In 1405, at Liujiagang in Suzhou, two hundred ships assembled on the river. The flagship stood as tall as a building. Aboard was a thirty-two-year-old eunuch named Zheng He, about to lead this vast fleet into the "Western Ocean".

You probably know the rest: seven voyages, as far as Africa.

But most people don't know that Zheng He brought back more than spices and giraffes. He brought back Sumali cobalt — a shipment of ore from Persia.

And it was this that made Yongle and Xuande blue-and-white so dazzlingly blue.

"Sumali" is not Chinese. It's a transliteration of a Persian word, referring to cobalt mined near Kashan.

This pigment fired into a blue unlike anything Chinese cobalt could produce.

Domestic cobalt is high in manganese: pale, greyish, like a lake under cloud. Persian cobalt is high in iron: deep, rich, almost indigo, with black spots where it pools thickest. Jingdezhen potters knew immediately — this was treasure.

On Zheng He's second voyage (1407), the fleet began bringing back Persian cobalt in quantity. For the next two decades, the imperial kilns of Yongle and Xuande ran on it.

In a sense, it was an accident. Zheng He sailed to project power and bring back spices. The cobalt was a by-product. But that by-product produced a peak in Chinese blue-and-white that has never been equalled.

Put a Yongle piece next to a Chenghua piece and the difference is immediate.

Yongle blue is deep and saturated, like ink dropped into water. Chenghua blue is pale and clear, like the sky after rain.

The first used Sumali cobalt; the second, domestic Pingdeng. One rich, one refined. Neither better — just different in character.

To spot Yongle-Xuande blue-and-white, look for three things:

Iron-rust spots. In the darkest passages, black crystallisations sink into the glaze, catching light with a metallic sheen. This is the signature of iron-rich cobalt.

Bleed. The blue spreads outward, like ink on rice paper. Bamboo leaves, clouds, lotus scrolls — all with soft, blurred edges.

Tonal variation. Within a single stroke, the colour shifts from deep to pale. Later imitations tend to look "too clean".

In 1435, the Xuande emperor died at thirty-eight. Two years earlier, Zheng He had died at Calicut on his last voyage.

Ming maritime policy tightened. No more treasure ships, no more routes, no more tributary states.

And no more cobalt.

Jingdezhen potters switched to domestic sources. Overnight, the blue shifted from rich to pale. The most brilliant chapter of Chinese blue-and-white quietly closed.

For centuries afterward — Qianlong, late Qing, Republic, right up to today — potters tried to imitate Yongle-Xuande. But Sumali cobalt was gone, and that blue could not be fired again. Every imitation is, in essence, a modern voice reciting a vanished line.