Kogarasumaru-Zukuri — Structure, Authentication, and Rare Historical Examples
* Image is for illustrative purposes only.Table of Contents
What is Kogarasumaru-zukuri?
"Kogarasumaru-zukuri (小烏丸造り)" is a form of tsukkurikomi (blade shape construction) of the Japanese sword, and its name derives from the celebrated sword of the Heian period, "Kogarasumaru (小烏丸)." The defining characteristic of this blade shape is that the front half of the blade from the kissaki (point) to the monouchi (striking edge) is double-edged (moroha), while the rear half near the nakago (tang) is single-edged (kataha)—that is, the standard structure of a Japanese sword.
Because it represents an intermediate form between the double-edged straight sword (chokutou) of ancient times and the single-edged curved sword (katana) established from the Heian period onward, it is positioned as an exceptionally rare construction embodying a "transitional blade shape." Existing kogarasumaru-zukuri swords number fewer than several dozen throughout all of Japan, making them special objects of interest for sword collectors and researchers.
Origin of the Name—The Legendary Sword "Kogarasumaru"
The sword "Kogarasumaru," from which the name kogarasumaru-zukuri originated, is an old sword (koto) said to have been crafted during the Heian period and is currently preserved as an imperial possession of the Imperial Household Agency (entrusted for safekeeping to the National Institutes for Cultural Heritage). According to tradition, it is said that Taira no Sadamori received and used it as a grant from the imperial court during the suppression of Taira no Masakado, and the name "Kogarasu" is believed to derive from the Yatagarasu (eight-legged crow), the messenger of Amaterasu Ōmikami. It is also said in origin legends that a messenger from the Ise Shrine granted it to Emperor Kanmu, but this tradition reflects a mythical pedigree and is a separate matter from the place of dedication.
The "Kogarasumaru" held by the Imperial Household Agency is a tachi with a blade length of 62.8 cm and curvature of 1.2 cm, with the first third of its length from the kissaki being double-edged. The form of this actual object became the origin of the name and definition of the blade shape known as "kogarasumaru-zukuri."
Detailed Structure
Examining the blade construction of kogarasumaru-zukuri in detail reveals the following characteristics:
| Item | Description |
|---|---|
| Form of the kissaki | The kissaki differs from ordinary Japanese swords (inokubi-kirisaki, ōkirisaki, nakakirisaki) and features a "moroha" (double-edged) point with edges on both sides. This is sometimes interpreted as a remnant of ancient double-edged straight swords (tsurugi). |
| Position of the ha-machi | The ha-machi—the boundary between blade and tang—exists as in ordinary Japanese swords, but in the portion ahead of the monouchi, an edge is also applied to the opposite side (the mune side). |
| Form of the curvature | Most existing kogarasumaru-zukuri swords possess the koshi-zori (waist curvature) seen in Heian-period tachi. This is considered a remnant of the transition from straight to curved swords. |
| Jigane and hamon | Most are works of the Heian to early Kamakura periods, with jigane often displaying the characteristic grain of old swords, such as ōitame (large wood-grain pattern) or masame (straight grain) flow. The hamon is primarily characterized by ko-midare (small irregular pattern) or suguha (straight pattern), differing in character from the bold hamon of later periods. |
Historical Positioning
The emergence of kogarasumaru-zukuri has its background in technical experimentation during the process of the establishment of the Japanese sword. The prevailing theory holds that during the Heian period, double-edged straight swords (tsurugi) brought from the continent gradually transformed into single-edged curved swords adapted to mounted warfare, and kogarasumaru-zukuri was temporarily produced as a form incorporating characteristics of both.
From the late Nara period through the early Heian period, there are records of moroha-zukuri (double-edged construction) swords being used as official weapons, and kogarasumaru-zukuri can be viewed as continuing this lineage. From the mid-Heian period onward, single-edged curved swords became completely dominant, and kogarasumaru-zukuri largely disappeared.
Existing Kogarasumaru-zukuri Swords
The kogarasumaru-zukuri swords currently identified are extremely limited in number, and include those designated as National Treasures or Important Cultural Properties. They are occasionally displayed at special exhibitions at the Tokyo National Museum and various regional sword museums, making opportunities to see the actual pieces rare.
Trading in the private sector is virtually nonexistent, and given its rarity and historical value, should it ever come to market, it would attract considerable attention both domestically and internationally.
Points for Authentication
When authenticating kogarasumaru-zukuri, the range and finish of the double-edged portion of the kissaki is of paramount importance. Since the possibility exists of later "altered pieces" deliberately given double-edged treatment, comprehensive judgment must be made considering the condition of the nakago, rust color, and consistency of the signature's period. In NBTHK authentication documents, the official designation of "kogarasumaru-zukuri" in the tsukkurikomi section formally certifies its rarity value.
Conclusion
Kogarasumaru-zukuri can be said to be a witness to an era when the Japanese sword still wavered between "tsurugi" (sword) and "katana" (sword). In the modern era, its rarity situates it as a special object of study and appreciation, and it remains an indispensable blade shape in any discussion of the formation history of the Japanese sword.
The actual Kogarasumaru held as an imperial possession is normally not on public display, but its existence is widely referenced by experts as a foundation point for Japanese sword research. This blade shape, which condenses the transformation of sword-making technology in the transition from ancient to medieval times into a single sword, continues to be an inexhaustible subject of inquiry not only for collectors but also for historians and art historians. The saying that possessing one kogarasumaru-zukuri sword is equivalent to touching the moment of the birth of the Japanese sword explains its significance. Precisely because of its rarity, the discovery of a previously unknown existing example becomes major news capturing the attention of the entire sword world.
