A 3000-year-old copper smelting site could be key to understanding the origins of iron

Research from Cranfield University sheds new light onto the transition from the Bronze Age to the Iron Age, showing how experimentation with iron-rich rocks by copper smelters may have sparked the invention of iron.

Published in the Journal of Archaeological Science, the work reanalyzed metallurgical remains from a site in southern Georgia: a 3000-year-old smelting workshop called Kvemo Bolnisi. During the original analysis in the 1950s, piles of hematite (an iron oxide mineral) and slag (a waste product of the metal production) were found in the workshop. Finding those iron oxides, the original excavators thought the workshop was an early iron smelting site.

However, new research shows that those assumptions were wrong. Rather than iron, workers at Kvemo Bolnisi were smelting copper using iron oxide as a flux—a substance added into the furnace to increase the resulting copper yield.

These discoveries give weight to a long-discussed theory that iron was invented by copper smelters. This evidence shows that ancient copper metalworkers experimented with iron-bearing materials in a metallurgical furnace, which was a crucial step towards iron smelting.

The importance of iron

While the Iron Age marked the beginnings of widespread iron production, the metal itself wasn’t a new discovery. Iron artifacts have been found dating from the Bronze Age, most famously an iron dagger with a gold and rock crystal hilt from the tomb of Egyptian king Tutankhamun.

But the earliest iron objects were forged from naturally occurring metallic iron found in meteorites, not extracted from iron ore through smelting. That rarity meant iron was, at that point in history, more valuable than gold.

The development of extractive iron metallurgy changed all this. Iron is one of the most abundant elements on Earth, even though naturally occurring iron metal is very rare. The ability to extract iron from iron ore and work it into useful materials such as tools or weapons is one of the defining technological transformations in human history.

The transition into the Iron Age was far from instantaneous, but it gave rise to the iron-wielding armies of Assyria and Rome and later the railroads and steel-frame buildings of the industrial revolution.

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Dr. Nathaniel Erb-Satullo, Visiting Fellow in Archaeological Science at Cranfield University, said, “Iron is the world’s quintessential industrial metal, but the lack of written records, iron’s tendency to rust, and a lack of research on iron production sites has made the search for its origins challenging.

“That’s what makes this site at Kvemo Bolnisi so exciting. It’s evidence of intentional use of iron in the copper smelting process. That shows that these metalworkers understood iron oxide—the geological compounds that would eventually be used as ore for iron smelting—as a separate material and experimented with its properties within the furnace. Its use here suggests that this kind of experimentation by copper-workers was crucial to development of iron metallurgy.

“There’s a beautiful symmetry in this kind of research, in that we can use the techniques of modern geology and materials science to get into the minds of ancient materials scientists. And we can do all this through the analysis of slag—a mundane waste material that looks like lumps of funny-looking rock.”