Glass was highly prized in the Roman Empire, especially the colorless, transparent version resembling a rock crystal. But the source of this testamentary material – known as Alexandrian glass – has long remained a mystery. Now, by studying the amounts of the element hafnium in the cup, researchers show that this valuable commodity really originated in Ancient Egypt.
During the Roman Empire, drinks and food were served in glass jars for the first time on a large scale, said Patrick Degriese, an archaeologist from KU Leuven in Belgium, who was not involved in the new study. “It was on every table,”
This whole cup had to come from somewhere. Between the first and ninth centuries AD, Roman glassmakers in the coastal areas of Egypt and the Levant filled sand kilns. The huge glass plates they created tilted the scales to nearly 20 tons. This cup is then dismantled and distributed in glass workshops, where it is melted and shaped into finished products.
But what many people really wanted was colorless glass, so glassmakers experimented with adding different elements to their batches. It is known that producers in the Levant have added manganese, which reacts with iron impurities in the sand. The manganese-treated glass still retains some color, says Grey Hoffman Barfod, a geologist at the University of Aarhus in Denmark who led the study, published this month in Scientific Reports. “It wasn’t perfect,” she said.
Glassmakers have also tried to add antimony, with much better results. “It made it completely crystal clear,” Dr. Barford said.
And expensive: The price list published by the Roman emperor Diocletian in the early fourth century AD calls this colorless glass “Alexandrian” and rates it at almost twice the price of manganese-treated glass. But the origin of Alexandrian glass, despite its name, has never been definitively anchored in Egypt.
“We have manganese-degreased glass factories, but we don’t have Alexandria glass factories,” said Dr. Barford. “History has been a mystery that historians have dreamed of solving.”
Motivated by this mystery, Dr. Barford and her colleagues analyzed 37 fragments of glass excavated in northern Jordan. The sarmas, every inch or two long, included Alexandrian glass and manganese-treated glass from the first to the fourth century AD. The sample also included other glass samples known to have been produced in Egypt or the Levant.
The researchers focused on hafnium, a trace element found in the mineral zircon, a component of sand. They measure the concentration of hafnium and the ratio of two hafnium isotopes in sarma.
Forged glass in different geographical regions had different hafnium signatures, Dr. Barford and her colleagues showed. Egyptian glass consistently contained more hafnium and had lower isotope ratios than Levant-produced glass, the team found.
These differences make sense, suggest Dr. Barford and her colleagues, because the zircon crystals in the sand are inadvertently sorted by nature.
After being expelled from the mouth of the Nile, the sand shifts east and north up the coast of the Levant, driven by water currents. The crystals in it are heavy, so they tend to settle early on the trip to the Egyptian beaches. This explains why glass forged in Egyptian furnaces usually contains more hafnium than Levantine glass, the researchers suggest.
When researchers analyzed sarma from Alexandrian and manganese-treated glass, they again found various differences in hafnia. The manganese-treated glass has hafnium properties consistent with Levant production as expected. And Alexandrian glass, clearer than clear glass when it came to clear glass, chemically resembled Egyptian glass.
It is being rewarded to finally cross the origins of Alexandrian glass, Dr Barford said, adding: “This has been an open question for decades.”
But it is still a mystery why the glasses from Egypt and the Levant show different ratios of hafnium isotopes. One possibility is that zircons containing certain isotope ratios are larger, denser or more voluminous, which affects their movement, Dr. Barford said. “We don’t know.”
An analysis of the chemistry of the Egyptian and Levantine sand beaches would be a logical way to confirm these findings, Dr. Barford said. “The next step will obviously be to go out and get sand from both places.”