Glass was highly valued in the Roman Empire, especially the colorless transparent version, reminiscent of rock crystal. But the source of this coveted material – known as Alexandrian glass – has long been a mystery. Now, by studying the trace elements of the element hafnium in a glass, researchers have shown that this valuable commodity really originated in ancient Egypt.
It was during the Roman Empire that drinks and food were first widely served in glass jars, said Patrick Degrise, an archaeologist from KU Leuven in Belgium who did not take part 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 areas of Egypt and the Levant filled the sands with sand. The huge glass plate they created weighed almost 20 tons. The glass was then broken and distributed to glass workshops, where it was melted down and turned into final products.
But what many wanted was colorless glass, so glass manufacturers experimented with adding different elements to their batches. It is known that Levant manufacturers add manganese, which reacts to iron impurities in the sand. However, the manganese-treated glass still retained some color, said Gray Hoffmann Barfod, a geologist at the University of Aarhus in Denmark who led the study, which was published this month in scientific reports. “It wasn’t perfect,” she said.
Glassworks also tried to add antimony, 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 evaluates it almost twice as much as glass treated with manganese. But the origin of Alexandrian glass, despite its name, was never definitively tied to Egypt.
“We have factories for manganese-stained glass, but we don’t have them for Alexandrian glass,” said Dr. Barford. “History dreamed of unraveling the historian.”
Motivated by this mystery, Dr. Barford and her colleagues analyzed 37 fragments of glass excavated in northern Jordan. Sherds, every inch or two long, included Alexandrian glass and manganese glass from the first to the fourth century AD. The sample also included other samples of glass, which are known to have been made recently either in Egypt or in the Levant.
Researchers have focused on hafnium, a trace element found in the mineral zircon, a component of sand. They measured the concentration of hafnium and the ratio of two hafnium isotopes in the sherds.
Glass forged in different geographical regions has different hafnium signatures, Dr. Barford and his colleagues have shown. Egyptian glass consistently contained more hafnium and had lower isotope ratios than Levant-produced glass, the team found.
These differences make sense, Dr. Barford and colleagues suggest, because zircon crystals in the sand are inadvertently sorted by nature.
After being expelled from the mouth of the Nile, the sand sweeps east and north up to the Levant coast, which moves by water currents. The zircon crystals inside it are heavy, so they tend to settle at the beginning of the journey on Egyptian beaches. This explains why glass forged in Egyptian furnaces tends to contain more hafnium than Levantine glass, the researchers said.
When the researchers analyzed the sarcasm of Alexandrian and manganese glass, they again found differences in hafnium. The manganese-treated glass had hafnium properties that corresponded to production in the Levant as expected. And Alexandrian glass, the brightest of the clear, when it came to clear glass, chemically resembled Egyptian glass.
Dr. Barford said it had been a final issue for decades.
But it still remains a mystery why the glasses from Egypt and the Levant have different ratios of hafnium isotopes. The possibility is that zircons that contain certain isotopic ratios are larger, denser, or bulkier, which affects their motion, Dr. Barford said. “We dont know”.
Dr. Barford said that analyzing the chemistry of the Egyptian and Levantine sand beaches would be a logical way to confirm these findings. “The next step, obviously, will be sand coming out of both places.”