The study focuses on the reconstruction of the technological process in the 16th–17th century lead smelter in Sławków based on chemical and petrographic analyzes of slags. There are three main types of material at the landfill: glassy, crystalline, and weathered. Glassy slags are made of amorphous phase in which crystals of pyroxene, willemite, olivine, wüstite, and lead oxide appear. Crystalline slags are composed of wollastonite, rankinite, melilite, anorthite, quartz, and Fe oxides. Weathered slags have a composition similar to glassy slags, but they also contain secondary phases: anglesite and cerussite. Chemical analyzes confirmed that the smelter used sulphide ores, which were roasted, and the main addition to the charge was quartz sand. The smelting process took place in a brick-built furnace, under reducing conditions, with varied oxygen fugacity ranging from WM to MH buffer. The slag characteristics show a knowledge of the workers in the field of smelting methods. The addition of SiO₂ allowed for the binding of elements that could contaminate the obtained lead, and at the same time, the low melting point of the material (1150 °C) and the melt viscosity (logη = 1.34 for 1150 °C) was maintained, enabling the effective separation of liquid lead.

Slags from the historic metallurgy of Zn-Pb ores are known for unique chemical and phase compositions. The oxides, silicates, aluminosilicates, and amorphous phases present therein often contain in the structure elements that are rare in natural conditions, such as Zn, Pb, As. The study focuses on processes occurring on the contact of the melted batch and the refractory materials that build the furnace, which lead to the formation of these phases. To describe them, chemical (X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS)) and petrological ((X-ray diffraction (XRD), electron probe micro-analyses (EPMA), Raman spectroscopy) analyses were performed on refractory material, slag, and contact of both. Two main types of reactions have been distinguished: gas/fluid- refractories and liquid- refractories. The first of them enrich the refractories with elements that migrate with the gas (Pb, K, Na, As, Zn) and transport the components building it (Fe, Mg, Ca) inward. Reactions between melted batch and refractory materials through gravitational differentiation and the melting of refractories lead to the formation of an aluminosilicate liquid with a high content of heavy elements. Cooling of this melt causes crystallization of minerals characteristic for slag, but with a modified composition, such as Fe-rich pyroxenes, Pb-rich K-feldspar, or PbO-As₂O₃-SiO₂ glass.

This study presents the first reconstruction of the smelting conditions in 16th‐ to 18th‐century smelters from Miedziana Góra (Holy Cross Mountains, Poland). Based on geochemical (inductively coupled plasma mass spectrometry/emission spectrometry, X‐ray fluorescence) and mineralogical analysis (X‐ray diffractometry, scanning electron microscopy, electron probe micro‐analysis) of historical slags, their chemical/phase composition and the basic smelting parameters (temperature, melt viscosity, and oxygen fugacity) were determined. Due to the differences in chemical and phase composition, slags from different smelting stages have been distinguished: hypocrystalline slags (MG6) from speiss/matte production and glassy (MG1–MG5) from matte conversion. In glassy slags, pyroxenes, quartz/cristobalite grains, and aggregates composed of metallic Cu and PbO are dispersed in the glass. Hypocrystalline slags are composed of wollastonites, anorthites, and metallic Cu. The temperature range at which the slags were formed was from ~1100°C (solidus temperature) to 1150–1200°C (liquidus temperature). The silicate melt's viscosity was from log η = 1.19 to 4.42 Pa s (at 1100–1200°C). The higher viscosity of MG1–MG5 slags indicates that, unlike MG6 slags, they were not formed during gravity separation. Information about the phase composition made it possible to determine the oxygen fugacity in the range of log fO₂ = −4 to −12 atm. High oxygen fugacity indicates the oxidizing nature of the smelting process.

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 $$\text{logP}_{{\text{O}}_2}$$ 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 $$\text{logP}_{{\text{S}}_2}$$ 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.