Atomic fingerprints of trade
The Roman city of Zeugma, located on the Euphrates River, possessed exceptionally detailed floor mosaics. For years, the origin of the intensely colored glass cubes, or tesserae, used in these artworks was a subject of speculation. Chemical analysis provided the answer, showing industrial-scale production and long-distance trade. Scientists used a technique called neutron activation analysis (NAA) to determine the elemental composition of the glass. By bombarding small samples with neutrons from a research reactor, they made the atomic nuclei within the glass unstable. As these nuclei decayed back to a stable state, they emitted gamma rays at energies characteristic for each element, creating a unique "atomic fingerprint" for the material.
The results of NAA, supplemented by other methods like Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), showed that the glass tesserae from Zeugma were made from soda-lime-silica glass. Specifically, their chemical profile, with low alumina and high soda content, matched glass produced using natron. Natron, a naturally occurring mineral mix of sodium carbonate decahydrate, was used as a flux to lower the melting temperature of silica sand. The primary source for this mineral in the Roman world was the Wadi El Natrun in Egypt. This discovery indicated that the raw glass was not made locally in Zeugma.
A two-stage industry
The chemical evidence supports a two-stage model for Roman glass production. Primary workshops, located in Egypt and the Levant where sand and natron were readily available, produced enormous slabs of raw, uncolored glass. These large glass ingots were then broken into chunks and shipped across the Mediterranean and overland to secondary workshops throughout the empire. Zeugma, a wealthy city on a major trade route, was one such secondary production center. Here, artisans would remelt the imported glass chunks, adding specific metal oxides as colorants to create a bright colors. For example, cobalt was added to produce deep blue hues, while copper could create red or turquoise. Antimony-based compounds were often used to make white and yellow tesserae opaque. These colored glasses were then cooled and cut into the small tesserae needed for complex mosaic designs. The analysis of Zeugma's mosaics shows the impressive scale of this trade network, with raw materials for a single floor traveling over 2,000 kilometers from their origin in Egypt to their final use in a Roman villa in Anatolia.