Comprehensive Chemical Characterisation of Byzantine Glass Weights

Abstract

The understanding of the glass trade in the first millennium CE relies on the characterisation of well-dated compositional groups and the identification of their primary production sites. 275 Byzantine glass weights from the British Museum and the Bibliothèque nationale de France dating to the sixth and seventh century were analysed by LA-ICP-MS. Multivariate statistical and graphical data analysis discriminated between six main primary glass types. Primary glass sources were differentiated based on multi-dimensional comparison of silica-derived elements (MgO, Al2O3, CaO, TiO2, Fe2O3, ZrO2) and components associated with the alkali source (Li2O, B2O3). Along with Egyptian and Levantine origins of the glassmaking sands, variations in the natron source possibly point to the exploitation of two different natron deposits. Differences in strontium to calcium ratios revealed variations in the carbonate fractions in the sand. At least two cobalt sources were employed as colouring agents, one of which shows strong correlations with nickel, indicating a specific post-Roman cobalt source. Typological evidence identified chronological developments in the use of the different glass groups. Throughout the sixth century, Byzantine glass weights were predominately produced from two glasses that are probably of an Egyptian origin (Foy-2 and Foy-2 high Fe). Towards the second half of the sixth century a new but related plant-ash glass type emerged (Magby). Levantine I was likewise found among the late sixth- to early seventh-century samples. The use of different dies for the same batch testifies to large-scale, centralised production of the weights, while the same die used for different primary production groups demonstrates the co-existence of alternative sources of supply. Given the comprehensive design of our study, these results can be extrapolated to the wider early Byzantine glass industry and its changes at large.

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

“…It should be borne in mind, however, that in some cases the CoO/NiO ratio of the colorant may be distorted for low cobalt concentrations due to overall elevated heavy element levels in certain base glass types. For instance, the nickel content in ancient colourless raw glass may vary from a few ppm in antimony decoloured glasses (3 ppm < NiO < 7 ppm) up to a few tens of ppm in HIMT (NiO ≈ 20 ppm) or Foy-2 high Fe samples (NiO ≈ 35 ppm) [25]. Manganese decoloured glass has nickel contents in the order of 10 to 15 ppm.…”

“…Recent analytical data from our laboratory have revealed the existence of cobalt colorants with comparatively low CoO/NiO ratios (3 < CoO/NiO < 10) in glasses dating to the sixth and seventh centuries CE. In a comprehensive analytical study of Byzantine glass weights, the cobalt signature was identified by the direct comparison of the chemical profile of two cobalt blue samples of the same base glass, probably deriving from the same batch, but with minor variations in elements related to the cobalt colorant [25]. Using the same computational model as before, the mixture of the colouring agent used for the cobalt blue Byzantine glass weights can be approximated as the difference between the non-coloured and cobalt coloured samples of the same primary glass production group (Table 4).…”

“…Byzantine glass weights that belong to the so-called Foy-2 glass type (n = 59) attributed to the sixth and seventh centuries have a median CoO/NiO ratio of about 8 (3 < CoO/NiO < 23). The more or less contemporaneous Levantine I glass weights (n = 24) have an even lower median of 3.3 and a very narrow range (2 < CoO/NiO < 5) [25]. Cobalt blue Viking beads and glass waste recovered from Ribe in Denmark (n = 38) dating mostly to the eighth century appear to break this pattern with a higher CoO/NiO median of 17 (5 < CoO/NiO < 30) [26].…”

“…The CoO/NiO ratios of these glasses is relatively low. At the same time, zinc contents increase to the extent that their concentrations are equal or higher than those of cobalt [19][20][21][22][23][24][25][26]. This transformation is best reflected in the compositional characteristics of Islamic soda-ash glass weights from Egypt dating to the eight to twelfth century in contrast to Byzantine glass weights from the sixth and seventh centuries ( Figure 6).…”

“…Despite some attempts to distinguish first millennium cobalt sources based on variations in CoO/NiO or CoO/ZnO ratios or on the manganese and antimony contents [19][20][21][22][23][24][25][26][27], the lack of reliable analytical data and the apparent high purity of the cobalt complicate the characterisation of cobalt and its distribution during this period. For the present paper, mostly published and forthcoming data of several glass assemblages dating to the fourth to eleventh century CE were collated and statistically evaluated.…”