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Celtic Hist. Newsletter: Ancient Copper in Ireland
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hist-@historicgames.com
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Nov 02, 2009 11:05 PST
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The Celtic History Newsletter
Brought to you by
The Celtic Croft
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MacGregor Historic Games
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Ancient Copper in Ireland
The many copper and bronze objects found in Ireland, indicate its
importance during the period between 2500 and 500 B.C. As a resource,
copper ore is not that common in Ireland and the Cork-Kerry districts
are where it is most concentrated. Of course, the ore first used by
the early Bronze Age peoples would have been "native copper," copper
that naturally occurs in metallic form, ready to be hammered in to
axes or other useful tools. Very little native copper can be found in
Ireland today, and scholars believe it was never a significant part of
the copper reserves on the island. Therefore most of the copper in
Ireland required smelting to make it useful.
Early man must have recognized that the greenish stain of copper
oxides in various rocks matched the patina that occurred on metallic
copper and perhaps made the connection that perhaps the metal could be
"cooked" out of the rock. Perhaps it was discovered by accident when a
piece of almost pure ore was heated by a fire, or a piece of metallic
ore was found inside a broken rock. However smelting was invented or
discovered, some of the purest copper oxides could be smelted in a
single step, but requires temperatures of 1080 degrees celsius to melt
copper, thus requiring charcoal as a fuel. (The remaining water and
sap/resins in even mostly-dry wood can create steam which limits the
temperature a wood fire can produce.) Eventually, these purer ores
were exhausted and sulfide ore had to be used which required a more
complex process. These sulfide ores needed to be "roasted" first at
lower temperatures (600 - 800 C) to drive off the sulfur before it can
be heated to smelting temperatures.
Unfortunately no Irish smelting pits have been found which can help to
recreate the exact methods used in the Bronze Age, but chemical
analysis of existing artifacts do offer some clues. Early Bronze Age
axes never show more than small traces of iron. This is characteristic
of a technique called "cobbing" where the richest parts of the ore
were broken from the rock, and the lower quality portions containing
the largest fragments of quartz and the host rock -which often
contained iron- where set aside. This is in contrast to the techniques
used in the Middle East and the Mediterranean where the ore was not
"cobbed" and in order to remove the iron, silica was added to mix.
Although this requires less hand sorting of the ore, the result was
copper with a higher iron content, and it produced much more waste
slag than cobbed ore. This reduced amount of slag from Irish copper
smelters may be part of the reason early smelting pits have yet to be
discovered. This lack of smelting pits also might make one wonder if
perhaps the Irish artifacts were not actually smelted in Ireland, but
other chemical analysis shows a "trinity" of arsenic, antimony and
silver in certain percentages in Irish copper that can provide a
"fingerprint" to identify Irish-made copper artifacts, and has been
used to identify ingots and finished artifacts which were exported to
Britain.
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