Mineralogy of slags: A key approach for our understanding of ancient copper smelting processes

Abstract

Copper was the first metal to have been smelted (extracted from its ore) some seven thousands year ago in the ancient Near East. For most pre-industrial periods, the documentation of copper smelting chaine operatoire relies mainly on investigations by archaeometallurgists of the metallurgical waste recovered during archaeological excavations, namely the copper slags. Copper slags are mostly an assemblage of crystals of oxides (iron, manganese, etc.), olivine (fayalite, etc.) and/or pyroxenes embedded in a polymetallic more-or-less glassy matrix. The mineralogy of the slags is directly related to the initial charge and the working conditions prevailing in the pyrometallurgical reactor. This chapter aims to give an overview of how copper slag mineralogy is investigated and the type of information it yields in order to help our understanding of past metallurgies and societies.

Reconstructing an ancient mining landscape: a multidisciplinary approach to copper mining at Skouriotissa, Cyprus

“…The most important ancient anthropogenic landmarks in the Skouriotissa area are the slag heaps . They are without doubt among the largest ancient slag heaps known in Europe (Bourgarit 2019). When mining restarted in the twentieth century, copper ores were not smelted on the island, but were instead exported as concentrates (Constantinou 1992b).…”

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

“…The majority of the fayalite has skeletal or 'hopper' habitus with hollow cores (Figs. 10,11 and 12) indicating slow cooling between 40 and 80 °C h −1 (Donaldson 1976;Faure et al 2003;Addis et al 2016;Bourgarit 2019). This common conclusion in the study of ancient slags was recently challenged by Pearce et al (2021) and somewhat earlier by Ettler et al (2009).…”

“…A rough estimate can be made with appropriate phase diagrams. Because the present slag, as many others from the Bronze Age, formed far from equilibrium conditions, the following can only be a very rough approximation (Hauptmann 2007;Bourgarit 2019). Moreover, ternary phase diagrams account for only a limited number of components (usually the main components), thus neglecting others (mainly minor components).…”

“…In contrast to tin, loss of copper to the slag was rather small, at least much less than for the smelting during the early stages of extractive metallurgy (Hauptmann 2007;. With about 12% by mass Cu 2 O in the centre region of the slag, the loss is well within the range of Early Bronze Age copper slags, but higher than for slags from the Late Bronze Age onwards (Bachmann 1980;Bachmann and Rothenberg 1980;Hauptmann 2007;Rehren et al 2012;Erb-Satullo et al 2015;Addis et al 2016;Bourgarit 2019). In combination with the proposed iron oxide fluxing, the copper content and the reconstructed process conditions suggest a developed smelting technology that fits into the metallurgical traditions of the mid-2nd millennium BCE.…”

“…The predominant granular morphology of magnetite or zinco-stanniferrous magnetite over dendritic growth characteristics nevertheless attests that the cooling could not have been too fast (Burger 2008, 97-98), as does the presence of melilites ((ferro)åkermanite-iron gehlenite-hardystonite) in the centre region (Ettler et al 2009;Vanaecker et al 2014). Crystal zoning, which would otherwise be an indication of growth under changing temperature conditions, is completely absent (Addis et al 2016;Bourgarit 2019). The observations thus confirm the general impression of slag solidification within a furnace without any signs of tapping, but moderate cooling and missing chemical zoning of olivines and spinels might suggest a larger reaction volume.…”