Isotopic evidence for the use of Caucasian antimony in Late Bronze Age glass making

Highlights

• New Sb isotopic analysis of stibnite ore and LBA metal objects.

• The only plausible source for Sb in the first glass making is stibnite.

• Sb ores involved in metallurgy are both stibnite and polymetallic ores.

• Sb used for glass making may be related to precious metal mining.

• Egyptian and Mesopotamian glassmaking were connected to Caucasian mining.

Abstract

Antimony (Sb) is considered a rare material in the archaeological record, found only in unusual circumstances. Nevertheless, antimony minerals were an important resource for several millennia, used in metallurgy and to opacify or decolour glass and glazes. In this way, Sb spread throughout the known world from the Chalcolithic onward. In glassmaking, stibnite was the only available resource that could provide in any measure the very pure Sb evident from trace element analyses of the earliest glass. Sb isotopic analysis has allowed Late Bronze Age Egyptian and Mesopotamian glass vessels and Caucasian Sb metallic beads to be compared to the possible ancient ore sources. The only known matches for the isotopic composition of the glass are stibnite ores from the Racha-Lechkumi district in the Caucasus (present-day Georgia), near the Zopkhito Au–Sb deposits, mined from the 17th century BCE. Conversely, the Sb metal beads represent several isotopic and trace element compositional groups, only one of which matches the Racha-Lechkumi stibnite. Sb extraction for glassmaking was likely unrelated to copper metallurgy, and may have been associated with the mining of precious metals.

Introduction

The vitreous materials industries know a long history of use of antimony (Sb). The widespread adoption of glass as a man-made material started during the 16th century BCE, and factory-based glassmaking has been placed in Mesopotamia and Egypt, with early finds in Syro-Palestine, Mycenae and the Caucasus as well (Shortland, 2012). Sb minerals are a major raw material used as an opacifier, combined with copper (Cu) or cobalt (Co) to make opaque blue glasses, or with calcium (Ca) and lead (Pb) antimonate for the production of opaque white (Ca₂Sb₂O₇ or CaSb₂O₆) and opaque yellow (Pb₂Sb₂O₇) glass, respectively (Shortland, 2002a). Early glass was an elite material, used as a substitute for semi-precious stone such as lapis and turquoise, whereas the earliest yellow glass may have been equivalent to gold (Au) (Shortland, 2012). Glass technology in Mesopotamia differed subtly from contemporary Egyptian sites, using different ways of opacifying and working glass. At least two, perhaps three, Near Eastern production sites may have existed in addition to the Egyptian sites (Shortland et al., 2018). These glass manufacturing areas are chemically and isotopically distinct (Shortland et al., 2007; Degryse et al., 2010), and raw glass was exchanged very early on, as can be seen from the discovery of 16th-15th century BCE Mesopotamian glass in Egyptian tombs (Kemp et al., 2020), or from the late 14th century BCE Uluburun shipwreck carrying ingots of Egyptian blue glass. Also the Amarna letters, a diplomatic archive of the House of the King's correspondence, include requests for glass. The presence of metals such as copper, lead and antimony in strong coloured, opaque glass triggers interesting questions concerning the origin of the raw materials used for glassmaking, and whether such sources are the same as the ones used in metallurgy.

The oldest copper objects containing variable levels of arsenic (As) and/or Sb (1–20% by weight) have been found in the southern Levant and by far predate the earliest glass. The finds of the Nahal Mishmar hoard are dated to 4600–3500 BCE (Shalev and Northover, 1993). Copper objects with significant Sb and As contents have also been identified at various sites in central Italy, dated from the early 4th millennium BCE until the Early Bronze Age. Also, Sb metal beads have been found in central Italy, where the oldest context at the site of Ponte San Pietro dates to 3635–3376 cal BC (Dolfini, 2014). In the Caucasus, copper objects containing up to 20% Sb by weight, sometimes also with As, date from possibly the end of the Early Bronze Age (late 4th millennium BCE) and definitely the Middle Bronze Age (early 2nd millennium BCE). They differ from the previously discussed Sb-rich metal objects in their minor to trace element composition. During the Late Bronze Age, from the middle of the 2nd millennium BCE onwards, the use of Sb-rich copper objects became widespread in the region, especially in the northern Koban area (Hauptmann and Gambaschidze, 2001; Meliksetian et al., 2003; Meliksetian and Pernicka, 2010; Pike, 2002). All the aforementioned Sb-rich Cu objects must be considered natural alloys, derived from smelting complex ores. Also metallic Sb objects have been widely reported in the southern Caucasus, the earliest finds dating to the middle of the 3rd millennium BCE, and becoming more prominent during the 2nd millennium BCE from sites in Armenia, Dagestan and Georgia (e.g. Chernykh, 1992; Meliksetian et al., 2003).

The most likely origin for the Sb used as an opacifier in ancient glassmaking throughout the Near East has therefore been suggested to lie in the Caucasus (Shortland, 2002b), where Sb-rich minerals were specifically mined during the Bronze Age. The earliest dated exploitation of stibnite there, the Zopkhito mine, is securely placed in the 17th century BCE (Chernykh, 1992), just before the start of widespread glass production. The area is more commonly known, however, for gold mining. Here, we use the Sb isotopic analysis to link the earliest glass to these Sb raw materials.