Shiny bronze in glassy matter: an inconspicuous piece of slag from the Bronze Age mining site of Mušiston (Tajikistan) and its significance for the development of tin metallurgy in Central Asia

Abstract

This paper aims at contributing to a better understanding of the beginnings of tin and bronze metallurgy in Central Asia by investigating a hitherto unique piece of a bronze slag. The object was originally discovered as a stray find only 4 km away from the large copper-tin deposit of Mušiston in Tajikistan. It contains many prills of bronze and copper as well as small charcoal particles. Radiocarbon dating of the charcoal places the slag in a period between 1900 and 1400 BCE and thus in the Late Bronze Age of the region. This date coincides with radiocarbon dates of relics from underground galleries of the Mušiston deposit. Chemical and microscopic examination demonstrated the slag to be a relic of a co-smelting process, in which a natural assemblage of tin and copper minerals was smelted simultaneously. Both the chemical and the tin and copper isotope compositions clearly link the slag to the nearby polymetallic ores from Mušiston, of which an extensive dataset is presented. The artefact’s lead isotope ratios and increased iron concentration in turn indicate intentional fluxing of the original ore charge with iron-dominated ores. These results are the first tangible evidence of a smelting process of tin ores in the entire region and therefore add a new dimension to the findings from previous mining archaeological investigations. At the same time, the results give significant information about the smelting process of secondary polymetallic ores from Mušiston and help in assessing the scientific data of Bronze Age bronze artefacts from Central Asia.

Bronze smelting activities during the Zhou dynasty: Analysis of slags from the Xincun and Baidian foundry sites in Central China

“…Based on these points, the central area of slag H98:3 is probably closer in chemical composition to the original tin ore than its outer area, and the original tin ore type could have been a calcium-rich vein type. The higher calcium content in the outer area may not be entirely derived from the ore (Berger et al 2022). The calcium content of the crucible was only 1.6%, which is much lower than that in the slag, indicating an introduction of considerable amount of external calcium during the smelting process.…”

“…The atmosphere of the furnace is another important factor affecting the smelting process. A strong reducing atmosphere is necessary for the reduction of minerals in the ores to metals, without remnant unreacted ore components (except for quartz) (Berger et al 2022). The analysis of the Xincun slag H91:4 suggested a weak reducing atmosphere during the smelting of the copper ores, resulting in incomplete ore reduction and leading to a heterogeneous microstructure with a mixture of malayaite and lead-bearing aluminate particles present (Fig.…”

“…Berger suggested that the predominance of tin and zinc (compounds) in the centre together with the numerous metallic inclusions limits the zone of ore reaction to this area, thus the sample may have experienced a low smelting temperature or insu cient smelting time (Berger et al 2022;Hauptmann 2007). Based on these points, the central area of slag H98:3 is probably closer in chemical composition to the original tin ore than its outer area, and the original tin ore type could have been a calcium-rich vein type.…”

“…The calcium content of the crucible was only 1.6%, which is much lower than that in the slag, indicating an introduction of considerable amount of external calcium during the smelting process. The poor homogeneity of the slag may be due to the uneven distribution of tin in the ore, resulting varying degrees of reaction between the ore and the calcareous uxes in different areas (Berger et al 2022).…”

“…This, on the one hand, may be due to the heterogeneous feature of smelting using an open crucible that may have created weak reducing atmospheres in some areas (Rovira 2004). On the other hand, oxidation may have occurred during the cooling process, with some of iron silicates being oxidised, and precipitated to hematite (Hauptmann 2007;Berger et al 2022).…”