Glass was a valuable and highly prized commodity within the Roman Empire, ancient China, and along the trade routes known collectively as the Silk Road. And yet archaeologists and scientists have been unable to come to a consensus on the origins and techniques for making various types of luxury glassware cherished during the Roman imperial period. Of particular interest is the location of workshops which created the crystal clear glass referred to as “Alexandrian glass.” Emerging technologies used for chemical and isotopic analysis combined with new archaeological discoveries are uncovering the sources, craftsmanship, and long-distance trade of this delicate commodity.
Ancient glass production required the heating of sand and lime with a flux (a substance which lowers the high melting point of the silica in the sand) in a kiln. Until the 9th century CE, this was usually a type of soda ash called natron. The technology has been around since the third millennium BCE within the ancient Levant (the area of modern Iraq and northern Syria, specifically at the Syrian site of Tell Brak); however, there remains many mysteries surrounding its primary production location. Although there is debate about the origins of the material, Mesopotamian artisans likely first crafted glass in order to make beads, glaze jewelry, and produce other small objects. By 1550 BCE, archaeological evidence surfaces for Egyptian glassworkers producing similar glass objects which would come to be particularly prized by pharaohs. Many of these vessels were a cobalt blue in imitation of stones such as lapis lazuli. Transparent glass was also a source of beauty and utility, being used for cosmetics bottles and decorative containers such as fruit bowls, but also used in ancient optics experiments and mechanical devices.
Back in 2005, archaeologists Thilo Rehren and Edgar B. Pusch, discovered a large number of artifacts with predominantly red-colored glass (a hue produced using copper) in them at a site on the Egypt’s Nile Delta called Qantir-Piramesses dating to 1250 BCE. The artifacts pointed to the fact that glass workers in the large, factory-like space would first heat raw materials within recycled beer jars. Next, the glass was colored and finally heated inside the crucibles in order to create round ingots.
These glass ingots could then be shipped elsewhere and later be turned into a litany of various glass containers, mosaic tiles, or window panes by glassblowers. The translucent “Alexandrian glass” mentioned in a Roman inscription describing prices for goods and services called the Price Edict of Diocletian (301 CE) notes this clear glass was the most expensive of the types listed. While some modern scholars have long believed “Alexandrian glass” was shorthand for all clear glass, scientists are now discovering through new processes of analysis connected to the isotopes found in sand that it is likely tied directly to the Egyptian city of Alexandria itself.
The use of isotope analysis is now allowing for new insights into this distinct, transparent type of glass with antimony added. Geoscientists from Aarhus University led by Gry Hoffmann Barfod and archaeologists from University College London and the Institute of Classical Archaeology in Münster recently published findings using ancient glass from the northern Jordanian site of Gerasa. The study indicates the utility of using the element hafnium (Hf) in tracing the provenance of ancient glass in order to isolate where it originally came from.
The isotopic analysis of the Gerasan glass allowed the scientists to pinpoint and separate out glass made in Egypt from that made in the Levant, and, in particular, to identify and document the production of antimony (Sb) — i.e. transparent — Roman glass to the area of Egypt. Manganese and antimony are the two elements that were regularly used in glass production in order to decolorize it. As the study concludes, the Nile and Atbara rivers brought minerals to the Nile Delta from Ethiopia, which controls the Neodymium (Nd) isotopic compositions of Nile sands. Analysis of these compositions can then allow for more certitude in identifying primary glass production sites in the Eastern Mediterranean.
This summer has seen a number of different approaches to sourcing the provenance of ancient and medieval glassware that are changing the field altogether. In a particular study from archaeologists at the University of Nottingham and the University of Science and Technology Beijing, scientists performed an isotopic investigation of sands, plants ashes, and Islamic glasses from Turkey and Syro-Palestine, this time using neodymium and strontium isotopes. The study addresses the provenance of the glass found in a famed 11th century CE shipwreck of a Byzantine merchant vessel at Serçe Limanı, a harbor on the southern coast of Turkey near Rhodes. The authors note they have used isotopic and chemical analysis to locate the origins of the glass found within the submerged Byzantine ship:
For the first time we have been able to provide a more secure provenance for the late 11th century CE Serçe Limani raw glass. It was made from Levantine coastal sand, or an equivalent source of geologically young sand, and has a Sr signature that suggests a production provenance in the Palestinian area, but not in established centres such as Tyre as has been suggested.
The Nottingham study has now established isotopic provenances for 9th- to 12th-century Islamic glass vessel fragments from al-Raqqa, Beirut and Damascus and demonstrated they “probably [coincide] with an area in or near Damascus. Two al-Raqqa vessel glasses with elevated potassium oxide levels and distinctive Nd and Sr signatures may have a central Asian origin.”
These newly published studies exemplify that knowledge of long-distance trade in antiquity and the middle ages is still evolving. Geochemists and archaeologists are now increasingly able to understand, chart, and then map out the isotopic “signatures” of various Mediterranean areas using glass, and in turn, to offer a broader understanding of the expansive trade networks at work in the Roman Empire and along the Silk Road. Who knew that one of the most fragile and luxurious of materials would become so valuable in exposing the bustling economy of the premodern world.