The kinkō (肥後金工) tradition comprises the metalworkers who flourished in province (modern Kumamoto) under the patronage of the Hosokawa house, from the period into the late period. Four families — Hayashi, Hirata, Nishigaki, and Shimizu — are regarded as representing the main line, and the first-generation masters of each were all retained by Hosokawa Sansai (Tadaoki), famed as a great commander and an accomplished master of the tea ceremony, and received his guidance. After the Katō family was dispossessed in 'ei 9 (1632) and Sansai's son Tadatoshi was enfeoffed with Kumamoto at 540,000 , Sansai entered retirement, choosing Yatsushiro Castle as his place of seclusion. Hirata Hikozō, founder of the Hirata school and descended from the Shōami line, had served Sansai in Kyoto and followed the Hosokawa through Tango and Buzen down to , dying there in 'ei 12 (1635); from among his followers emerged Shimizu Jingō (his nephew) and Nishigaki Kanshirō, who together with Hayashi Matashichi of Owari lineage formed the four major schools of metalwork. Matashichi (1613–1699), born in Kumamoto to a family of gunsmiths who had served Katō Kiyomasa before entering Hosokawa service, is regarded as the founder of the Hayashi line. The broader tradition also embraces avocational works by warriors, including by Sansai himself and by Miyamoto , who served Hosokawa Tadatoshi from 'ei 17 (1640).
Shaped by Sansai's direction, the school as a whole is characterized by its combination of outstanding practical function suited to the warrior's taste and a dignified, elevated artistic quality. The Hayashi manner centers on well-forged, lustrous iron — often likened to the deep tone of yōkan — worked in ground openwork () with fine engraving and gold ; Matashichi's "never give the slightest impression of flatness," conveying an exquisite sense of , and his son Shigemitsu's work displays the beautiful purplish patina (murasabi) characteristic of Hayashi-school . The Hirata artisans inherited much of the Shōami tradition: while iron grounds are known, they favored such as , , brass, and — admired as "Hirata-gane" and "Hikozō-gane" — adding elegant decorative effect through file treatments such as Amida-yasuri and , on rare occasions employing cloisonné inlay (shippō-zōgan); Hikozō also originated the distinctive rim mounting known as the Odawara . Shimizu Jingō displays the most intense individuality among metalworkers, boldly placing birds of prey, owls, and rain dragons in brass suemon-zōgan upon hammered, fire-finished iron, while intentionally reserving broad open space on the reverse to deepen the feeling of . The Nishigaki line continued under Nagahisa, who trained in the Gotō Kenjō house before serving the Hosokawa as a retained craftsman. Cherry blossoms and warabite are motifs frequently adopted for , with Matashichi regarded as the pioneer.
Grounded in the aesthetic of the Hosokawa school of tea while inheriting the bold, vigorous spirit of , metalwork occupies a singular position in the tosogu canon, extending from to complete mountings furnished with fittings bearing the Hosokawa kuyō crest. Matashichi's iron has been "praised without cease" since early times, and his authority was such that other craftsmen were said to "keep their distance by three sha"; because most masters worked unsigned, the few extant signed examples are treasured as precious documents of each hand. Many celebrated works remained in the Hosokawa family itself, passing to the Eisei Bunko collection — testimony to a tradition, later continued by the Kamiyoshi line, whose quiet dignity and refined elegance remain the very embodiment of the warrior aesthetic.