Prehistoric Metallurgy

The Chalcolithic/Copper Age Metals sporadically occur in nature in pure enough concentrations that they were recognized and exploited in prehistoric times. Softer metals like gold and copper can be worked by hammering without heating. It is surmised that ceramic production created the technological foundation for the smelting of metal.

Copper Mining Copper ores come in two basic forms: sulphides and oxides. Sulphide ores contain a metal combined with sulphur. Early miners were attracted to oxides because they were easily recognizable, accessible, and easier to prepare for smelting. Sulphide ores have to be roasted before they are smelted.

The Mines of Ross Island, Killarney, Co. Kerry, Ireland

Irish Prehistoric Copper Mines

The Blue Hole

Early miners were limited to metal deposits occurring on the surface.

Ore Preparation: beneficiation Once extracted, it was necessary to pound the ore to separate the metal ore from the gangue – the worthless host rock.

Smelting The melting point of copper is 1100˚ Celsius. The earliest smelting used charcoal and blow pipes. The copper would be melted in the furnace and collected after cooling in the form of prills, or round droplets. These would then be melted in a crucible.

Bowl furnaces from Ross Island

Roasting/Smelting

Bowl furnaces and an Early Medieval Shaft Furnace

Artifact making Molten copper would be poured into an open mold of stone. The earliest copper artifacts in the Middle East were trinkets such as bracelets and beads. In northern Europe tools such as axes and awls were made. These quickly replaced their stone and bone counterparts.

As metal demand increased, and higher grade ores became exhausted, smelters turned to slagging processes to increase production and efficiency. Fluxes like sand and manganese were added to enhance the separation of the silica rich slag from the metal.

Alloying It was discovered that adding arsenic or tin to copper produced a metal that was harder. The resulting metal is bronze. Tin is a rarer metal than copper, and trade routes came into being to transport tin over long distances.

Bronze Age techniques More complex tools and weapons were made during the Late Bronze Age, enabled by clay molds and the lost wax technique.

Iron Age Vietnamese bronze drum made by the lost wax technique.

Analysis of Metallurgical Remains X-ray florescence: An object is bombarded with x- rays. This excites the atoms and causes them to fluoresce (glow). Differing atoms are excited at different energy levels. One can determine which elements are present, and determine the quantities.

Analysis of an object from a Bronze Age site. C-221 Co. Clare Ireland

Metallographic Examination Petrography: Microscopic examination of thin sections A technique that reveals the types and configurations of minerals and that make up a sample. A slice is made of a rock that is so fine that light can penetrate the grains. It is examined under a petrographic microscope under polarizing light.

Iron and Steel Iron melts at 1550˚ celcius, therefore smelting required a shaft furnace. Remains of Romano-British iron smelting furnace with forging hearth in the foreground, East Sussex, UK

The molten iron collects at the bottom of the furnace in a mass that is called a bloom. Because early furnaces could not reach very hot temperatures, the iron mass would contain ample impurities.

The technique for extracting these impurities was to heat the bloom and strike it repeatedly with hammers. Reenactment of Viking iron working

The shaping of iron into tools was by smithing, rather than casting The shaping of iron into tools was by smithing, rather than casting. Because long-distance trade was not necessary to make iron, it was a more democratic metal. Re-enacters making tools using iron age techniques and tools.