Changes in the Signature of Cobalt Colorants in Late Antique and Early Islamic Glass Production

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.

Smalt production in the Ore Mountains: Characterization of samples related to the production of blue pigment in Bohemia

“…A series of works (Gratuze et al . 1995, 2018; Roo de 2004; Perez Arantegui et al . 2009; Giannini et al .…”

The Color Palette of the Mosaics in the Roman Villa of Noheda (Spain)

“…The nature of cobalt-bearing raw materials used in antiquity for coloring blue glasses is still unclear. A recent investigation distinguished two different cobalt sources according to the CoO/NiO ratios, which changed from higher to lower ratios sometime during the fourth century CE [70]. Interestingly, the cobalt blue samples from Noheda exhibit two distinct correlations of cobalt to nickel (Figure 7a), suggesting that two different cobalt sources were used, thereby confirming that the transition from the ancient cobalt source to the exploitation of a nickel-rich cobalt source occurred in the fourth century CE.…”

The Supply of Glass at Portus Ilicitanus (Alicante, Spain): A Meta‐Analysis of HIMT Glasses

“…Cobalt colourants tend to introduce only a limited number of impurities, which during the Roman period pertain mostly to iron, copper and lead (Gratuze et al . ). The ratios between iron and cobalt in the cobalt colourants calculated by subtracting the iron content of the vessel body from the iron content of the cobalt blue drops (6 < Fe 2 O 3 /CoO < 11) are consistent with the ratios of iron to cobalt typically encountered in Roman glasses (median of 7.6; Gratuze et al .…”

“…The ratios between iron and cobalt in the cobalt colourants calculated by subtracting the iron content of the vessel body from the iron content of the cobalt blue drops (6 < Fe 2 O 3 /CoO < 11) are consistent with the ratios of iron to cobalt typically encountered in Roman glasses (median of 7.6; Gratuze et al . ). The base glass composition of the blue drops can thus be assumed to have been made of HIMTa glass similar to that used for the vessel body, to which a cobalt source rich in copper, lead and iron was added (Table S1 and Fig.…”