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The Mystery of the Clear Glass

Photo - Smithsonian Magazine

Interesting articles often come up when we are researching our podcast topics. One such article is from the New York Times (NYT) in 2020 and it highlights research into the origins of Alexandrian Glass. For those who listen to the podcast, Alexandrian glass is the crystal-clear glass prized by the Romans that Dr. Dimos mentioned in the episode.

But it’s obviously more than a podcast reference. Alexandrian glass was highly valued in the Roman Empire. It was both colorless and transparent and arose at a time when glass was being used all over the empire for serving food and wine and for mosaic tiles and windows. According to the NYT article by Katherine Kornei, glass makers in Egypt and further up the coast of the Mediterranean (Jordan, Lebanon, Syria, for example) would fill furnaces full of sand, which would vitrify into glass and those glass slabs would be shipped around the Roman Empire, broken up and distributed to glass workshops to be remelted and shaped into dishes, bowls, tiles, glasses and all sorts of items. Something that I found fascinating was that the evidence suggests that there was market domination of glass production by a small number of large glass centers and that those centers provided glass throughout the Roman Empire and beyond and that the glass was recycled often. (2)

The challenge came in how to make the glass colorless and transparent. The Romans were used to adding metal-containing minerals to make colored glass. Cobalt was used for an opaque blue, lead antimonate for yellow (sprinkle a little copper in there to get green) and calcium antimonate for white.(2) To get rid of the innate color of the glass, caused by traces of minerals and metals, two different additives were used, manganese and antimony. Of the two, antimony was said to be better at de-colorization. Both elements would oxidize any trace iron in the glass, leading to removal of any tint of color. Antimony had the added advantage of removing dissolved gasses as well and so was better at making a transparent glass.(3) It was this antimony-treated glass that was dubbed, “Alexandrian Glass”.

As interesting as the history and science of transparent glass is, that’s not why the NYT did an article about it. While it has been known for some time that the major glass manufacturing areas of the Roman Empire were Egypt and the Levant (Syria, Jordan, Lebanon, Palestine, Israel and parts of Turkey), it was not known which region produced the prized Alexandrian glass. Now, you may be tempted to think that since it was called Alexandrian glass, that of course it was from Egypt. While that’s a logical assumption, we also know that names sometimes take on a life of their, particularly in a culture where so much trade was happening.

To get to the bottom of this mystery, Dr. Gry Hoffman Barfod, a geoscientist at Aarhus University decided to analyze glass fragments from a site that was excavated in northern Jordan. The shards were fairly tiny, but held a lot of information. They consisted of both Alexandrian glass and manganese-treated glass. As you might imagine, there are many components of glass. For this research, the scientists focused on hafnium. Hafnium is a trace element that is found in zircon, which is a part of sand, which becomes the glass.

Hafnium itself is an interesting element. It’s a lustrous, silvery gray metal that is often found with zirconium minerals. Another cool fact about hafnium is that it was predicted by Mendeleev when he was compiling his periodic table of elements though it wasn’t officially discovered until over 50 years later. It’s currently used in filaments and electrodes and in control rods for nuclear power plants. Pertinent to the glass analysis, 36 isotopes of hafnium have been recorded, with five stable isotopes. Fun fact – 182Hafnium has a half-life of 8.9 million years and is used as a tracker isotope for the formation of planetary cores. So cool! (5)

OK, back to the glass analysis. Dr. Barford analyzed the amount of hafnium and 2 of its isotopes in the glass shards found at an excavation site in Jordan, looking at the concentration of hafnium and the ratio of the two isotopes. This became a chemical signature of the different types of glass and where they were made. Egyptian glass had more hafnium and a lower isotopic ratios then glass produced in the Levant. To explain this, the scientists looked at the movement of sand up the east coast of the Levant after exiting the Nile. The zircon crystals in the sand are heavy so they would settle out early along the Egyptian coast, hence more hafnium (from the zircon) in the Egyptian glass. (1)

After establishing a geographic fingerprint for the glass, Dr. Barford and colleagues examined the hafnium signature of the Alexandrian and manganese-treated glass. The manganese-treated glass had a hafnium signature similar to glass produced in the Levant, while the Alexandrian glass had a signature similar to Egyptian produced glass. So Alexandrian glass really is from Alexandria, or at least from Egypt.(1)

Another piece of evidence that corroborates this finding is the observation that sodium-rich, calcium-poor glass dissolves antimony more completely and leads to a better clear glass. Increasing the calcium in the glass leads to antimonate-calcium crystals that turn the glass opaque. This sodium-rich, calcium-poor glass was predominantly made in Egypt.(3)

It’s always satisfying when a mystery is solved, especially when that mystery involves science and art. I hope you enjoyed your trip down this particular rabbit hole with me and maybe learned a fun fact along the way. I know that I for one will never look at sand quite the same way again.



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