On a wakizashi of his debut years Kotetsu cut his own biography into the nakago: a native of Echizen who, past half a hundred years (半百), came to live in Edo of Musashi and gave himself to the smith's craft. Nagasone Okisato, called Sannojo, who after taking lay orders signed Kotetsu Nyudo, was originally an armorer (katchu-shi) of Echizen; around Meireki 2 (1656), when he was about fifty years old, he moved to Edo and turned swordsmith, a conversion the published commentary restates on blade after blade. Dated work runs from that same Meireki 2 to Enpo 5 (1677), the year before his death: one of the most celebrated careers in the Japanese sword fits inside two decades. The sources explain the sudden fame concretely: he brought from the armorer's forge a rare skill in treating iron, he excelled at carving, and he won his name by devising a strikingly novel hamon, the juzu-ba[[c:1]], the proven sharpness of the edge and the beauty of the carving[[c:2]] doing the rest. The modern commentary calls him the most popular smith of the entire Shinto age[[c:3]].
The standing formula of the published sources gives his work in one sentence: the jigane is strong, and the ji and ha are bright and clear[[c:4]]; most of his blades open with a yakidashi[[c:5]], the short straight start of the temper above the machi. Above it, in the mature work, runs the hamon that made the name: the juzu-ba, the peculiar gunome-midare[[c:6]] whose round-headed gunome[[c:7]] link so evenly that the crests stand almost in one line, like a string of prayer beads. Thick ashi enter frequently, the nioi is deep, ko-nie lies thick, fine kinsuji and sunagashi run through the ha, and the nioiguchi is bright and clear. The boshi is typically sugu with a small round return, and in his later years he often burned a gunome across the yokote itself, a habit one text calls the idiosyncratic gesture of Kotetsu[[c:8]].
The jigane is the armorer's legacy, and the sources say so directly. Of a kanmuri-otoshi wakizashi of Meireki 2 or 3, his first manner, the commentary writes that the forging of the jigane is good even among his own works, well kneaded and extremely strong[[c:9]]; in those early blades a large open hada (大肌) surfaces toward the middle, and the so-called tekogane (テコ鉄), the armorer's billet iron, appears in the ji. In the prime the forging tightens to a ko-itame knit to the utmost, ji-nie thick and dust-fine, chikei fine, the steel clear and bright, while flecks of jifu (地斑) still surface here and there. The horimono are his own hand: the early kurikara in splendid openwork (欄間透) is read straight from the Echizen carving of the Kinai school (記内彫), the add-mei moves from dosaku horinokore (同作彫之) to horimono dosaku (彫物同作), the shinkitae-saku addendum (真鍛作) marks selected forgings, and even unsigned carvings are given to him outright on the fineness of the chisel.
The NBTHK dates his blades almost by the signature alone. He signed first with the old-iron characters (古鉄), a form with almost no parallel; then in the hane-tora manner; and from the eighth month of Kanbun 4 (1664) in the hako-tora form, both occurring within that single month. The early period belongs to the hyotan-ba, shallow gunome paired in twos[[c:10]] whose fused silhouettes suggest the double gourd, which one Juyo text dates as the hamon often seen from the end of Manji into the first years of Kanbun[[c:11]]. The later period carries the juzu-ba over the typical Kanbun-shinto sugata[[c:12]]: mihaba tapering markedly from base to tip, shallow sori, a compact chu-kissaki. The texts set the summit late, from Kanbun 10 to about Enpo 2 or 3[[c:13]], call the sober blades of about Kanbun 8 mostly tight in the nioiguchi, short of flamboyance but solid[[c:14]], and praise a wakizashi of Enpo 3 or 4 as a work of complete ripeness[[c:15]]. Two registers stand aside. Suguha is comparatively rare for him, and when he took it up the nioiguchi as a rule tightens[[c:16]]. Tanto are rarer still, probably not reaching ten extant pieces[[c:17]]; in them the jifu stands out, the nioi runs deep with coarse nie, sunagashi and kinsuji mingle, and the feeling of Soshu is strong, one text reading the aim as lying in the direction of Go (江辺).
The cutting tests are nearly a second signature. Many of his designated blades carry gold-inlaid attestations by Yamano Kaemon no Jo Nagahisa and Yamano Kanjuro Hisahide, the leading authorities on test-cutting of the day[[c:18]], recording two-body and three-body cuts[[c:19]] dated to the day. These kinzogan inscriptions sit on the nakago beside his own mei as certifications of the edge, never as signatures. That wakizashi outnumber katana, with frequent masterpieces among them, one text lays to commissions from wealthy merchants more than from the warrior class[[c:20]]. Honma Junji's verdict stands in the Juyo Bijutsuhin records: among the Kanbun-shinto smiths, Kotetsu of the east is, with Sukehiro and Shinkai of the west, beyond question of the first rank[[c:21]]; the same note adds that a market price overtaking the old masterpieces[[c:22]] goes too far. The Nagasone forge outlived him: Okimasa, his adopted son, succeeded as the second-generation Kotetsu, with Okinao and Okihisa completing the gate, Okinao read as a daisaku hand during the master's lifetime.
Fujishiro rates him Sai-jo saku, the top grade. The official record carries 129 designated works: five Important Cultural Properties, ten Tokubetsu Juyo and 102 Juyo, 112 in those two tiers together, and ten prewar Juyo Bijutsuhin. The record is almost entirely signed, 115 signed blades against a single unsigned piece, so the mei chronology above doubles as the connoisseur's map of the work. The recorded provenance runs through the great houses: the Nabeshima of Hizen, the Shimazu, the Okubo of Odawara, the Uwajima Date, the Maeda of Kaga and the Imperial Family; one blade passed through the house of Yamada Asaemon, the prewar premier Inukai Bokudo owned the wind-and-thunder-gods wakizashi of Enpo 5, his last year, and a blade of Kanbun 1 rests in the Tsukamoto Museum of Art. The five Important Cultural Properties are patrimony, preserved beyond the market; nearly everything else stands in the Tokubetsu Juyo and Juyo registers, long held in private collections, so a genuine Kotetsu surfaces only rarely and commands the very top of the market when it does. For the connoisseur the recognition runs ahead of the nakago: a juzu-ba over a strong, bright jigane in a Kanbun-shinto sugata reads as Kotetsu before the signature is ever examined.
