Koretsugu was born in Kan'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 Chikuzen who in Meireki 1 (1655) went up to Edo by his lord's order, studied the Bizen tradition under Musashi 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 Chikuzen Nobukuni school as one of the two pillars of Chikuzen shinto. 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 choji of the Kamakura Fukuoka Ichimonji, recovered two and a half centuries later in shinto steel, and the published sources call him most skilled at exactly that, a smith who "excelled above all in choji-ba in the manner of the Ichimonji school, and was extremely skillful"[[c:1]].

His characteristic temper is a broadly hardened choji-midare set high over the shinogi in places, mixing gunome, ko-gunome and pointed elements within the clove heads, with ashi and yo entering well to produce a florid, varied midare. The temper is nioi-dominant with ko-nie gathered along it, fine sunagashi run through, and the nioiguchi is bright and clear. On the finest work of the two cousins the heads tighten and lean, the choji taking on a saka or reverse-slanting tendency that the published sources name the group's particular feature, forming the distinctive pouch-shaped fukuro-choji, the "unique pouch-shaped fukuro-choji in which their special character lies"[[c:2]]. A second recurring tell sits within the ha: 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 shinogi, and which they read as a salient mark of the same workshop and lineage.

The jigane is where the revival is told apart from its Kamakura model. He forges a packed itame or ko-itame that flows strongly in the lower half and tends almost to masame, carrying fine ji-nie like minute particles, delicate chikei, and a standing utsuri that is sometimes a straight, sugu-like reflection and sometimes a midare-utsuri. This masame-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 Bizen, together with the tightened nioiguchi with ko-nie and the sunagashi, while noting that on his blades "the appearance of utsuri is slight"[[c:3]]. The boshi answers the ha: deeply tempered, entering in a midare-komi to a small round, the tip finishing in hakikake. The shape is the Kanbun shinto bearing, shinogi-zukuri with mitsu-mune, a rather wide mihaba with clear taper, a thick kasane and usually a shallow sori.

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 saka inclination in its choji, 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 choji but by the masame-tinged jigane with standing utsuri, the broad yakihaba reaching toward the shinogi, the deep boshi and the ika no atama, the points the sources call the salient features of the same 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 omote; several add a date and one a kinzogan date of Kanbun 11, so that a smith of the shinto age is read off his own inscriptions rather than off attribution. The largest is a votive katana of Kanbun 6, exceptionally wide and long with deep sori, 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 same Kanbun shinto period, often kept the deeper sori, Koretsugu by contrast more often made the shallow Kanbun construction, the difference the published sources single out for attention, observing that he "frequently made constructions with the shallow sori typical of Kanbun-shinto"[[c:4]]. His teacher Korekazu, the Musashi Daijo of the Edo Ishido, gave him the masame-tinged jigane and the saka-leaning choji that distinguish the whole revival from the Kamakura Ichimonji it looks back to. Against true old Bizen his work is told by the fine masame, the tightened nioiguchi with ko-nie and the sunagashi; against the showier Osaka Ishido it keeps the Chikuzen jigane and the deep-tempered boshi. The published sources affirm his finest as a representative work of the school and "a masterpiece among his works"[[c:5]], naming one "a typical work of Chikuzen Ishido"[[c:6]].

For the collector Koretsugu is a Juyo-rank shinto name rather than a designated-treasure one. He has no National Treasures and no Important Cultural Properties; his record runs entirely through the Juyo Token rank, seven signed blades passing the shinsa across sessions from the thirteenth to the fiftieth. Provenance is thin in the surviving record, and no museum or daimyo house can be grounded on his blades, though one Kanbun 9 katana was made on commission for a member of the Kozai clan of Sanuki, retainers of the Hosokawa since the Nanbokucho period, and survives with a black kawari-nuri uchigatana koshirae 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-sori Kanbun pieces rather than the grand votive blades. When one does appear it is a self-documenting shinto Ichimonji revival in good order, the kind the judges call a blade "in which Koretsugu's true strengths are fully exhibited"[[c:7]], and a clear window onto how the Fukuoka choji was forged again in the Kanbun age.

Koretsugu was born in Kan'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 Chikuzen who in Meireki 1 (1655) went up to Edo by his lord's order, studied the Bizen tradition under Musashi 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 Chikuzen Nobukuni school as one of the two pillars of Chikuzen shinto. 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 choji of the Kamakura Fukuoka Ichimonji, recovered two and a half centuries later in shinto steel, and the published sources call him most skilled at exactly that, a smith who "excelled above all in choji-ba in the manner of the Ichimonji school, and was extremely skillful"[[c:1]].

His characteristic temper is a broadly hardened choji-midare set high over the shinogi in places, mixing gunome, ko-gunome and pointed elements within the clove heads, with ashi and yo entering well to produce a florid, varied midare. The temper is nioi-dominant with ko-nie gathered along it, fine sunagashi run through, and the nioiguchi is bright and clear. On the finest work of the two cousins the heads tighten and lean, the choji taking on a saka or reverse-slanting tendency that the published sources name the group's particular feature, forming the distinctive pouch-shaped fukuro-choji, the "unique pouch-shaped fukuro-choji in which their special character lies"[[c:2]]. A second recurring tell sits within the ha: 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 shinogi, and which they read as a salient mark of the same workshop and lineage.

The jigane is where the revival is told apart from its Kamakura model. He forges a packed itame or ko-itame that flows strongly in the lower half and tends almost to masame, carrying fine ji-nie like minute particles, delicate chikei, and a standing utsuri that is sometimes a straight, sugu-like reflection and sometimes a midare-utsuri. This masame-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 Bizen, together with the tightened nioiguchi with ko-nie and the sunagashi, while noting that on his blades "the appearance of utsuri is slight"[[c:3]]. The boshi answers the ha: deeply tempered, entering in a midare-komi to a small round, the tip finishing in hakikake. The shape is the Kanbun shinto bearing, shinogi-zukuri with mitsu-mune, a rather wide mihaba with clear taper, a thick kasane and usually a shallow sori.

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 saka inclination in its choji, 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 choji but by the masame-tinged jigane with standing utsuri, the broad yakihaba reaching toward the shinogi, the deep boshi and the ika no atama, the points the sources call the salient features of the same 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 omote; several add a date and one a kinzogan date of Kanbun 11, so that a smith of the shinto age is read off his own inscriptions rather than off attribution. The largest is a votive katana of Kanbun 6, exceptionally wide and long with deep sori, 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 same Kanbun shinto period, often kept the deeper sori, Koretsugu by contrast more often made the shallow Kanbun construction, the difference the published sources single out for attention, observing that he "frequently made constructions with the shallow sori typical of Kanbun-shinto"[[c:4]]. His teacher Korekazu, the Musashi Daijo of the Edo Ishido, gave him the masame-tinged jigane and the saka-leaning choji that distinguish the whole revival from the Kamakura Ichimonji it looks back to. Against true old Bizen his work is told by the fine masame, the tightened nioiguchi with ko-nie and the sunagashi; against the showier Osaka Ishido it keeps the Chikuzen jigane and the deep-tempered boshi. The published sources affirm his finest as a representative work of the school and "a masterpiece among his works"[[c:5]], naming one "a typical work of Chikuzen Ishido"[[c:6]].

For the collector Koretsugu is a Juyo-rank shinto name rather than a designated-treasure one. He has no National Treasures and no Important Cultural Properties; his record runs entirely through the Juyo Token rank, seven signed blades passing the shinsa across sessions from the thirteenth to the fiftieth. Provenance is thin in the surviving record, and no museum or daimyo house can be grounded on his blades, though one Kanbun 9 katana was made on commission for a member of the Kozai clan of Sanuki, retainers of the Hosokawa since the Nanbokucho period, and survives with a black kawari-nuri uchigatana koshirae 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-sori Kanbun pieces rather than the grand votive blades. When one does appear it is a self-documenting shinto Ichimonji revival in good order, the kind the judges call a blade "in which Koretsugu's true strengths are fully exhibited"[[c:7]], and a clear window onto how the Fukuoka choji was forged again in the Kanbun age.