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Centralized production of Roman glass

by Bruno Van de Casteele

May 12, 2013

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Donate Sometimes we underestimate the capacities of our ancestors. Take for instance the ancient Romans, who developed the craft of blowing glass, instead of casting it. The results are sometimes phenomenally beautiful, as the Wikipedia page on Roman glass attests.

The knowledge of glass blowing was well spread accross the Roman Empire, with some items being found in Afghanisten or even China. A very fine example turned up in a Merovingian cemetery (7th century CE) a couple of kilometers from my home. (I guestblogged about this on Aardvarcheology Blog.)

It was already known that the glass blowing technique was quickly spread across the Roman Empire. But chemical analysis showed that most of the glass between the first and eight century was of a very similar composition. One leading theory tries to explain this by placing all production of glass itself (from sand) in the Syro-Palestine and Egyptian region, with chunks of glass being traded across the Empire and processed locally into a finalized product. As Stern points out, this split of the industry in two distinct branches really defined the industry as it used the extensive trade network. It probably made production more industrialized and efficient, while allowing for diversification for local tastes.

Researcher Monica Ganio from the University of Louvain, Belgium, has brought an additional argument for that theory in her doctoral thesis, to be published this month. In one of those interesting cases where multiple branches of science work together, she has done an isotope analysis on almost two hundred shards of glass. Specifically, she was looking for the isotopes of strontium and neodymium.

Indeed, it seems that colorless glass originated in the eastern part of the Mediterranean, and it can even be shown that beach sand was used for it (strontium can be found in sea water). By crosschecking the isotope signature with other databases, Ganio shows that the sand probably originated in the eastern Mediterranean, although the western Mediterranean is not entirely ruled out. She can even pinpoint different source materials used, including antimony (for decoloring the glass), probably from limestone (and not from seashells, who have a different chemical and isotopic signature).

This is in alignment with other finds, for instance from Fleming who points out that glassware has inscriptions of their makers on it, in Greek or Jewish (a sort of "made in China"). But it is not the final words. Indeed, ancient Roman authors like Pliny the Elder (who died studying the Vesuvius eruption in Pompeii in 79 CE) have referenced glass production in Italy, Spain and the Lower Countries. No such sites have been excavated yet, but the search continues. Furthermore, Ganio has indicated (link in Dutch) that Italian sand resembles closely sand from the Eastern Mediterranean, and that a standardized recipe or even ingredients for creating glass could also be used in a lot of places.

So, as usual, this is not the final word. The initial research project on which Ganio made her doctorate had as aim to prove the Italian origin of glass found in Herculaneum and other Italian sites. Credits to Ganio (who is of Italian origin) that she followed the evidence that confirmed the leading theory. And credits, too, for searching alternative solutions and willing to do further research.

by Bruno Van de Casteele

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