Moroha-zukuri: Double-Edged Japanese Sword Blades — Form, History, and Notable Examples

What is double-edged construction?

The construction (tsukurikomi) of Japanese swords is classified by the cross-sectional shape of the blade and the method of edge attachment. Most Japanese swords have a basic structure of a single edge on the front side, but "double-edged construction (moroha-zukuri)" is an extremely distinctive form that has cutting edges on both the front and back of the blade.

Structurally, the spine (mune) is thinly ground, and both sides function as edges. Unlike normal shinogi-zukuri or hira-zukuri, the spine side often has hamon, and the overall cross-section is thin and sharp with a diamond shape. Its origins can be traced to the "ken" (straight swords) before the Heian period, and it is a form that appears in contexts of ritual, religious ceremony, and weapons.

Ancient double-edged swords—from ken to warabite-tō

The oldest examples of double-edged swords in Japan trace back to the "ken" that came from the continent during the Yayoi and Kofun periods. The swords of this period were strongly ritualistic and magical in nature, and were treasured in contexts separate from combat use. In the Nara and early Heian periods, double-edged swords were stored in the Shōsōin, where they occupied a special place in court sword culture.

In the Tōhoku region, a unique sword called "warabite-tō" developed. It is known for its curved tip that resembles the frond of a young fern, but both double-edged and single-edged examples exist for the blade section. After the Ezo subjugation by Sakanoue no Tamuramaro (801), this sword, said to have been used by the Ezo (Emishi) who faced the ritsuryō state, is also an important transitional form for understanding the later shift to the curved shape of Japanese swords.

Examples of double-edged construction in the Kamakura and Nanboku-chō periods

While Japanese swords developed rapidly from the late Heian through the Kamakura period, double-edged construction fell out of the mainstream, though some swordsmiths continued to make them for special purposes. From the Kamakura period onward, double-edged construction was seen mainly in the forms of tantō and ken, and was often transmitted as shrine donations or treasured items of the warrior class.

Records remain of large double-edged swords close to ken form being made in the Nanboku-chō period (14th century) amid trends toward larger and more magnificent forms. However, surviving examples are rare, and those designated as National Treasures or Important Cultural Properties are limited. A characteristic feature is that many double-edged swords survive in relatively good condition, perhaps because they were more often intended for shrine dedication and ritual purposes than for actual combat use.

The relationship between Shintō ritual and double-edged blades

Double-edged construction is particularly common in the realm of "shintō" (sacred swords) dedicated to shrines. In Shintō, the sword is used as a symbol of "misogi" (purification) and "harae" (purification), and there is an aspect where double-edged swords were preferred for the meaning of purifying pollution from both sides.

The "kusanagi no tsurugi" (Grass-Cutting Sword), said to be one of the Three Sacred Treasures, is also positioned as a continuation of the double-edged "ken" in antiquity. Among the sacred bodies and divine treasures transmitted to the Jingū and Taishas, some include double-edged swords, most of which are not open to the public.

Even in modern times, double-edged swords are used in Shintō rituals, and there are not a few examples of double-edged form in sword-shaped blades used as "katashiro" (substitutes) in the igushi and kagura.

Classification and the difficulty of forging

In the classification of Japanese sword construction, double-edged construction is also written as "moroha-zukuri." In the context of sword research, it is treated as a separate form from single-edged swords (tachi, uchigatana, wakizashi, tantō) and is often classified as "ken-rui" (swords).

The following types of hamon placement exist.

• Both sides have hamon
• One side has hamon omitted

In authentic double-edged construction, quenching is applied to both sides, but this is technically extremely difficult and is a process that tests the swordsmith's skill. The hagane (blade steel) on one side is easily affected by the heat of the quench on the opposite side, and temperature control and cooling control are required to a greater degree than in ordinary single-edged swords.

In double-edged construction, clay (applied before quenching) must be applied to both sides, so special technique is required for uniform clay thickness and application method. There are the following differences in quenching methods, with each swordsmith's workshop employing different approaches.

• Swordsmiths who employ a "one side at a time" method, quenching one side after the other
• Swordsmiths who attempt to quench both sides at once

Appreciation and collection—the market value of double-edged construction

At sword markets and art auctions, double-edged antique swords from before the Edo period are sometimes offered for sale, and there is a certain demand due to their rarity and unique form. From a collector's perspective, the beauty of a construction that differs from the mainstream form and its deep relationship with Shintō and Buddhism are the axes of evaluation.

The following points are important evaluation criteria in authentication.

• The quality and uniformity of the hamon on both sides
• The beauty of the shinogi (ridge line)
• The shape of the nakago (tang) and presence or absence of a signature

In the assessment by the Society for the Preservation of Japanese Art Swords (NBTHK), double-edged construction is also within the scope of normal assessment, and application for rankings such as Preservation, Special Preservation, Important, and Special Important are possible.

Modern swordsmiths also sometimes forge double-edged swords as custom works or dedication swords. Under the Cultural Agency's swordsmith registration system, the making of ken (double-edged form) is permitted within the same qualifications as tachi and tantō, and is counted within the production limit (24 per year). In understanding the diverse constructions of Japanese swords, double-edged construction continues to be an important subject of study for both experts and enthusiasts as a bridge connecting "pre-Japanese sword" sword culture and the "establishment of the Japanese sword."