8th-century glass fragments with unusual cobalt blue. Trace element analysis links the colorant to mines in Oman, proving Indian Ocean trade reached this far inland.
Daderot, Public domain, via Wikimedia Commons
A Sogdian city's chemical secret
In the Jizzakh Region of Uzbekistan, the archaeological site of Sharistan reveals the remains of a major city of the Sogdian civilization. Known in the 6th to 9th centuries CE as a political and cultural center of the Ustrushana principality, its ruins include a governor's palace, temples, and residential areas. Among the artifacts recovered from an 8th-century layer are fragments of intensely colored cobalt-blue glass. For decades, the source of this specific cobalt was a mystery. The chemical makeup of the glass itself is typical for the region and period: a soda-lime-silica glass made using plant ash as a flux, identified by its relatively high magnesia (MgO) and potash (K2O) content. Early Islamic glassmakers inherited Roman traditions but shifted from using mineral natron to plant ash as the source of soda, a change that is well-documented in the archaeological record.
The blue color, however, tells a different story. The bright hue comes from adding small amounts of cobalt oxide (CoO) during the glass melting process. The specific cobalt used at Sharistan contains a unique fingerprint of trace elements.
The Omani connection
Scientific analysis, likely using methods such as Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), allows for the precise measurement of trace elements within the glass. This technique can detect elements present in parts-per-million, creating a distinct chemical signature for the raw materials.
When researchers analyzed the Sharistan glass, they found that the trace element profile of the cobalt colorant did not match known sources from Persia, such as the mines at Qamsar, which were significant producers of cobalt for ceramics and glass. Nor did it match the chemical signatures of cobalt sources from Egypt, which were used extensively during the Bronze Age. Instead, the analysis revealed a strong correlation with cobalt ores mined in the Ar-Radvah region of Oman.
This finding provides physical evidence of a previously unconfirmed long-distance trade route. Sharistan is located deep within the Asian continent, hundreds of miles from the nearest sea. Oman, on the other hand, is a maritime nation on the southeastern coast of the Arabian Peninsula, with a long history of Indian Ocean trade. The presence of Omani cobalt in a Sogdian glass workshop means that this raw material traveled from coastal Oman, likely through the Persian Gulf, and then overland along the Silk Road or its tributaries to reach Central Asia. This demonstrates a interaction between maritime and terrestrial trade networks, connecting distant mining, trade, and craft production centers in the 8th century.
💡Fun Facts
Sharistan, also known as Bunjikat, was the capital of the Ustrushana principality and may have had a population of 10,000 people with a water supply and sewage system.
Cobalt has been used to color glass blue since at least the 2nd millennium BCE in Egypt and the Near East.
The type of glass made in this period is called "plant ash glass" because soda and potash, used as a flux to lower the melting temperature of silica, were derived from the ashes of halophytic (salt-tolerant) plants.
The Silk Road was not a single route but a vast network of interconnected land and sea trade routes that connected East Asia with Europe and Africa.
