How Copper was Mined
What was the copper mining workflow?
The workflow involved in the process of transforming copper ore to a copper object involves a series of steps which can be summarized as
Prospection
Preparation - gathering wood for firesetting, collecting items which could be used or modified for use as tools such as cobbles to make hammerstones, antlers to make picks, cattle bones (e.g. the scapula) to make shovels
Mining – breaking the rock face and transporting the rock fragments to the sorting area
Sorting - separating the copper-rich rock from the ‘gangue’ i.e. the waste rock which is visible today as spoil heaps
Smelting – extracting metal from the rock – a waste product called slag is produced
Casting – making moulds into which to pour the molten metal
To learn more about the transformative impact of copper mining, see ‘Bronze Age Copper Mining on Iveragh – a time of transformations’
Prospection
The people prospecting for copper evidently knew where to look. Among the various means employed in detecting the presence of copper was the identification of rock faces with a turquoise-green staining (Figure 1. Copper staining on rock). This staining, which be visually striking and strongly contrasting in colour with the surrounding rock surface, is caused by oxidation of the copper mineral and is therefore an indication that copper ore is present within the rock. Even if no evidence of mineralization is visible on the rock surface, the occurrence of Stereocaulon lichen indicates that the rock is metal rich (Figure 2. Stereocaulon lichen on rock). Salt-loving plants such as thrift and sea campion, which are generally found only at the coast, can grow in association with copper with many such instances at Muckross near Ross Island (Figure 3. Salt-loving plants such as thrift and sea campion, which are generally found only at the coast, growing by a copper mine).
Copper staining on rock
Stereocaulon lichen on rock
Salt-loving plants such as thrift and sea campion, which are generally found only at the coast, growing by a copper mine
Turquoise-green staining, Sterocaulon lichens and salt-tolerant plants can indicate the presence of copper.
Preparation
Where ‘firesetting’ would have been used, a ready supply of wood would have been essential (Figure 4). It is estimated that at Ross Island 4000–15000 tons of wood were required to extract 4000 tonnes of rock1. Cobblestones, rounded stones whose edges have been smoothed by river or sea water, were sourced for use as hammerstones. Cobbles that fit comfortably in the hand and are therefore suitable for pounding would have been selected. If the stone was to be attached to a handle (‘hafted’), grooves would have been made in the stone to enable it to be better secured.
Firesetting was used at Staigue copper mine
Mining
The transition from the Stone Age to the Bronze Age was gradual. Stone age technology was used to mine copper in the early part of the Bronze Age which is known as the Chalcolithic or Copper Age. Where the rock breaks easily and wood is scarce, the ore could be extracted using only cobbles and picks made from antlers. As firesetting was used only in early Bronze Age copper mining in Ireland, evidence for its use is an important clue to the time in which the mine was in use1. Firesetting causes thin sheets of rock to shed from the surface leaving a rock face with the rounded smooth profile characteristic of the technique2. One such example can be seen on the south of the Iveragh peninsula at the Bronze Age copper mine at Behaghane which is known locally as St Crohán’s Hermitage (Figure 5). St Crohán is said to have lived in this shallow cave and to have left an impression of his head in the rear wall. This folklore may well have protected the ‘print’ of the earlier mining from being erased by copper prospectors in the late 19th century who mined very close by. This man-made cave is possibly shallower now than when it was first mined nearly 4,500 years ago.
Interior of Behaghane copper mine with the smooth walls characteristic of firesetting.
Sorting
Using larger stones as anvils, copper-rich rocks would be broken to separate the mineral rich fragments from the waste rock. Cobbles would have been used as hammerstones. (Figure 6). At Ross Island, where evidence for mining spans 500 years from about 4,400 years ago (2,400 BC), some hammer stones were hafted, having wooden handles much like that of a modern-day hammer. Hafting involves an investment of time and effort. Where mining activities were short-lived, hammer stones only were used1.
Cobblestone with impact marks
Smelting
Smelting processes vary according to the mining processes employed, which are themselves dictated by the kind of rock which was being mined, some of which can be smelted at lower temperatures than others. The copper-rich rock would have been placed in a stone bowl known as a crucible which would then be placed on a fire which would have been lit in a shallow pit. As high temperatures were required, air from a bellows was pushed into the pit to increase the rate of combustion and hence the heat output. The heat from the fire would initiate the smelting process whereby the copper minerals in the rock melts to form black molten metal which later cools and hardens to copper.
Copper slag formed as a waste by-product of some smelting processes is flat and distinctive in appearance and known as plate slag or by the German term, plattenschlacken. Slag initially occurs as a liquid that solidifies upon cooling, the ‘flow lines’ from its liquid state occasionally visible in the plate slag such as this example recently found on the lake shore near Muckross House in Co. Kerry by Dr Paul Rondelez (Figure 7).
Copper slag – notice the ‘flow’ lines in the rock
Casting
Molten metal from the furnace would be poured from the crucible into a mould. Moulds were made of either clay or stone and could be single-sided or ‘bivalve’ - double-sided. They were used either to create a finished product or to create an ingot (a block of metal which is typically oblong in shape). Once the metal cooled, the metal object could be removed from the mould. Examples of pure copper ingots include axe-shaped ingots made from Ross Island copper which were discovered in Brittany at the start of the last century3.
Experimental archaeology is a term that describes modern attempts to understand ancient processes by replicating something from a particular time in the past e.g., making bread or building a house, using only the tools and technology available at that time. Umha Aois (meaning Bronze Age) is a collaboration of arts and archaeologists who cast bronze using and clay crucibles and moulds to make new artwork and replicate ancient forms. Outdoor charcoal-fired pit furnaces like those thought to have been used in the Bronze Age are used to smelt the bronze which is poured directly from the crucible into the mould. You can watch a video of a Bronze Age-style casting here: http://www.umha-aois.com/.
References
Ross Island – Mining, Metal and Society in Early Ireland. (A multidisciplinary study of the copper mining complex at Ross Island, Lough Leane, Co. Kerry) Author: William O’Brien. 2004.
The Use of Fire in Prehistoric and Ancient Mining : Firesetting. Paléorient. Vol. 26, No. 2
Author(s): Gerd Weisgerber and Lynn Willies. 2000.Prehistoric Copper Mining in Europe: 5500-500 BC. Oxford University Press. Author: William O’Brien. 2015.
