The Roman Empire pioneered many technical and scientific feats at the height of its power. While aqueducts, architecture and roads are most often associated with the ancient civilization, they also did something else very well—glassmaking.
Indeed, the Roman glass industry was prolific, mass producing wares for drinking and dining, window panes and wall mosaics. One of the most outstanding achievements, though, was “Alexandrian” glass—a colorless glass favored for high-quality drinking ware. The name of the glass, as declared by Roman Emperor Diocletian in the fourth century, led researchers to believe it had Egyptian origins. However, large amounts of Roman glass are known to have been made in Palestine, where archaeologists have uncovered furnaces for colorless glass production. But, no such furnaces have been found in Egypt.
So, despite archeologists’ best efforts, the origins of Alexandrian glass have been muddled—until now. Researchers with Aarhus University (Denmark) employed an ion chromatography-based method that revealed a high level of the rare isotope hafnium in the glass, providing compelling evidence that the glass was indeed made in Egypt.
The isotopic evidence from the researchers’ study, which was recently published in Scientific Reports, reveals multiple aspects of the Roman glassmaking industry. Isotope concentrations led the researchers to divide the glass into three categories—Egyptian, Levant (Palestine, Israel, Lebanon and Syria) and recycled Roman glass. While strontium (Sr) and neodymium (Nd) isotopes appear in all the types of glass, the presence of hafnium (Hf) in Alexandrian glass sets it apart.
Chromatographic analysis revealed a clear line of demarcation that shows hafnium isotopes can distinguish between natron glass made in Egypt versus natron glass made in the Levant. Specifically, the values of the hafnium isotopes place the production of Alexandrian Roman glass in Egypt.
Since hafnium and neodymium in natron glass are controlled by minerals in the sands used for glass production, the researchers traced how the common elements of strontium and neodymium would be expected to flow along the entire Mediterranean coast. Their analysis of sand movement and sediment deposition confirmed the link between the level of isotope concentrations in each type of glass and the glass’ place of origin.
“Hafnium isotopes have proved to be an important tracer for the origins of sedimentary deposits in geology, so I expected this isotope system to fingerprint the sands used in glassmaking,” said study co-authors Gry Barfod and Charles Lesher. “The fact that this expectation is borne out by the measurements is a testament of the intimate link between archaeology and geology.”
While hafnium isotopes have been used in geology before, this study marks the first time they have been used to examine man-made materials in an archeological context.
“These exciting results clearly show the potential of hafnium isotopes in elucidating the origins of early materials. I predict they will become an important part of the scientific toolkit used in our investigation of the ancient economy,” said study co-author Ian Freestone, a professor at the University College London.
Photo: One of the colorless Roman glass sherds from Jerash, Jordan, analyzed in this study. Purple splashes are iridescence due to weathering. Credit: Danish-German Jerash Northwest Quarter Project.