12th-century bronze coins reveal role of Northern Mozambique in Swahili trade history

A recent study by Dr. Ignacio Montero-Ruiz and his colleagues examined the provenance of ancient copper artifacts discovered on the Island of Ibo off Mozambique’s coast. The study, published in Azania: Archaeological Research in Africa, provides new insights into the role small coastal villages played in the early development of the Indian Ocean trade.

For much of the Swahili trade history, the role of northern Mozambique has been considered marginal.

However, in 2016, a test pit dug on the island of Ibo revealed various artifacts linking the area to the wider Swahili trade, including Persian Gulf or Iranian glazed ceramics, a fragment of a soft-stone ware possibly from Iran or Oman, and glass beads traded from India and Central Asia.

Among the finds were also three bronze coins, a chain, and metal fragments. The metal objects were subject to elemental analysis by X-ray fluorescence (XRF) and lead isotope analysis.

The elemental analysis revealed that the coins had a unique elemental composition unlike any other known Swahili coins.

The Ibo coins were made up of an unusually high tin-lead ratio and, unlike Swahili coins, contained no arsenic. The researchers proposed that this may indicate a shift in the source of copper (needed to make bronze) for Swahili coins during the 11th and 12th centuries.

Lead isotope analysis was conducted on two of the coins and a fragment of the coin to determine the geological origin of the copper.

Using a specialized mass spectrometer, the researchers compared the isotope signatures with sources across Africa and Eurasia.

The results showed that none of the metals had originated in the Islamic world, as their alloy composition did not match. Neither were they recycled metal.

Instead, the researchers hypothesized that the most likely source of the copper was the Copperbelt region in Congo-Kinshasa (possibly the Shinkolobwe mine).

This would make the copper used to create the Ibo artifacts the most distant source of copper to date.

While the poor condition of the coins and the fact that the researchers could not claim they belonged to a Swahili mintage made linking them to any particular place difficult, the researchers were able to establish a link between the Ibo coins and coins struck in Kilwa.

The Quirimbas Archipelago, which is part of Ibo, was part of the gold trade network that connected Kilwa to areas in Limpopo (South Africa) and the Zambezi, located in the Middle Limpopo Valley.

The Middle Limpopo Valley was a significant political entity that controlled long-distance trade in gold and ivory. Within it were several polities, including Mapela (ca. 1055–1450 AD), Mapungubwe (ca. 1220–1300 AD), and Great Zimbabwe (ca.1300–1450 AD).

This long history of trade between the southern areas of Africa and the Swahili coast helped introduce gold, tin, bronze, and brass metallurgy into these southern areas around the second millennium.

This trade connection between Ibo and the polities in the Middle Limpopo Valley has been demonstrated through the analysis of a gold bead also excavated at Ibo, says Prof. Maria Ruiz-Galvez Piergo, one of the researchers on the study.

“According to the provenance analysis, this gold came either from Great Zimbabwe or South Africa, since it isn’t easy to distinguish, based on trace elements composition, between the gold ores of the Greenstone Belts and those of the Witwatersrand Basin.”

While much of the ore, including copper and bronze, used in these polities was mined locally in South Africa, Zimbabwe, and Botswana, some came from the Copperbelt in the Congo-Kinshasa.

The research team proposes that the copper used in the Ibo coins and artifacts likely originated from the Congo region before being redirected eastward to the coast through the Middle Limpopo Valley, suggesting that they followed a similar route to the gold trade network.

“Taking into account that Sofala was considered the outlet for the Zimbabwean gold, we considered the Limpopo River, connected to Sofala, as the most probable route [of copper],” said Prof. Ruiz-Galvez Piergo.

While the evidence isn’t conclusive, the study opens the possibility that copper mining districts in Congo-Kinshasa were linked to Swahili-oriented trade routes.

Copper, along with other valuable materials, may have been traded through the Quirimbas before entering into the wider Swahili trade network, highlighting the important role small local villages may have played in the early development of the Indian Ocean trade route system.

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