Glass was highly prized throughout the Roman Empire, especially a colorless, transparent version that resembled rock crystal. But the source of this coveted material – known as Alexander glass – has long remained a mystery. Now, by studying the trace amounts of the element hafnium inside the glass, researchers have shown that this truly priced product originated in ancient Egypt.
It was during the time of the Roman Empire that beverages and food were served in glass vessels for the first time on a large scale, said Patrick Degryse, an archaeologist 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 glassmakers in the coastal regions of Egypt and the Levant filled sand kilns. The large glass plates they created dropped the scales to nearly 20 tons. That glass was then shattered and dispersed in glass workshops, where it was remarried and molded into final products.
But what many people really wanted was colorless glass, so glass manufacturers experimented with adding different elements to their bundles. Manufacturers in the Levant are known to have added manganese, which reacts with iron impurities in the sand. The manganese-treated glass still retained some color, however, said Gry Hoffmann Barfod, a geoscientist at Aarhus University in Denmark who led the study, which was published this month in the Scientific Reports. “It was not perfect,” she said.
Glassmakers also tried to add antimony, with much better results. “That made it completely clear,” Dr Barfod said.
And expensive: A price list issued by the Roman emperor Diocletian in the early fourth century AD refers to this colorless glass as “Alexandrian” and rates it at almost twice the price of manganese-treated glass. But the origin of Alexander glass, despite its name, was never finally ascended to Egypt.
“We have factories for the decolored manganese glass, but we do not have them for the Alexandria glass,” Dr Barfod said. “It has been a mystery that historians have dreamed of solving.”
Motivated by that enigma, Dr. Barfod and her colleagues analyzed 37 glass fragments excavated in northern Jordan. Wines, each an inch or two long, included the Alexander glass and the manganese-treated glass from the first to the fourth centuries AD. The sample also included other specimens of glass known to have been produced recently in Egypt or the Levant.
The researchers focused on hafnium, a trace element found in mineral zircon, a component of sand. They measured the concentration of hafnium and the ratio of the two hafnium isotopes in the shoots.
The wrought glass in different geographical regions had different signatures of hafnium, Dr. Barfod and her associates pointed out. 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 inside the sand are inadvertently sorted by nature.
After being expelled from the mouth of the Nile, sand sweeps east and north on the Levant coast, pushed by water currents. The zircon crystals inside it are heavy, so they tend to settle early when traveling on Egyptian beaches. This explains why forged glass in Egyptian kilns tends to contain more hafnium than Levantine glasses, researchers suggest.
When the researchers analyzed the glass shafts treated by Alexandria and Manganese, they again found distinct differences in hafnium. Manganese treated glass had hafnium properties consistent with production in the Levant, as expected. And Alexandrian glass, the purest of light when it came to transparent glass, chemically resembled Egyptian glass.
Rewarding rewards for finally noting the proximity of the Alexandria glass, Barfod said, adding, “This has been an open question for decades.”
But it is still a mystery why the glasses from Egypt and the Levant display different ratios of hafnium isotopes. One possibility is that zircons containing certain isotopic ratios are larger, denser or larger, which affects their movement, Dr Barfod said. “We do not know.”
Analyzing the sand chemistry of the Egyptian and Levantine beaches would be a logical way to confirm these findings, Dr Barfod said. “The next step will definitely be to go out and get sand from both places.”