Glass was highly valued throughout the Roman Empire, particularly a clear, colorless version that resembled rock crystal. But the source of this coveted material, known as Alexandrine glass, has long remained a mystery. Now, by studying small amounts of the element hafnium inside the glass, researchers have shown that this precious product actually originated in ancient Egypt.
It was during the time of the Roman Empire that drinks and food were served in glass containers for the first time on a large scale, said Patrick Degryse, an archeometrist 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 1st and 9th centuries AD, Roman glaziers in the coastal regions of Egypt and the Levant filled the kilns with sand. The huge glass slabs they created tipped the scales to almost 20 tons. That glass was broken and distributed to glass workshops, where it was melted and transformed into final products.
But what many people really wanted was colorless glass, so glassmakers experimented with adding different elements to their batches. Levante producers are known to have added manganese, which reacts with iron impurities in the sand. However, manganese-treated glass still retained some color, said Gry Hoffmann Barfod, a geoscientist at Aarhus University in Denmark who led the study, which was published this month in Scientific Reports. “It was not perfect,” he said.
The glaziers also tried to add antimony, with much better results. “That made it completely crystal clear,” said Dr. Barfod.
And expensive: a price list issued by the Roman emperor Diocletian in the early 4th century AD. C. refers to this colorless glass as “Alexandrine” and values it at almost double the price of manganese-treated glass. But the provenance of Alexandrian glass, despite its name, had never been conclusively established in Egypt.
“We have the factories for manganese bleached glass, but we don’t have them for Alexandrine glass,” said Dr. Barfod. “It has been a mystery that historians have dreamed of solving.”
Motivated by this puzzle, Dr. Barfod and her colleagues analyzed 37 glass fragments excavated in northern Jordan. The sherds, each an inch or two long, included Alexandrine glass and manganese-treated glass from the 1st to the 4th century AD. The sample also included other glass specimens known to have been produced more recently in Egypt or the Levant. .
The researchers focused on hafnium, a trace element found in the mineral zircon, a component of the sand. They measured the hafnium concentration and the ratio of two hafnium isotopes in the pots.
Glass forged in different geographic regions had different hafnium signatures, Dr. Barfod and her collaborators showed. 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 within the sand are inadvertently classified by nature.
After being expelled from the mouth of the Nile, the sand stretches east and north along the Levante coast, driven by water currents. The zircon crystals inside are heavy, so they tend to settle early on the journey on Egyptian beaches. That explains why glass forged in Egyptian kilns tends to contain more hafnium than Levantine glass, the researchers suggest.
When the researchers analyzed the Alexandrine and manganese-treated glass fragments, they again found clear differences in hafnium. The manganese treated glass had hafnium properties consistent with production in the Levante as expected. And Alexandrian glass, the clearest of the gaps when it came to clear glass, chemically resembled Egyptian glass.
It’s finally gratifying to determine where the Alexandrine glass came from, Dr. Barfod said, adding: “This has been an open question for decades.”
But it remains a mystery why the glasses from Egypt and the Levant display different proportions of hafnium isotopes. One possibility is that zircons containing certain isotopic ratios are larger, denser, or more voluminous, which affects their movement, Dr. Barfod said. “We do not know”.
Analyzing the chemistry of Egyptian and Levantine beach sand would be a logical way to confirm these findings, Dr. Barfod said. “The next step obviously would be to get out and get sand from both places.”
