Sumali cobalt — the pigment Zheng He's fleet brought back, which made Yongle and Xuande blue-and-white so dazzlingly blue.

But where exactly did it come from?

To answer that, we first need to figure out what "Sumali cobalt" actually is.

The four characters "苏麻离青" are not Chinese. They are a transliteration.

Scholars generally agree it corresponds to the Persian sulaimani or the Arabic sumawi — referring to a cobalt-producing place in central Persia.

That place is called Kashan.

Kashan lies in today's central Iran, about 200 km south of Tehran. In the medieval period it was Persia's most important cobalt source, and a ceramics centre in its own right — producing blue-and-white pottery from as early as the twelfth century.

Cobalt is not rare in the earth's crust, but high-grade cobalt ore is.

Central Persia — especially around Kashan — happened to have abundant high-grade cobalt, high in iron and low in manganese. Fired, it produced a blue of extraordinary depth and intensity, quite unlike any Chinese pigment.

More importantly: Persians did not just have ore. They had been firing blue-and-white pottery at scale since at least the ninth century, in the Islamic period.

In other words, Persia was itself one of the birthplaces of blue-and-white technology. Their understanding, mining, and processing of cobalt preceded China's by four or five centuries.

So when Zheng He's fleet reached the Persian Gulf, it encountered a mature industry — with mines, workshops, merchants, and markets already in place.

From Kashan to Jingdezhen was a journey across half of Asia.

First, overland. Cobalt moved from Kashan to Isfahan, or south to Persian Gulf ports — above all Hormuz.

Second, by sea. From Hormuz, across the Arabian Sea to Calicut on India's west coast. Calicut was one of the most important transit ports of the Indian Ocean trade — Zheng He stopped there on nearly every voyage.

Third, further east. From Calicut through Ceylon and Sumatra to Malacca. Malacca was a key base Zheng He established in Southeast Asia.

Fourth, into China. North from Malacca through the South China Sea to Guangzhou or Quanzhou, then overland or by inland waterway to Jingdezhen.

The whole route could take more than a year one way. That is why Sumali cobalt was so expensive — the transport cost alone was astronomical.

In 1433, Zheng He made his last voyage and died at Calicut.

After that, the Ming turned to maritime prohibition. Treasure ships stopped being built, and the ocean routes were abandoned.

But "maritime prohibition" was only the immediate cause. The deeper reason: the political value of the route had declined.

During Yongle and Xuande, the Ming needed the voyages to build a tributary system and procure exotic goods. After Xuande, the court judged the expenditure no longer worthwhile — enormous cost, limited return.

The route stopped. The cobalt stopped.

Jingdezhen potters turned to domestic pigments. Pingdeng cobalt and Zhe cobalt entered the scene. Blue-and-white shifted from rich to pale, and Chinese porcelain entered another era.

The story of Sumali cobalt ended there.