Reconstructing Ancient Copper Smelting Practices in the Eastern Part of the Srednogorie Metallogenic Zone (SE Bulgaria) Using Slag Mineralogy and Chemistry

Abstract

This study reconstructs the smelting conditions of ancient copper production in the eastern Srednogorie metallogenic zone (SE Bulgaria) based on the mineralogical and chemical composition of archaeological slags. Slag samples from three ore fields were investigated: Rosen (Propadnala Voda, Korucheshme and Rosen deposits, and Atiya metallurgical centre), Zidarovo (Krushevets archaeological site) and St. Ilia (Boyadzhik archaeological site). Mineralogical observations, bulk chemical analyses, SEM-EDS data and phase-diagram evaluation were applied. The slags are dominated by iron-bearing phases (fayalite, magnetite/maghemite, wüstite, laihunite and kirschsteinite), with copper occurring mainly as metallic droplets and sulphide matte inclusions. Slags from Rosen ore field and Krushevets indicate smelting of sulphidic copper ores under moderately reducing conditions, whereas Ca-rich Boyadzhik slags lacking sulphides suggest smelting of copper ore from the oxidation zone of a copper deposit. The slag viscosity and melt characteristics varied significantly between metallurgical sites. Estimated smelting temperatures range from ~1050–1100 °C (Rosen ore field) to ~1150–1200 °C (Krushevets) and ~1200–1300 °C (Boyadzhik). These results reveal several metallurgical regimes in the eastern Srednogorie region and provide new archaeomineralogical evidence for technological variability in ancient copper production. All studied slags contain critical and strategic elements. Their behaviour and distribution between the melt and the crystalline phases formed during cooling are directly governed by smelting conditions, a thorough understanding of which is essential for developing an appropriate methodology for their extraction.