Purpose of the last Ming sea prohibition
"Ming Ban" refers to a period during which all activities on the high seas was prohibited by the court, in the Ming dynasty. Actually, there were several such sea prohibitions in place, during different Ming reigns, and for very different reasons.
This term is mostly used to refer to the last period of sea prohibition; that is the only sea prohibition that would have affected the trade with the European seafaring nations, but it was not the only one.
European ships arrived in China only in the early 16th century, and the Ming dynasty ended in 1644.
This single sea prohibition stopped virtually all exports and all trading activities between China and the various East India Companies. Maritime trade was only reinstated during the reign of Qing emperor Kangxi.
This specific sea prohibition was in part enacted in order to reduce piracy along the coast of the Chinese empire. Following the seven voyages to the west by the Chinese fleet, under admiral Zheng He, during the Yongle reign (1403-1424), no large ships were built anymore and the Ming empire isolated itself once again. The fleet was abolished.
During the following reigns the empire had no naval resources to defend its interests at sea. It resorted to the Ming ban in order to starve pirates of the resources, which attracted them to raid the Chinese coast.
Piracy and Coastal Raids
The Ming court tried to stop rampant piracy along the coastal area in vain. It is necessary to understand, that these pirates did not only attack ships and ports, but also settlements that lay some stretch inland. Probably due to the unpredictability of such attacks, it was too difficult for the imperial troops to react swiftly.
The Chinese word for these pirates meant actually "Japanese pirates". Even now many documents wrongly mention them as "Japanese pirates".
True, initially the pirates were of Japanese origin, but it appears that both the government of the Qing dynasty as well as that of Korea were aware that the original Japanese piracy found its real cause on the continent, with Yuan dynasty China, and that most of the later pirates were Chinese and Koreans.
Reason for the Japanese piracy
During the Yuan dynasty, when China was under Mongol rule, the Yuan government attacked Japan with the help of its allies in the Korean peninsula. Although they miserably failed to conquer Japan, they sacked the Japanese island of Tsushima and other outlying places and, it appears, that according to Mongol practices of the time, most of the population was killed.
The survivors turned to piracy because they had lost their livelihoods and families; apparently there weren't enough people anymore to till the fields. They started attacking the Korean coast to free any captives taken by the attackers, and to take supplies for their survival. The survivors became pirates out of necessity!
Later on, Koreans and Chinese started participating in these raids and finally made up most of the pirates' numbers, while that of the Japanese dwindled to a small percentage. These professional pirates started raiding the coast of the Pohai sea, and down the southern coastal areas at least as far as the southern Guangdong, at the coast of the South China Sea.
As the Ming court had destroyed its own naval ships earlier, after admiral Zhenghe's last voyage, it was unable to deter the pirates. Thus, it tried to deter them by starving the pirates of the goods they were coming to loot. They did that by forbidding all activities on the high seas by the inhabitants of the Chinese coastal area. It seems that it was expected that the inhabitants would retreat farther inland, creating an empty zone along the coast. The ban was ineffective, however, because the pirates would travel way inland for their attacks.
While the Ming ban did little to affect piracy, it increased the hardships for the coastal population and many people working in the ceramics and other export related industries lost their livelihood because of this.
More than likely this is also the cause that many kilns artisan's emigrated to work in kilns in Southeast Asia, e.g. in today's Vietnam. Many Vietnamese ceramics of the time show clearly Ming characteristics.