Researchers have long debated exactly where ancient Rome’s prized, totally transparent glass was made. Now, a new study has used chemical analysis to match the empire’s crystalline wares to the Egyptian sands from which they came, reports Katherine Kornei for the New York Times.
In 301 A.D., Emperor Diocletian issued an Edict on Maximum Prices that refers to a particularly pricey, colorless glass known as “Alexandrian.” Many archaeologists have interpreted this name as a sign of the glass’ Egyptian origins.
According to Ross Pomeroy of RealClearScience, artisans made Alexandrian glass—the material of choice for high-class chalices—by adding antimony oxide during the glassmaking process. This oxidized the iron present in the sand needed to produce glass, clearing up its blue-green tint by transforming ferrous to paler ferric.
For those who wanted to see the color of their wine but were unable to afford Alexandrian vessels, clear manganese glass (then known as Judaean glass) cost about a third less. Though archaeologists have identified numerous factories in the Levant known to have produced this lower-grade glass, no definitive evidence had previously linked Alexandrian glass to Egypt.
Determined to solve the mystery, Gry Hoffmann Barfod, a geoscientist at Denmark’s Aarhus University, and her colleagues analyzed 37 glass fragments found at an archaeological site in northern Jordan. The shards included enigmatic Alexandrian glass, manganese-processed glass, and glass made more recently in either Egypt or the Levant.
Earlier studies comparing manganese glass produced in the Levant and Alexandrian glass of inconclusive origin had failed to differentiate the samples because they contained similar ratios of the isotopes strontium and neodymium, the researchers write in the journal Scientific Reports. To avoid this issue, Barfod and her colleagues decided to look into the relative ratios of isotopes of the element hafnium.
Per the paper, the analysis determined that ratios of hafnium isotopes can be used to differentiate Alexandrian glass from Levantine glass decolorized with manganese. The researchers also report that Alexandrian glass exhibits hafnium isotope ratios comparable to that of glass known to come from Egypt, seemingly settling the longstanding mystery.
“These exciting results clearly show the potential of hafnium isotopes in elucidating the origins of early materials,” study co-author Ian Freestone, an archaeologist at the University College London, says in a statement. “I predict they will become an important part of the scientific toolkit used in our investigation of the ancient economy.”
As the Times notes, the researchers will next test the chemical makeup of the sand itself to see if the ratios found in the glass samples correspond with grains found on local beaches in Egypt and the Levant.