The Romans called it Alexandrian glass. Where does it actually come from

(ORDO NEWS) — Trace amounts of isotopes hint at the true origins of a type of glass that was highly prized in the Roman Empire.

Glass was highly prized throughout the Roman Empire, especially the colorless, transparent version that resembles rhinestone. But the source of this coveted material, known as Alexandrian glass, has long been a mystery. Now, by studying the trace amounts of the element hafnium in glass, researchers have shown that this valuable commodity actually originated in ancient Egypt.

During the Roman Empire, drinks and food were first served in glass vessels on a large scale, said Patrick Degris, of Leuven University in Belgium. Glass has also been used in windows and mosaics.

Between the first and ninth centuries A.D. glass blowers in the coastal regions of Egypt and the Levant filled furnaces with sand. The huge glass slabs they created weighed up to 20 tons. Then this glass was broken and sent to glass workshops, where it was melted into finished products.

But many wanted clear glass, so glass manufacturers experimented with adding different elements. Producers in Levant are known to have added manganese, which reacts with iron impurities in the sand.

“Glass treated with manganese still retains a little color,” said Grie Hoffmann Barfod, a geologist at Aarhus University in Denmark who led the study. “It wasn’t perfect,” he said.

Glassmakers have also tried adding antimony with much better results. “It made it completely transparent,” said Dr. Barfod.

And expensive: the price list issued by the Roman emperor Diocletian at the beginning of the 4th century. N.E., Calls this colorless glass “Alexandrian” and estimates it almost twice as expensive as glass treated with manganese. But the origin of the Alexandrian glass, despite its name, was never fully established in Egypt.

When researchers analyzed Alexandrian and manganese glass vessels, they found clear differences in hafnium. The manganese treated glass had hafnium properties matched to that produced in the Levant, as expected. And Alexandrian glass, the most transparent, chemically resembled Egyptian glass.

Dr. Barfod said it would be gratifying to finally identify the origin of Alexandrian glass, adding: “This question has remained open for decades.”

But it still remains a mystery why glasses from Egypt and the Levant show different ratios of hafnium isotopes. One possibility is that zircons containing certain isotopic ratios are larger, denser or bulkier, which affects their movement, Dr. Barfod said. “We do not know”.


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