methods used, usually do not include sulphur but that sulphur should be evident metallographically.

Pigott reported on the Thailand Archaeometallurgy Project (TAP), specifically the site of Non Pa Wai in central Thailand, dated to the early second millennium BC. This village had a burial that contained pieces of a broken mold. Crucibles about six inches in diameter have been found but no tuyeres. Perhaps the furnaces, which were movable smelting chimneys, were wind blown. Rostocker has shown that mixed oxide-sulphide smelting was exothermic and produced copper in a matte envelope. This low-tech approach would need no charcoal, only dry wood fuel. The ingot molds have unique shapes; slag has turned up that fits them but no metal. But small metal ingots of a size suitable for remelting have been found, and in special shapes that are thought to be coded for use in trade. The picture here is of part-time, dry season local production active in exchange. Joosten et al. estimated iron bloomery furnace output in the Netherlands between the 4th and 11th centuries AD from slag remains, based on a model confirmed by experimental reconstructions that for a 1:1 fuel:ore ratio gave a ore:slag:iron ratio of 1.8:1:0.4 for slagpit furnaces and 1.4:1:0.2 for slag-tapping furnaces. They also added írattlestones’ to our vocabulary. These are a kind of bog ore in which goethite has formed an envelope around pebbles that rattle inside.

Several promising examples of the application of highly sensitive analysis were presented. Ferrari et al. showed the data from Greenland ice cores that has received so much recent attention. Ice there is laid down at a rate of about 40cm per year. They took cores weighing about 250kg, which they analyzed down to picogram per gram levels. They detected increased signals of copper and lead during Classical Antiquity that correlate well with the production figures published by the late Clair Patterson. They hope eventually to be able to do quantitative studies of metal production. Shotyk et al. measured lead deposition in a Swiss bog as far back as 12370 years. By indexing against scandium, the beginnings of smelting in the Mediterranean could be detected at 3000BP in the peat record, as could the Roman empire and its decline. Killick suggested that lower lead values after the peak around 1300AD might have been an effect of silver imported from the New World.

The social context of early metal production was discussed by Shennan, who reviewed recent hypotheses that have replaced the application of what were essentially modern economic principles, as in the cementing of relations by gifts replacing profits from trade. He referred to some of this scholarly activity as ‘ethnographic parallel-picking,’ but remarked that you do have to make assumptions and some assumptions are more fruitful than others. Gillis discussed tin foil covered ceramics found as grave goods in the Aegean LBA. Replication experiments have shown that heated tin foil can oxidize to a golden color. She suggested the burials may have been of smiths, asserting that they held a high position amongst craftsmen. Both she and Olmsted identified tin as a status metal. Olmsted presented Gaulish smiths of the LIA also as having status; they are known by name, were not slaves, and their work was not specialized. Westover is studying the relationship of metallurgical sites and sacred sites in the Aegean and the Near East, for which she detects different purposes. Perea’s paper on the study of technological change using gold in Spain as the paradigm sparked much discussion; data without theory was declared meaningless. But it was asked, where in this construct are the ‘getting acquainted’ studies?

The session on the ethnography of metallurgy opened with Killick’s review of the many forms that iron bloomery technology takes as it still exists in Cameroon and Nigeria. It was evident from the variety of details that it is not easy to know what needs to be recorded. He warned that because iron making skills need constant practice they are very soon lost. This was also emphasized in Blair’s report of replication experiments relating to iron making in the Alps. These lead him to believe that there was a whole array of specialist workers. Baldia related the trade in copper in early northern Europe to the rise of megalithic tombs, relating their location to causeways and their intersections, river crossings and harbors. In North America the recycling of metal from copper kettles manufactured in Europe was reported by Moreau. Fragments of kettles were cut and shaped by the Amerindians, one form being little cones that were sewn to clothes or other objects to make a tinkling sound.

Much of what was useful in this symposium were reminders of what is not so, or may not be, supplying a necessary balance against the natural urge to drive interpretation of results just as far as they can be made to go. Gale reminded us that slag is not always the product of local ore. The finding by Skinner et al. that debasement of tin in English tokens was far too late to be ascribed to the collapse of the tin industry upon the Black Death was presented as an warning against simplistic economic conclusions from technological factors. And so on. Suzanne Young and Paul Budd, the organizers of this symposium, are to be congratulated for a highly stimulating meeting. We look forward to the volume of proceedings now in preparation.