Koretsugu was born in 'ei 5 (1628) and died in the third month of Tenna 1 (1681) at the age of fifty-three, a domain smith to the Kuroda house of who in Meireki 1 (1655) went up to by his lord's order, studied the tradition under Daijo Sakon Korekazu of the Ishido line, and after three years returned to Fukuoka. With his cousin Moritsugu he is named by the published sources as a leading master of the Fukuoka Ishido, the line they set alongside the school as one of the two pillars of . He worked under the common names Hansanbyoe and Ippei, and because his heir Toshitsugu died before him, Moritsugu succeeded to the main line. His is a revival hand: the brilliant of the Fukuoka , recovered two and a half centuries later in steel, and the published sources call him most skilled at exactly that, a smith who "excelled above all in -ba in the manner of the school, and was extremely skillful".
His characteristic temper is a broadly hardened set high over the in places, mixing , and pointed elements within the clove heads, with and entering well to produce a florid, varied . The temper is -dominant with gathered along it, fine run through, and the is bright and clear. On the finest work of the two cousins the heads tighten and lean, the taking on a or reverse-slanting tendency that the published sources name the group's particular feature, forming the distinctive pouch-shaped -, the "unique pouch-shaped - in which their special character lies". A second recurring tell sits within the : a distinctive edge form the judges call ika no atama (烏賊の頭), a "squid head" that appears precisely where the hardening reaches broadest toward the , and which they read as a salient mark of the workshop and lineage.
The is where the revival is told apart from its model. He forges a packed or that flows strongly in the lower half and tends almost to , carrying fine like minute particles, delicate , and a standing that is sometimes a straight, -like reflection and sometimes a . This -tinged steel is the trait he took directly from Korekazu, and the published sources make it the first point by which his work is separated from true old , together with the tightened with and the , while noting that on his blades "the appearance of is slight". The answers the : deeply tempered, entering in a to a small round, the tip finishing in . The shape is the bearing, with , a rather wide with clear taper, a thick and usually a shallow .
His surviving record reads as one revival manner held across a spread rather than as separate periods, and the judges draw the variation themselves. Several of his dated blades are noted as departing from the school formula: although Fukuoka Ishido generally shows the inclination in its , on these pieces that tendency is not conspicuous, and they differ somewhat in character. On just such a blade the school is confirmed not by the reverse but by the -tinged with standing , the broad reaching toward the , the deep and the ika no atama, the points the sources call the salient features of the hand. The work is uniformly signed, all seven recognized blades carrying a long signature cut with a thick chisel in a distinctive hand on the ; several add a date and one a date of 11, so that a smith of the age is read off his own inscriptions rather than off attribution. The largest is a votive of 6, exceptionally wide and long with deep , which the judges hold a typical work showing no lack of control despite its scale.
Within the line his place is set by contrast with his cousin rather than by any borrowed comparison. Where Moritsugu, working in the period, often kept the deeper , Koretsugu by contrast more often made the shallow construction, the difference the published sources single out for attention, observing that he "frequently made constructions with the shallow typical of -". His teacher Korekazu, the Daijo of the Ishido, gave him the -tinged and the -leaning that distinguish the whole revival from the it looks back to. Against true old his work is told by the fine , the tightened with and the ; against the showier Osaka Ishido it keeps the and the deep-tempered . The published sources affirm his finest as a representative work of the school and "a masterpiece among his works", naming one "a typical work of Ishido".
For the collector Koretsugu is a -rank name rather than a designated-treasure one. He has no National Treasures and no Important Cultural Properties; his record runs entirely through the rank, seven signed blades passing the across sessions from the thirteenth to the fiftieth. Provenance is thin in the surviving record, and no museum or house can be grounded on his blades, though one 9 was made on commission for a member of the Kozai clan of Sanuki, retainers of the Hosokawa since the period, and survives with a black whose fittings are given to Yanagawa Naoaki. With every recorded example signed and most long held, a Koretsugu reaches the market only from time to time, and chiefly his shallow- pieces rather than the grand votive blades. When one does appear it is a self-documenting revival in good order, the kind the judges call a blade "in which Koretsugu's true strengths are fully exhibited", and a clear window onto how the Fukuoka was forged again in the age.