From Japan to 1600: A Social and Economic History, by William Wayne Farris (U. Hawai‘i Press, 2009), pp. 94-96:
Commerce grew to become a vibrant sector, primarily because Japan was located next to the most dynamic economy on earth: that of Sung China. Sung Chinese invented gunpowder, the compass, and mass printing. The country also had advanced carbon-stoked iron furnaces producing high-grade ferrous products and a cotton industry producing everything from ships’ sails to military uniforms. The population grew by leaps and bounds during the Sung period, as the “rice bowl” of southern China was more intensively cultivated and regional craft and trade specialization took place as never before.
Trade between China and Japan, exclusively for the archipelago’s elite, was already underway in the tenth century. By the late eleventh and twelfth centuries huge Chinese junks called even more regularly at Hakata, Kamizaki, and other Kyushu ports. By 1100, a community of overseas Chinese took up residence in northern Kyushu cities such as Hakata. They held rank at the Japanese court and some even attended the funeral of an important official in northern Kyushu in 1097. In 1151, two samurai attacked the overseas Chinese there, and the fleeing merchant families numbered more than sixteen hundred. Archaeological evidence also points to a dramatic increase in commerce with China during the twelfth century, as the number of sites in Japan containing shards of Chinese porcelains grew exponentially. Besides Kyushu, Chinese traders also called on ports along the northwestern coast of Honshu. By 1180, some daring Japanese captains attempted the passage to southern China as well.
Chinese merchants traded their silk, spices, and porcelain for northeastern Japan’s furs and gold. The Chinese especially coveted gold; a Chinese trader wrote in 1118 “the country of Japan … in its earth has a wealth of precious products.” Perhaps for this reason, the dynamic Sung state, populated by wealthy consumers, ran a balance of trade deficit with Japan. Piles of Sung cash were soon helping to remonetize the Japanese economy. By 1150 there were signs that the outflow of Sung cash was causing the economic giant problems. In 1199, the Chinese government tried to ban the use of its coins in trade with Korea and Japan. A significant increase in the amount of Sung coins in Japanese sites took place beginning in the 1170s. People wrote of a “cash sickness” in 1179, and then the court banned the counterfeiting of Sung coppers. These proscriptions were apparently ineffective, because the court repeated them in 1187, 1189, and 1192. Along with the cash came an inflationary price spiral, beginning in the 1170s, helping to further destabilize an already teetering social pyramid.
The Ise Taira built a trading empire in western Japan during their tenure as the military arm of the court from 1159 to 1180. They controlled bases such as Fukuhara in modern Kobe, Itsukushima along the Inland Sea, and Kamizaki in northern Kyushu. The Taira made allies of the seafaring families in western Japan. They were so involved in the Sung trade that in 1180 ex-emperor Takakura, born of a Taira mother, was induced by Kiyomori to sail from Fukuhara to Itsukushima aboard a Sung junk.
In addition, the Koryŏ dynasty (918–1258) exchanged goods frequently with Japanese merchants. Following the collapse of the Silla kingdom, relations between the Japanese court and Korea improved. Between 1050 and 1090, Japanese merchants visited Korea in sixteen trade missions, bearing weapons, screens, and precious metals for the Koryŏ court.
This strong external stimulus, combined with the modest demographic recovery, led to a rebound in Japanese domestic commerce between 1050 and 1180. As had occurred during the eighth century, the capital and Kinai constituted the core of commercial activity, because that region had a large number of consumers and the remnants of an advanced transportation system. Commerce was more dynamic in western Japan and probably less important in eastern Honshu. Long-distance exchange, however, enabled the elite to acquire the marvelous products of northern Japan, such as gold and wild horses. The elites also still received most commodities in kind from their on-site landlords and tax farmers, and peasants bought and sold at markets only occasionally, yet demographic and economic recovery supported and was assisted by the return of a more vital market system.