‘Alexandrian’ glass confirmed by hafnium isotopes

Abstract

Archaeological glass contains information about the movement of goods and ancient economies, yet our understanding of critical aspects of the ancient glass industry is fragmentary. During Roman times, distinct glass types produced in coastal regions of Egypt and the Levant used evaporitic soda (natron) mixed with Nile-derived sands. In the Levant, furnaces for producing colourless Roman glass by addition of manganese have been uncovered, whereas the source of the desirable antimonydecolourised Roman glass remains an enigma. In the Edict of Diocletian, this colourless glass is listed as "Alexandrian" referring to Egypt, but its origin has been ambiguous. Previous studies have found overlapping strontium and neodymium isotope ratios for Levantine and Egyptian glass. Here, we confirm these findings and show for the first time, based on glasses from the ancient city of Gerasa, that hafnium (Hf) isotopes are different in Egyptian and Levantine natron glasses, and that Sb Roman glass is Egyptian. Our work illustrates the value of Hf isotopes in provenancing archaeological glass. We attribute the striking difference in Hf isotopes of Egyptian versus Levantine glasses to sorting of zircons in Nile sediments during longshore drift and aeolian transport along the southeastern Mediterranean coast leaving behind a less juvenile fraction. The Roman glass industry underwent a massive expansion over the first century CE. At its peak it supplied not only tablewares for households across the Empire but also furnished major public buildings with many tonnes of glass for windows and mosaics 1,2. The raw glass was made by fusing Egyptian evaporitic soda (natron) and sand to produce large glass slabs in tank furnaces with capacities of 8-20 tonnes 3,4. These were broken up and distributed to glass workshops where the glass was remelted and shaped into objects for use 5,6. This division of production continued until at least the ninth century, when a change from a mineral soda flux over to plant ash occurred bringing about the end of the Roman glassmaking tradition 7,8. The technological achievements of the Roman glass industry were precocious and not surpassed until the rise of the European industries in the eighteenth century. In particular, the Romans produced large quantities of an expensive and highly valued glass, described by Pliny 9 as "colourless or transparent, as closely as possible resembling rock crystal" (Fig. 1), where the iron from the sand was oxidised from blue Fe 2+ to very pale Fe 3+ by the addition of antimony oxide, Sb 2 O 3 10,11. In the Price Edict of Diocletian, this colourless glass is listed as "Alexandrian" thereby referring to Egypt 12. Despite this, the production site for this so-called Sb Roman glass is unknown but several authors have suggested, on the basis of circumstantial evidence, that it was in Egypt 13,14 (see Supplementary Information for details). Strong evidence that the primary glassmaking factories melting sand and natron to glass were predominantly located along the coast of the eastern Me...

A glass workshop in ‘Aqir, Israel and a new type of compositional contamination

“…GV-1 has high soda and low alumina and was decolourised using antimony (Table 3). It is typical of Roman-Sb glass of the first-fourth centuries, probably originating in Egypt (Gliozzo 2017;Schibille et al, 2017;Barfod et al, 2020). GV-2 which contains c. 1%…”

Chemical analyses on late antique glass finds from Histria, Romania

“…One can state, within the PGAA detection limits, that this object was manufactured from fresh glass, as its 'clean' compositional pattern does not suggest recycling. According to some recent literature data, a likely Egyptian origin for the glass from which this beaker was shaped can be proposed (Barfod et al, 2020). Sb-decolorizing procedures were widespread during the early Roman period, being mainly used until the fourth c. AD (Gliozzo, 2017).…”

“…This classification has been continuously amended, as the body of studied Roman and Late Antique glass finds has been constantly on the rise. These refinements also demonstrate a better understanding of the data resulting from the numerous archaeometric studies published during the recent years (e. g. Barfod et al, 2020). The chemical groups identified in the Late Antique glass finds reflect the changes in raw materials (various sands and/or decolorizers) that took place in time, issues related to different provenances or manufacturing practices (Freestone, 2021).…”

Emerging Glass Industry Patterns in Late Antiquity Balkans and Beyond: New Analytical Findings on Foy 3.2 and Foy 2.1 Glass Types

“…In addition, elevated hafnium concentrations of AD-A-8 and AD-VC-1 (1.2 and 1.4 ppm, respectively) compared to the pertaining AD/N1 group (0.81 ± 0.18, excluding the outlier AD-A-11 with 6.6 ppm of hafnium) conform better to Roman antimony glass AD/N2 (1.13 ± 0.22 ppm), and MSG1c group from Padova (1.41 ± 0.81 ppm), classified as Foy 3.2, both with an Egyptian provenance. As shown by Barfod et al [ 22 ], the hafnium concentration in sands decreases from the Nile delta towards the Levantine coast during the longshore transport of the Nile sediments. This indicates that AD-A-8 comes from a location that is likely closer to the Nile delta from the origin of the majority of AD/N1 glasses.…”