The origins of bronze tools

The earliest metal goods probably came to Britain from Ireland. Paul Budd reports :-

Squeezing through the labyrinth of tortuous passages carved out of the solid rock at depths of 70 metres or more, it is difficult not to spare a thought for Britain's prehistoric copper miners. Some of the tunnels beneath the Great Orme's Head, near Llandudno in North Wales, are so small that they could only have accommodated children.

The experience is fascinating and the conditions brought vividly to light. Crawling on hands and knees by the light of a dimly burning taper clenched between the teeth; huddled in the gloom and pounding at the fire-softened rock with a cobblestone hammer; the work must have been excruciating. And yet, 20 years ago this and other astonishing evidence for the earliest copper mining in Britain did not exist and the passages lay undiscovered.

Today, we are entering a new phase of research on Britain's earliest copper mines. Much of the excavation and recording has taken place, telling us when and how the mining and ore processing was carried out and, to some extent, about the people who did it; but where did all the copper go? A long-standing objective in archaeometallurgy has been to try to link Bronze Age metal tools and weapons to their sources. Now, perhaps for the first time, scientific techniques are beginning to tell us something about this vital key to understanding the organisation of prehistoric metal production and exchange.

Until the early 1980s only one prehistoric copper mining site was known in the British Isles. The site, Mount Gabriel on the Mizen peninsular in the far south-west of Ireland, was simply a cluster of primitive opencast workings and shallow galleries dug into the hillside. The mine was considered unique, perhaps owing its survival to the extremely poor quality of its copper ores and therefore to the lack of subsequent interest in mining them. When it was investigated in the 1960s, it was generally agreed that virtually all the evidence for early copper mining in Britain had been obliterated by later activity. Copper mining peaked in the late 19th century, by which time mechanised techniques were responsible for radical alteration of many mining landscapes. Survival of prehistoric evidence seemed unlikely. Today, this pessimism has been dispelled.

Thanks very largely to the (often unpaid) efforts of a small number of dedicated field workers, some 30 probable or definite prehistoric copper mining sites have now been identified in the British Isles, of which the Great Orme, with its visitors' centre and guided tours of the Bronze Age mine workings, is the most impressive. Many of these sites survive, despite all the odds, on surface outcrops of copper which, in the historic period, became well known and highly productive. In addition to Mount Gabriel and the Great Orme the best known and best investigated sites to date are at Parys Mountain in North Wales, Cwmystwyth in central Wales, Alderley Edge in Cheshire and Ross Island near Killarney in South-West Ireland.

Over the last decade or so, the antiquity of mining at these sites has been firmly established, mostly by radiocarbon measurements on charcoal and sometimes bone sealed within the mining waste. In addition to the mine, Ross Island also features a `work camp' area from which radiocarbon dates have been obtained. All of the sites were in use during the Bronze Age. Ross Island appears to be the earliest, with dates clustering in the second half of the 3rd millennium BC. This is just prior to the beginning of the Early Bronze Age in Ireland - the period associated with the introduction of metallurgy in the British Isles. The other sites were all in use at much the same time spanning the Early Bronze Age and earlier Middle Bronze Age from c 1900-1200BC. But what of the evidence for the copper they produced?

Bronze Age metalwork has an enduring fascination and has been the subject of study for two centuries or more. In the latter half of the current century, typological classification of metalwork has given way to a developing interest in its composition in the hope that stylistic or regional metal groups would share characteristic patterns of trace elements which might then be linked to particular ore sources. In the British Isles, significant changes in the impurity pattern of copper and bronze metalwork do occur over time and between different regions, but attempts to relate this to the general pattern of copper mineralisation in the British Isles have always been inconclusive.

Now, with the mines identified, it is becoming possible to develop a clear idea of the impurity patterns likely to have resulted from smelting the ores from particular places. A detailed mineralogical survey by Rob Ixer at the University of Birmingham is now revealing just such information. The work is painstaking and highly skilled. A detailed understanding of metallogenesis and ore geology are required even to select representative mineral samples for further study. Ore petrography is then used to identify the mineral suite and build up a picture of the formation process (or processes) and subsequent geological history of the deposit. Only with this level of understanding is it possible to identify the ore that was actually mined from a particular site.

The results of Ixer's analyses are fascinating. With one exception, all of the sites investigated can only have produced virtually pure (impurity free) copper. This contrasts strongly with the Bronze Age metalwork for which common impurity patterns have emerged.

The earliest metalwork, with a primary distribution in South-West Ireland, often contains considerable arsenic - sometimes several per cent - with lesser amounts of antimony and silver. Later, at roughly the time that mines such as the Great Orme and Cwmystwyth were in use, these compositions give way to copper with a higher nickel content. By this stage the copper is most often alloyed with tin to form bronze and has a wider distribution across upland areas of Britain.

Of the mines investigated, only Ross Island, the earliest, produced copper with a significant arsenic-antimony-silver impurity pattern. Could it be that Ross Island, perhaps together with as yet undiscovered mines in the region, was the dominant source of the earliest copper before it became mixed and diluted with the pure copper from Wales and northern England? Were Killarney's Beaker Culture people our first metallurgists? If so, where did the nickel come from in the later metal? Does it represent the growing status of alternative groups of metallurgists with their own as-yet-undiscovered copper supply?

Answers to some of these questions are now emerging from lead isotope analyses of the ores. The isotopic composition of lead within an ore deposit relates to its geological formation process and age, with the result that different deposits can have characteristic values (although they sometimes overlap). Lead isotopes are unchanged by the smelting process so that the signature of the ore is carried by the finished copper.
Brenda Rohl, working at Oxford University's Isotrace Laboratory, has analysed ores from many of the newly discovered mines as well as numerous Early and Middle Bronze Age copper and copper-alloy artefacts. Some of the earliest, `type A', metal tools do have isotopic signatures which match ores from Ross Island, but the mine is unusual in having two types of ore with quite distinctive isotope signatures. Some `type A' tools have isotope ratios which suggest they were made by mixing the two.

For the later metalwork analysed by Rohl the picture is more complex with a pattern indicative of the mixing of copper from multiple sources. Only at Ross Island is there evidence of prehistoric metal processing in Britain, and in general we do not know how far ores were transported for smelting. However, the mixing is less likely to have resulted from the original smelting process, and was probably rather the consequence of later melting-down and recycling of artefacts from different sources. This is a relatively simple operation and may have been performed, perhaps from an early date, more commonly than is traditionally thought.

Occasionally, however, very distinctive patterns emerge from which specific conclusions can be drawn. In one case analysis of five of the nickel-rich `type B' artefacts shows them to have a highly unusual lead isotope composition resulting from uranium associated with the ore. There are only a handful of deposits, all in Cornwall, where such nickel- and uranium-bearing copper ores occur.

The discovery of the copper mines has undoubtedly given a boost to archaeometallurgy in the British Isles, at last allowing us to bridge the gap between artefacts and their sources. Clearly much remains to be done, but interesting results are already emerging which reinforce the suggestion of an early metallurgical focus in South-West Ireland. Their products were soon joined by, and mixed with, those of other miners working the copper deposits of Wales, northern England and, almost certainly, Cornwall, where prehistoric mines may yet be awaiting discovery.

Dr Paul Budd is NERC Advanced Research Fellow in Science-based Archaeology at the University of Bradford

Issue no 36, July 1998