From Empire of the Winds: The Global Role of Asia's Great Archipelago, by Philip Bowring (I.B. Tauris, 2018), Kindle pp. 170, 172:
Trade was very much two-way, with Chinese manufactures in strong demand in return for the exotic products of the south and west. Trade was also highly profitable for the government, which normally took 30 per cent of imported products to sell. Yet Chinese merchants were not involved in this burgeoning overseas trade. As in earlier times, as previously noted, it was foreign ships which carried Chinese monks and pilgrims to Sumatra or Java and then on to Sri Lanka or India. The expansion of trade led the central government to appoint a special official, the Commissioner for Trading with Foreign Ships, to regulate it and try to ensure that revenues were collected by the state and not by local officials. Foreign captains were required to register their names and cargoes and were imprisoned for attempting smuggling, though bribery of officials charged with inspecting cargoes was commonplace. But for a long time Chinese ships were not permitted to sail overseas.
This ban began to be relaxed during the Tang era. Local officials in the south were likely to be more sympathetic to trade than those in the capital. Later, members of the imperial family itself as well as senior officials may have engaged in it via middlemen. Nonetheless, the official ban combined with the volume of trade explains why Guangzhou came to be host to a huge population of foreign merchants and others to support trading activities, a population of Arabs, Persians, Parsees, Tamils, Jews and so on that may have numbered more than a hundred thousand. There was also a sizeable settlement at Yangzhou at the southern terminus of the Grand Canal. But relations were not always easy, and rich foreigners were sometimes a target. Many were massacred in Yangzhou at the time of a major rebellion against the Tang in 760, and thousands were said to have been killed in Guangzhou during the 878 rebellion. This shut down trade for several years.
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Noting the profits, Song rulers finally saw the benefits of allowing Chinese to go overseas. As they did so, the need for the old state-focused tribute system declined. The Confucian disdain for trade had long been an elite facade, and now it was stripped away and the nobility participated directly in trade. The arrival of Chinese traders indeed helped the lesser ports of Nusantaria to develop, undermining the dominance of Srivijaya. There was a surge in shipbuilding, with Song-era wrecks found off the coasts of Fujian and Zhejiang revealing vessels up to 34 metres in length with a beam of 9 metres. Unlike Nusantarian designs, they had bulkheads and were joined by iron nails, not wooden dowels.
During the Song period China made advances in shipbuilding, advances that included the construction of large, ocean-going vessels which could reach India and beyond. However, it is not clear how much they were used in trade, which continued to be dominated by Arabs, with Persians, Tamils and Nusantarians all playing roles. With so little archaeological evidence to go on, it is difficult to track the evolution of ship designs and how far Song China borrowed ideas from the Nusantarian, Tamil and Arab vessels with which they were familiar – or vice versa. What we can be sure of is that China’s seagoing progress in the Song period, driven by the combination of commercial enterprise and the tax needs of the state, made possible the avowedly expansionist policy of its successor, the Mongol Yuan dynasty.
The main export cargoes were ceramics, which became common throughout Nusantaria and also in Japan. According to early thirteenth century Chinese sources, the ceramics trade was mainly in foreign hands. Ports that have now vanished thrived.
