Ancient Bronze Age metalworkers producing copper may have been unknowingly generating iron as well. Robert Downes of the University of Oxford argues that iron metallurgy has its roots in copper smelting furnaces of the Near East. There, iron was an unintended but increasingly visible byproduct, long before anyone set out to produce it deliberately.
The study, published in Archaeological Perspectives on Materials and Technologies, centers on a series of full-scale furnace experiments. Downes fired charges of copper and iron oxides inside a reconstructed shaft furnace, matching ancient copper production conditions.
Iron entered the copper metal in every single experiment. Its quantity tracked directly with how much iron was present in the original ore charge.
Iron found its way into copper in every experiment
A key finding involves what happened during casting. When furnace copper was melted and poured into ingots, the iron content dropped dramatically. In one experiment, iron loss during a single casting exceeded 70 percent.
Downes argues this explains a longstanding puzzle about why ancient copper artifacts contain so little iron. The casting process was quietly stripping most of it away each time.
This also means accidental iron production was far larger than previously assumed. At Timna in southern Israel, Downes estimates ancient smelters produced roughly 350 kilograms (772 pounds) of iron per year as a byproduct.
At Wadi Nasib in the Sinai, copper production over the 2nd millennium BC may have totaled around 5,000 tonnes. Unintended iron output there could have reached hundreds of tonnes over two centuries.
Bronze Age metalworkers produced iron on a massive scale
At those volumes, workers would have noticed. Once the iron content in the copper exceeded around 10 percent by weight, the gray metallic phases became visible to the naked eye. Beyond 30 percent, the iron was hard to miss. It even disrupted the casting process through rapid oxidation.
Despite this, Downes rules out the theory that workable iron could have been pulled directly from a copper smelt. The experiments showed that iron became too entangled with the copper to consolidate into a usable bloom. No clean separation occurred at any point during firing.
Why a direct route to iron simply did not work
Instead, Downes proposes that repeated observation of this anomaly pushed ancient metalworkers to experiment gradually. Through trial and error, they learned to reduce copper minerals in the ore charge.
Eventually, they began firing iron-bearing minerals alone. Two further steps were then required. Workers needed to master the retrieval of the iron bloom at high heat and then forge weld it into a usable mass.
These developments, Downes concludes, laid the foundation for bloomery iron smelting methods. That technology spread across the Near East by the early 1st millennium BC.
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