Ishidō Unju Korekazu, commonly known as Masatarō, was the nephew of Katō Chōunsai Tsunatoshi, a domain-employed smith of Yonezawa in Uzen Province. He first studied the techniques of sword forging under Tsunatoshi, and later succeeded as the seventh-generation head of the Ishidō house, adopting the name Korekazu. His period of work extended from the closing years of Tenpō through the early Meiji era, and he died in Meiji 24 (1891) at the age of seventy-five. One notes that he lived "during the era of the Meiji sword prohibition and was unable to fully exercise his skills, which is regrettable." Comparatively many of his works survive today, and they include a collaborative piece () with the Aizu smith Kakuda- Gengō, in which Gengō forged the blade and Korekazu applied the tempering -- though "in terms of appearance, it is a blade that seems entirely to be the work of Korekazu."
Korekazu's works divide broadly into two modes: - and Sōshū-den. "Nevertheless, his true strength lay in -, which may be called the hereditary specialty of the Ishidō house." The defining characteristic that sets him apart from his contemporaries is repeatedly identified by the : "whereas many -tradition smiths of the period tempered in , he instead produced it in ; in this one can discern his originality." The is typically mixed with , , , and elements tending toward pointed forms; enter well and long, with intermingled; the is deep; adheres thickly, with occasional coarser ; and vigorous and run throughout the temper. His early works, "like those of his teacher Tsunatoshi, show a that is tight, with containing "; in later years, however, "he tempered this in small , introducing a further level of variation within the ." The forging ranges from tightly packed with dense and fine to mixed with and , at times achieving so fine a surface as to become "plain-like" (). His construction tends toward robust proportions -- wide , thick , long blades, and often an -- conveying a powerful, heroic that shows "no sense of imbalance" even at exceptional lengths.
The evaluative language of the converges on a portrait of controlled originality within inherited tradition. Individual blades are praised as "a quintessential example" (), as works in which "his true strengths are fully manifested," and as pieces displaying "abundant internal activity" () within the temper. The is described as "bright" and "keenly clear" (), and the combination of with conspicuous and is identified as the signature by which his hand may be recognized. An engraved "Jōi" (Expel the Barbarians) inscription on one blade, together with dedications to retainers of the Sendai Date house, enable one "to grasp the temper of the age" in which these swords were made. Korekazu stands as the foremost figure of the late Ishidō lineage, a smith whose reinterpretation of the tradition constitutes the most distinctive individual voice among -period revivalists.