Origin of Historical Ba-Rich Slags Related to Pb-Ag Production from Jihlava Ore District (Czech Republic)

Abstract

The aim of this study was to characterize historical slags which originated during silver production from the Jihlava ore district, Czech Republic. The area was among the head producers of silver within the Lands of the Czech Crown in 13th–14th centuries. The mined ores had complex composition, being formed mostly by pyrite, sphalerite, galena, chalcopyrite, and accessory silver-rich minerals such as silver-bearing tetrahedrite (freibergite) or pyrargyrite, with gangue represented by quartz and Mn-rich carbonates or baryte. Large volumes of slags with contrasting composition were generated during the Pb-Ag production. Altogether, two main types of slags were identified in the district. The first type is characterized by high BaO contents (up to 34.5 wt.%) and dominancy of glass, minor quartz, and accessory amounts of Ba-rich feldspar (up to 93 mol.% of Cls), metal-rich inclusions, Ba-Pb sulphates and only rare pyroxene, wollastonite and melilite. The composition of the second group belongs to fayalitic slags containing glass, Fe-rich olivine, accessory pyroxene, feldspar, quartz, and inclusions of various metallic phases. Fluxes were derived from gangue (quartz, carbonates, baryte) or local host rocks for both types of slag. The calculated viscosity indexes reflect (with minor exceptions) medium-to-high effectivity of metal separation. Smelting temperatures were estimated from a series of ternary plots; however, more reliable estimates for both types of slags were obtained only from experimental determination of melting temperature and calculations using bulk/glass compositions (~1100–1200 °C).

Efficiency of Portable X-ray Fluorescence for Distinguishing Lead Slag

“…Notably, barium, occurring naturally in the local mineralization gangue as barytes, is recognized as an effective flux in Pb-Ag metallurgy. 28 Similar to findings in Jihlava, 16 where barium was detected in certain slags, we identified the presence of 7.5% barium in seven out of the 14 slags subjected to SEM analysis.…”

“…33 resulting in an average melting temperature of 1150 °C. Notably, the temperatures do not deviate between gray and black slags and align with those estimated in Jihlava 16 and Massa. 29 In line with the conclusions drawn from the aforementioned studies, furnace ventilation would likely have been forced, although it remains uncertain at this stage of research whether this was linked to a hydraulic system.…”

“…14,15 Moreover, these types of slags may have broader implications, likely representing slag commonly produced between the 12th and 14th centuries. Indeed, visually similar slags have been identified in the Czech Republic, in proximity to the Jihlava mines, 16 as well as in Italy in the metallurgical workshops of Cugnano and Montieri. 17 However, Italian black and gray slags differ, as "matteless slags" are not commonly found in Tuscany.…”

“…This decision was based on the similarity in calculated melting temperatures among the black and gray slags, despite their distinct compositions. 16 Similarly, recent slag discoveries in England dating from the 15th century could also correspond to Case 3, exhibiting similar compositions. 21 Regarding the Mont Lozère slag, two hypotheses have been proposed: 14 (i) The gray slag may correspond to the ore roasting phase, while the black slag represents the reduction stage (Case 1); (ii) the two types of slag could be a consequence of the same reduction process, with the gray slag resulting from poorly purified ore treatment and the black slag from nearly pure ore (variant 1 of Case 2).…”

“…The smelting of polymetallic ores is also underscored in the case of Jihlava, where barium is utilized to facilitate the smelting of local polymetallic ores, renowned for their abundance in sphalerites, pyrites, and Cu ores. 16 Similarly, in Tuscany, the smelted ores are highly polymetallic, and the use of a flux, namely alumina, is strongly suspected. 17 Nonetheless, in both Tuscany and Mont Lozère, the precise number of stages involved in the creation of the two slag types remains inadequately understood.…”