Glass was highly prized throughout the Roman Empire, particularly a colorless transparent version that resembled rock crystal. But the source of this brave material – known as Alessandrian glass – remained a mystery. Now, by studying trace quantities of the element of hafnium in glass, researchers have shown that this prized product really originated in ancient Egypt.
It was during the time of the Roman Empire that drinks and food were served in glass vessels for the first time on a large scale, said Patrick Degryse, an archaeometer at KU Leuven in Belgium, who was not involved. in the new study. “It was on every table,” he said. Glass was also used in windows and mosaics.
All that glass had to come from somewhere. Between the first and ninth centuries AD, Roman glass producers in the coastal regions of Egypt and the Levant kilns filled with sand. The huge glass slabs they created doubled the scales to almost 20 tons. That glass was then broken down and distributed in the glass workshops, where it was rebuilt and formed into final products.
But what most people wanted was colorless glass, so glass producers experimented by adding different elements to their lots. Producers in the Levant are known to have added manganese, which reacts with iron impurities in the sand. The manganese-treated glass still retained a bit of color, however, said Gry Hoffmann Barfod, a geoscientist at the University of Aarhus in Denmark who led the study, which was published this month in the Scientific Reports. “It wasn’t perfect,” she said.
Glassware also tried to increase antimony, with much better results. “It made it completely crystal clear,” Dr. Barfod said.
And expensive: A price list issued by the Diocletian Roman emperor in the early fourth century AD refers to this colorless glass as “Alexandria” and values it at almost double the price of glass treated with manganese. But the provenance of Alessandrian glass, despite its name, was never conclusively connected to Egypt.
“We have factories for manganese decolorized glass, but we don’t have the glass for Alexandrian,” Barfod said. “It was a mystery that historians dreamed of solving.”
Motivated by that enigma, Dr. Barfod and her colleagues analyzed 37 fragments of glass excavated in northern Jordan. The sheds, every inch or two long, included Alexandrian glass and manganese-treated glass from the first to fourth centuries AD. The sample also included other samples of glass known to have been produced more recently in Egypt or the Levant.
The researchers focused on hafnium, a trace element found in the mineral zirconium, a component of sand. They measured the concentration of hafnium and the ratio of two isotopes of hafnium in the sherds.
Counterfeit glass in different geographical regions had different hafnium signatures, showed Dr. Barfod and her collaborators. Egyptian glass consistently contained more hafnium and had lower isotope ratios than glass produced in the Levant, the team found.
These differences make sense, Dr. Barfod and her colleagues propose, because the zircon crystals in the sand are inadvertently chosen by nature.
After being expelled from the mouth of the Nile, the sand sweeps east and north to the Levant coast, driven by water currents. The zircon crystals inside them are heavy, so they tend to settle early in the trip on Egyptian beaches. This explains why forged glass in Egyptian kilns tends to contain more hafnium than Levantine glass, the researchers suggest.
When the researchers analyzed the Alexandrian sherds and manganese-treated glass, they again found distinct differences in hafnium. Manganese-treated glass had product-consistent hafnium properties in the Levant, as expected. And Alexandrian glass, most clearly from the clear when it came to transparent glass, was chemically resembling Egyptian glass.
It is gratifying to finally list the provenance of Alexandrian glass, Dr Barfod said, adding, “This has been an open question for decades.”
But it is still a mystery why glasses from Egypt and the Levant show different proportions of hafnium isotopes. One possibility is that zircons containing certain isotopic proportions are larger, denser or thicker, which affects their movement, Dr. Barfod said. “We don’t know.”
The analysis of Egyptian and Levantine beach sand chemistry would be a logical way to confirm these findings, Dr. Barfod said. “The next step obviously would be to get out and get sand out of both places.”