Kogarasumaru (小烏丸)—The Imperial Household Agency Treasure: How the Taira Clan's Heirloom Sword Tells of the Birth of the Japanese Blade

What is Kogarasumaru—The "Missing Link" in Sword History

Kogarasumaru occupies an extraordinarily unique position in the history of the Japanese blade. This sword, transmitted to have been crafted in the early Heian period (around the 9th century), existed at the most crucial turning point in Japanese blade history: the transition period from the straight blade (chokuto) to the curved blade (wanto).

Currently housed as a precious imperial treasure (gyobutsu) at the Imperial Household Agency's Sanomaru Shozo-kan (within the East Imperial Garden at the Imperial Palace in Tokyo), it is rarely displayed to the public. However, its unique form and the mystique of its transmission have been preserved and discussed with great interest by sword enthusiasts and researchers.

The Lineage of Kogarasumaru—The Taira Clan's Heirloom Treasure Sword

The "House Blade" of the Kanmu Heishi

Kogarasumaru is transmitted as the "heirloom treasure blade" of the Kanmu Heishi (Kanmu Taira clan). It is said to have been passed down through successive generations by the heads of the Taira clan, who were descendants of Emperor Kanmu (reigned 781–806).

According to legend, it is said to have been bestowed upon Taira no Takamochi (founder of the Kanmu Heishi who went to eastern provinces and became a warrior) by the deity of Ise Grand Shrine. As a sword with a mythological lineage, it was deeply connected with the authority and sanctity of the Taira clan.

With the Heike—Before and After the Battle of Dan-no-ura

At the Battle of Dan-no-ura (1185), the final decisive battle of the Genpei War, the entire Heike clan threw themselves into the sea and perished. At this time, one of the "Three Sacred Treasures," the Kusanagi no Tsurugi (Grass-Cutting Sword), is also said to have sunk beneath the waves.

How the Heike clan's heirloom treasure blade Kogarasumaru came to be transmitted to later generations is unclear; however, at least one blade known by the name "Kogarasumaru" was passed down to later generations, and in modern times it was presented to the imperial household to become a precious imperial treasure.

The Form of Kogarasumaru—The Mystery of the "Intermediate Between Double-Edged and Single-Edged"

A Unique Blade Structure

The greatest distinguishing feature of Kogarasumaru is its unique blade structure. It possesses an "intermediate form" that is neither a typical Japanese blade (single-edged with curve) nor an ancient straight blade (double-edged or single-edged without curve).

Its specific morphological characteristics are as follows.

Fusion of Double-Edged and Single-Edged Forms: Approximately one-third of the blade near the point (kissaki) is double-edged (with cutting edges on both sides, giving it a sword-like form), while most of the blade body is single-edged (with a cutting edge on one side only). This represents an intermediate form between the "morohashira-style" and "single-edged style," which is extraordinarily rare in the history of Japanese blades.

The Presence of Curvature (Sori): The blade possesses curvature and is not a perfectly straight blade. It exhibits a waist curve, demonstrating the prototypical form of the Heian-period tachi.

Blade Length: The cutting edge length is transmitted to be approximately 71 cm (about 2 shaku 3 sun 5 bu).

This form embodies the transition period "from straight blades (double-edged swords) to curved blades (single-edged swords with curvature)," making it an extremely important physical artifact for understanding the formation process of Japanese blades.

The Technical Level of the "Pre-Kotou Period"

The period of Kogarasumaru's manufacture (early Heian period, around the 9th century) corresponds, in the standard historical periodization of Japanese blades, to the transition from the "Jokotou period" to the "Kotou period." Jokotou refers to straight blades from before the Nara period, while Kotou refers to curved blades (such as tachi and uchigatana) from the mid-Heian period onward.

Kogarasumaru, with its form serving as a "bridge" between these two periods, is truly the missing link of Japanese blade history.

The Mystery of Japanese Blade Origins Revealed by Kogarasumaru

Why Did the Japanese Blade Develop Curvature?

One of the greatest mysteries in the history of Japanese blade formation is "why the straight blade evolved into the curved blade." The existence of Kogarasumaru provides one answer to this mystery.

The Mounted Warfare Adaptation Theory: According to this theory, as samurai in the Heian period came to engage primarily in combat from horseback, the "waist-curved blade" that was suited for drawing and slashing from horseback proved advantageous. The waist curve of Kogarasumaru embodies this "adaptation to mounted warfare."

The Forging Technology Evolution Theory: According to this theory, as folding and differential hardening techniques advanced, "natural curvature" occurred due to differences in carbon distribution in the steel. The difference in thermal expansion between the edge (high-carbon steel) and spine (low-carbon steel) creates the curve.

The Independence from Continental Influence Theory: The transition from imitating continental (Tang) designs in the Nara period to developing uniquely Japanese blade forms.

The form of Kogarasumaru—with its double-edged point, single-edged body, and waist curve—can also be interpreted as an "experimental form" that embodies the trial and error of this transition period.

Kogarasumaru in Modern Times—Its Value as a Cultural Treasure

As an Imperial Treasure Held by the Imperial Household Agency

Today, Kogarasumaru is a precious imperial treasure (gyobutsu) held by the Imperial Household Agency's Sanomaru Shozo-kan. Public exhibitions are limited, though it is occasionally displayed at special exhibitions.

Its value as a cultural treasure is immeasurable. As the oldest surviving example of a curved blade from the early Heian period, it is an irreplaceable resource in the study of Japanese blade formation history.

Its Position in Sword Research

Sword researchers and historians regard Kogarasumaru as "physical evidence of the establishment of the Japanese blade." The survival of blades with similar forms is extraordinarily rare, giving it great scholarly significance as a direct record of forging techniques and blade design standards from the early Heian period.

Moreover, its unique form as an "intermediate between double-edged and single-edged designs" is also noted from a comparative cultural perspective for showing the intersection between continental sword culture and uniquely Japanese blade culture.

Legends and Culture Surrounding Kogarasumaru

Connections to Myth and Legend

As its name "Kogarasu" (little crow) suggests, Kogarasumaru is said to be connected with the sacred crow (divine messenger, the eight-span crow). The tradition that it was bestowed by the deity of Ise Grand Shrine embodies the Japanese cultural concept that blades are not mere weapons but "vessels for sacred power."

Kogarasumaru in Kabuki and Literature

The name Kogarasumaru appears in kabuki and Japanese literature. In the kabuki play "Genpei Nunobiki no Taki" (The Genpei Waterfall), it appears as the famous blade of Taira no Kiyomori, with its mystical lineage depicted dramatically.

Attributed to Amakuni and "Kissaki-Morohashira Style"—Two Keys to Understanding Kogarasumaru

Essential to discussing Kogarasumaru are the traditions about its maker and the nomenclature of its blade form.

Although Kogarasumaru bears no maker's signature, it has long been transmitted as "attributed to Amakuni." Amakuni is a legendary swordsmith said to have been active from the Nara through early Heian periods and is counted among the founders of the Japanese blade. However, since Amakuni is known to have created signed blades, there has long been scholarly debate about whether the unsigned Kogarasumaru can truly be attributed to Amakuni, and it is most appropriate to understand him as the "attributed maker" in transmission rather than the confirmed maker.

The blade form is called "kissaki-morohashira-zukuri" (point double-edged style). Only a certain range near the point is double-edged (morohashira), while the lower portion is single-edged—a unique structure produced during the transition from straight to curved blades. Because Kogarasumaru is the representative work of this style, kissaki-morohashira-zukuri is also known by the alternative name "kogarasu-zukuri."

The fact that a blade form is named after the individual blade itself eloquently speaks to Kogarasumaru's symbolic position in blade history.

Recent research has also supplemented details of its provenance. It was granted by the imperial court to Taira no Sadao, who suppressed the Taira Masakado Rebellion, and subsequently became a Taira family treasure. After the Muromachi period, it passed to the Ise clan, and later to the Sō clan, lords of Tsushima. In 1882 (Meiji 15), the head of the Sō clan presented it to Emperor Meiji, making it an imperial treasure, where it has remained until the present day.

Conclusion—Kogarasumaru as Witness to the Birth of the Japanese Blade

While Kogarasumaru is often described as a "famous blade of the early Heian period," its fundamental significance lies in being "an extant blade that embodies the transition period from straight to curved forms." Its form, with both double-edged and single-edged sections and a waist curve, eloquently tells how the Japanese blade was born.

Preserved as an imperial treasure by the Imperial Household Agency, this single blade is living evidence of the moment over a thousand years ago when Japanese smiths created the Japanese blade, now one of the world's most renowned.

If you are interested in the history and provenance of blades, please also visit our column list.

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