The use of predominance area diagrams (PAD) to determine the oxygen and sulfur fugacities prevailing during historical metallurgical processes: the case of fifteenth to seventeenth century copper slags from Polichno (Old Polish industrial district)

The study presents the first use of predominance area diagrams (PADs) to determine oxidation–reduction conditions during reconstructing historical copper smelting processes. The smelting temperature and oxygen and sulfur fugacities during smelting were determined based on experiments and the geochemical (ICP-MS/ES, XRF) and mineralogical (SEM, EPMA) analyses of the fifteenth to seventeenth century slags from Polichno (Holy Cross Mountains, Poland). Results obtained during high-temperature experiments allowed to determine the slags' solidus and liquidus temperatures. The liquidus temperature was in the range of 1100–1200 °C, and the solidus temperature was in the range of 800–1100 °C. Data on temperature conditions were used in thermodynamic calculations to construct predominance area diagrams and then to determine the ranges of oxygen and sulfur fugacities in which the formation of slags was possible. Slags from Polichno were formed with the oxygen fugacity in the range of logP_O_2 = − 4.30 (POL1, POL4 at 1200 °C) to − 14.08 atm. (POL3 at 1090 °C). In turn, the sulfur fugacity during slag formation ranged from logP_S_2 = − 2.50 (POL5 at 1200 °C) to − 6.92 (POL4 at 1060 °C) atm. The relatively high sulfur availability confirms using sulfide ores without prior roasting. The wide range of sulfur and oxygen fugacity indicates the process's heterogeneity.