Iron in copper metallurgy at the dawn of the Iron Age: Insights on iron invention from a mining and smelting site in the Caucasus

Despite enormous interest in the origins of the iron, the world's quintessential industrial metal, the technological foundations of the invention and innovation of extractive iron metallurgy remain unclear. While fundamental aspects of geology and thermodynamics favor a model for the invention of iron by copper smelters, empirical archaeological evidence to support this model is lacking. Reanalysis of the smelting workshop at Kvemo Bolnisi, originally published as an iron smelting site in the 1960s and dated to the late 2nd millennium BC, offers insights by which copper smelters recognized and experimented with iron oxides. Chemical and microscopic analysis of slags and minerals samples via optical microscopy and SEM-EDS conclusively shows that metalworkers at the site were smelting copper rather than iron. However, our analyses, coupled with a reassessment of the excavation report, show that iron oxides were deliberately stockpiled and added to the furnace as a separate component of the charge to flux the silica-rich host rock. These discoveries make Kvemo Bolnisi arguably the earliest unequivocal example of the deliberate use iron oxide fluxes in copper metallurgy. The knowledge and behaviors reflected in the Kvemo Bolnisi copper smelting technology have important implications for theories about the invention of iron metallurgy by copper smelters. • There is a lack of empirical evidence for the mechanics of iron invention • Analysis of copper smelting site reveals unambiguous, deliberate use of hematite flux • Iron-rich slags preserve relict ore and flux fragments, but ores lack iron • Results have significant implications for iron-from-copper models of invention • Use of flux was key for developing thermodynamic knowledge of iron and its compounds