Abstract
Prior to the eighteenth century, cobalt was exclusively employed as a colouring agent for vitreous materials, and its use appears to be concurrent with the earliest large-scale production of glass during the Late Bronze Age (LBA). LBA cobalt deposits with a distinctive elemental signature have been identified in the oases of the western Egyptian desert, while cobalt mines in Kashan (Iran) and in the Erzgebirge (Germany) are known to have been exploited during the later Middle Ages. For most of the first millennium BCE and CE, however, the identity of cobalt sources and their supply patterns remain elusive. The aim of this study is to characterise the chemical composition of cobalt colorants used during the first millennium CE. Compositional variations indicate the use of different raw materials and/or production processes, which in turn has implications for the underlying exchange networks. Using mainly correlations between cobalt, nickel and zinc as discriminants, our results show that the compositional signature of cobalt underwent two major changes. An increase in the CoO/NiO ratios occurs between the late fourth and the beginning of the sixth century, while a new zinc-rich source of cobalt begins to be exploited during the second half of the eighth century in the Islamic world.
Highlights
The use of cobalt as a colorant for vitreous materials appears to have more or less coincided with the advent of the large-scale production of glass during the Late Bronze Age (LBA) both in Egypt [1,2]as well as in Mesopotamia [3]
One of the main characteristics of the cobalt colorants used during the first millennium CE is their seemingly high purity
To determine the chemical signature of the cobalt colorant itself, it is necessary to first reconstruct the composition of the raw glass used for the production of the cobalt-blue glass
Summary
The use of cobalt as a colorant for vitreous materials appears to have more or less coincided with the advent of the large-scale production of glass during the Late Bronze Age (LBA) both in Egypt [1,2]as well as in Mesopotamia [3]. The use of cobalt as a colorant for vitreous materials appears to have more or less coincided with the advent of the large-scale production of glass during the Late Bronze Age (LBA) both in Egypt [1,2]. In the European Iron Age, deep blue glass became the prevailing colour for the production of Celtic beads and bracelets from the time of La Tène C [4,5]. Cobalt is exceptional among the glass colorants, because in contrast to other colouring elements such as iron, copper, antimony, tin, lead and manganese, cobalt was exclusively used for its colouring properties. Whatever its origin and mineralogical nature, the use of cobalt as a colouring agent in glass and ceramic workshops implies the establishment of extraction processes and refinement techniques.
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