EMAP_2012_Report_6_1.pdf (7.3 MB) - The Heritage Council

3.2: Raw materials, processes, manufacturing
Although a wide variety of objects were made of copper-alloy, there is substantially less
evidence for the production of copper-alloy than for iron. The first stage involved sourcing
metal ores from their primary contexts. Copper ore has been found at Lagore (Hencken 1950,
240-41), and lead ore has only been noted at Ardcloon (Rynne 1956, 208). Smelting slag
containing copper has been found at Cooltubbrid East, Co. Waterford along with a copper
smelting hearth which the excavator identified as ‘experimental copper-smelting’ (Tierney
2008, 208). The recent discovery of two early medieval smelting furnace pits at the Ross
Island copper mines is the first evidence for the extraction and processing of copper ores
from their source in this period (O'Brien 2004). Three slag deposits were radiocarbon dated
to the late sixth/early seventh century. The possible remains of clay tuyères and furnace wall
materials were also found (O'Brien 2004). There was no evidence for moulds or crucible
remains indicative of metal casting and this suggests that Ross Island was a primary
processing site. The meagre evidence appears to suggest that the processing of copper ore is
more likely to have taken place at its source and transported in the form of ingots or cakes to
settlement sites. Bronze or copper ingots are found on at least seven settlement sites
(Appendix 1.2) as well as at Cathedral Hill, Armagh and Clonmacnoise (Ryan 1988, 43, King
2009, 341-43).
Smelting is the process whereby copper and other base metals such as tin were extracted
from their ores. The process involved a furnace which was heated up to very high
temperatures by the use of bellows or blow-pipes and produced some waste slag, though not
in the same quantities as iron smelting. Crucibles may have been used as part of ‘the initial
reduction of ore minerals in a controlled smelting process’ as well as ‘in the refining and
alloying of metal and in the casting of final metal’ at later stages in the process (Comber
2008, 139). After smelting, the copper may have been purified by melting it in a clay crucible
and stirring it. Two lumps of metallic tin have also been found at Garranes (Ó Ríordáin 1942,
100-2). The smelted tin may have been added at this stage to form bronze and the metal
was then ready to be cast in stone or more frequently clay moulds (Edwards 1990, 90).
Non-ferrous metal-working furnaces can be difficult to interpret when all that survives is a
spread of burnt clay and charcoal and it can be easy to confuse this evidence with domestic
hearths (Comber 2008, 135). Early medieval furnaces were typically of a primitive bowl type
and could have been used for smelting or re-melting a range of metals and alloys (Comber
2008, 139). A metal-working area on the west side of Moynagh Lough produced a bowlshaped
furnace associated with five clay-nozzles fragments, 67 crucible sherds, three heating
tray fragments, an antler motif and over 600 clay mould fragments and one kilogram of slag
(Bradley 1993, 77-80). The excavator believed that it was used for melting copper rather
than smelting copper, on the basis that there was very little slag on the site (Bradley 1993,
77-80) though Comber (2008, 138) has cautioned that ‘a relatively pure ore does not produce
a large amount of slag, while the melting of smelted and refined metal should produce very
little’. A furnace at Movilla Abbey may also have been used for melting as it was found in
association with crucibles and scrap copper alloy (Ivens 1984b, 77). Iron slag was definitely
identified on the site (Yates 1983, 62), but it is less clear whether there was copper slag as
well so it is difficult to establish if it was used for the smelting of metal ores. De Paor (1970)
described the discovery of a ‘bronze working furnace’ at Iniscealtra. It was described as a
scorched hollow with a clay dome associated with large quantities of ‘cupric’ slag (De Paor
1997, 62). No crucible or mould fragments are listed in association with the structure and the
slag has not as yet been examined by a specialist. Copper globules, two copper fragments
and a bar ingot were found at Castlefarm, County Meath but were not further reported on (O’
Connell & Clark 2009, 52). A small fragment of slag from the melting of a zinc based copper
alloy was also found at Dunnyneil Island in association with crucibles (Young 2006a).
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