Jokoto: The Straight Blades Before the Japanese Sword — Nara, Early Heian Excavations, and the Shosoin Swords

What Is Jōkotō? Periodization and Definition

The history of the Japanese sword is conventionally divided into five major periods: jōkotō (ancient swords), kotō (old swords), shintō (new swords), shinshintō (newly revived swords), and gendaitō (modern swords). The term jōkotō refers to blades produced before the mid-Heian period — roughly before the 10th century CE — when the distinctively curved, single-edged Japanese blade had not yet fully emerged. Jōkotō are characterized above all by their straight or nearly straight profile, reflecting the strong influence of continental straight swords imported from China and the Korean Peninsula.

Surviving examples of jōkotō are extraordinarily rare. Most that exist today are preserved as sacred treasures at the Shōsōin repository at Tōdaiji Temple in Nara, or at major shrines such as Kasuga Taisha and Atsuta Jingū; others survive only as fragmentary remains excavated from kofun burial mounds and archaeological sites. Fewer than a few dozen intact examples are believed to exist worldwide, making jōkotō the most historically important yet least understood chapter in Japanese sword history.

Continental Technology Transfer: The Arrival of Iron Culture

Full-scale iron culture reached the Japanese archipelago during the late Yayoi period (around the turn of the Common Era) through the Kofun period (3rd–7th centuries CE). Through immigrants from the continent — known as toraijin — iron raw materials and smithing techniques arrived from the Korean Peninsula. Initially, iron tools and agricultural implements dominated production, but before long, weapon manufacture began in earnest, including iron swords and long swords.

Iron swords and blades excavated from kofun burial mounds closely resemble forms from the southern Korean Peninsula (the Gaya and Silla regions), suggesting either direct imports or local production by continental-trained smiths. Blades are straight, either single- or double-edged, typically 60–90 centimeters in length; wooden handles rarely survive due to organic decay.

A landmark piece of evidence is the gold-inlaid iron sword (kokuhō, National Treasure) unearthed in 1968 from Inariyama Kofun in Saitama Prefecture. Dating to the year 471 CE (the xinhai year), it bears 115 characters in gold inlay recording the genealogy of a man called Ōwake-no-omi, demonstrating that sophisticated smithing and decorative metalwork coexisted by the late 5th century. A contemporaneous sword from Eta Funayama Kofun in Kumamoto Prefecture also bears silver inlay inscription, confirming that continental written culture and smithing technology arrived on the archipelago as an integrated package.

Nara-Period Swords: What the Shōsōin Collection Reveals

The finest direct evidence for Nara-period (710–794 CE) swords is the collection preserved in the Shōsōin at Tōdaiji Temple in Nara. The Shōsōin holds approximately 9,000 treasures, the core of which were dedicated to the Great Buddha by Empress Kōmyō upon the death of Emperor Shōmu in 756 CE (Tenpyō Shōhō 8).

Among these treasures are several swords, all of straight-blade form with lavish Chinese-style koshirae (sword mounts). The most celebrated is the Kingin-den-sō Kara-tachi (Gold-and-Silver-Decorated Tang-Style Tachi), measuring 98.2 centimeters overall with a blade length of 73.7 centimeters and no curvature. Its scabbard is decorated with raden (mother-of-pearl inlay) and gold-and-silver metalwork of exceptional refinement. These objects demonstrate that swords in this period functioned not merely as weapons but as prestige goods in diplomatic and ritual contexts.

The specifically Japanese smithing techniques that would define the mature Japanese sword — the combination of hard outer steel (kawagane) and softer core steel (shingane), the formation of the hamon (temper line) through differential hardening — were still in a developmental stage during the Nara period. Most swords in the Shōsōin are either imported Tang-Chinese pieces or domestic reproductions of Tang forms, with a clear technological discontinuity from the later Japanese sword tradition.

The Early Heian Transition: From Straight to Curved Blade

In the 9th century (early Heian period), a dramatic transformation occurred in blade form: the shift from straight swords to swords with sori — the characteristic curvature of the Japanese blade. This transition is one of the greatest mysteries in sword history. Scholarly consensus now points to a combination of interacting factors.

The spread of cavalry tactics: The protracted wars against the Emishi people of northeastern Honshū (the Emishi Expeditions, 780–811 CE) led court armies to adopt cavalry tactics on a large scale. Practical battlefield experience demonstrated that a curved blade was superior to a straight one for drawing and cutting on horseback.

Evolution of materials and forging technology: The development of tatara ironmaking — using the tamahagane steel produced from domestic satetsu (iron sand) — nurtured a distinctly Japanese material culture. Japanese iron sand yields a relatively homogeneous, low-impurity steel; it was likely discovered empirically that differential cooling during quenching (the blade edge cooling faster than the spine) produced natural curvature as a byproduct of the hardening process.

Changing social demands: The weakening of the ritsuryō state and the rise of provincial warlords and warrior elites created demand for more practical weapons. Compact, portable, single-edged curved blades suited real combat better than heavy ceremonial straight swords.

Among the earliest known curved swords is work attributed to Hōki Yasutsuna, a legendary smith of the late 9th century. His most celebrated work, the Dōjigiri Yasutsuna (National Treasure, Tokyo National Museum), displays fully developed sori and a beautiful hamon, and is often cited as one of the earliest examples of the "completed" Japanese sword form.

The Technical Level of Jōkotō: A Modern Metallurgical Assessment

A modern metallurgical and archaeo-scientific perspective has added considerable nuance to our understanding of jōkotō technology. Fluorescent X-ray analysis and microstructural examination of excavated swords are gradually revealing carbon content, impurity composition, and the presence or absence of heat treatment.

Results show that swords predating the Nara period are a mixture: some bear evidence of repeated folding and welding (orikaehi-tanren) — the technique of improving steel homogeneity by folding the billet — while others consist of relatively homogeneous low-carbon steel, probably inexpensive mass-produced items made by local smiths.

Notably, some 5th-century kofun blades show the characteristic layered microstructure produced by repeated folding, providing direct physical evidence of advanced continental smithing transplanted to the archipelago. This is not a direct precursor to classical Japanese sword forging, but it represents the germinal stage of a local technological evolution that would, over centuries, produce distinctly Japanese blade-making traditions.

The Legacy of Jōkotō for the Modern World

For too long, jōkotō was dismissed as mere prehistory to the "real" Japanese sword. Recent scholarship has reversed this assessment: without this formative era, the Japanese sword culture that followed could not have emerged.

The process of receiving continental smithing technology and then transforming it through Japan's own materials — iron sand, charcoal, water — and geographic conditions (a humid climate, abundant forest resources) was what ultimately crystallized into the kotō tradition from the 10th century onward. The gold-and-silver-decorated straight swords sleeping in the Shōsōin are not merely objets d'art; they are witnesses to Nara-period Japan as a crossroads of East Asian civilization.

For contemporary sword scholars and connoisseurs, the chance to handle a jōkotō in person is virtually nonexistent. Yet the results of excavation research and the annual Shōsōin Exhibition (held each autumn in Nara) offer at least a glimpse of their existence. Knowing the origins of the Japanese sword reminds us that this art form was never a singular invention — it was the product of deep continental exchange and Japan's own unique natural and social environment.