Originally published November 2009
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 into 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.