Rare element solves mystery of Roman clear glass

The Edict of Diocletian, aka the Edict on Maximum Prices, was issued in 301 A.D. in a failed attempt to control the runaway inflation that plagued the empire after decades of currency debasement during the Crisis of the Third Century. It established maximum wages for labor and maximum prices for more than 1,200 products from kidney beans to timber to the famous purple dyed silk (150,000 denarii a pound for that bad boy, more than twice the price of gold). Today the edict survives in fragmentary inscriptions only, but enough of it is extant to serve as a uniquely rich source of information about Roman economics, trade goods and buying power.

Among the products on the list are different types of glass. Alexandrian clear glass is the most expensive at 24 denarii a pound, 30 for cups and vessels. Judaean clear glass cups and vessels cost 20 denarii a pound. Pliny noted in Natural History that “the highest value is set upon glass that is entirely colourless and transparent, as nearly as possible resembling crystal” and Alexandrian was the top of the line in clear glass.

Roman-era glass furnaces have been discovered in the Levant, producers of the Judaean glass. No such furnaces have been found in Egypt so archaeologists have long debated whether the so-called Alexandrian glass was really made there. Researchers have only been able to generally infer ancient glass is Egyptian because it doesn’t match the products from the Syro-Palestinian furnaces and from higher concentrations of titanium dioxide found in Egyptian sand. The first criterion is passive at best, and the second excludes high quality sand deliberately chosen for its low levels of iron oxides (and therefore titanium dioxide) that was available to Egyptian glassmakers during the Roman period.

That obstacle has now been surmounted with the help of the rare element hafnium. Researchers subjected Roman glass sherds to strontium, neodymium and hafnium isotope analyses. They found distinct hafnium isotope signatures in the glasses made with Egyptian sand and the ones from the Syro-Palestine coast.

“Hafnium isotopes have proved to be an important tracer for the origins of sedimentary deposits in geology, so I expected this isotope system to fingerprint the sands used in glassmaking”, states Gry Barfod. Professor at Aarhus University Charles Lesher, co-author of the publication, continues: “The fact that this expectation is borne out by the measurements is a testament of the intimate link between archaeology and geology.”

Hafnium isotopes have not previously been used by archaeologists to look at the trade in ancient man-made materials such as ceramics and glass. Co-author Professor Ian Freestone, University College London, comments, “These exciting results clearly show the potential of hafnium isotopes in elucidating the origins of early materials. I predict they will become an important part of the scientific toolkit used in our investigation of the ancient economy.”

The results of the study have been published in the journal Scientific Reports and can be read here.

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Comment by norm
2020-07-13 07:25:07

“The fact that this expectation is borne out by the measurements is a testament of the intimate link between archaeology and geology.”
This is a very true statement. Many of the major Maya cities were built on chert deposits. I use those samples to examine Maya trade routes. I’m using color and structure but the above sounds right on.

 
Comment by Lory
2020-07-15 19:30:21

Perhaps the reference for “kidney beans” was really for “fava”, since beans are a New World food, with the exception of the chick pea and fava, and unknown in ancient Roman times.

 
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