The Science of Strength in kotō, shintō, and guntō — What Is Known and What Is Not

What Produces "Never Breaks, Never Bends, Cuts Superbly"

The reputation of Japanese swords for never breaking, never bending, and cutting superbly derives not from the properties of a single steel material but from a structure that combines steels of differing hardness. The kawagane (jacket steel) that covers the outside of the blade is high in carbon content, hard but brittle, while the internal shingane (core steel) is low in carbon content, soft but tough. Research conducted by metallurgist Tawara Kuniichi during the Taishō era using a metallurgical microscope revealed that this composite structure of kawagane and shingane, combined with the coexistence of martensite (hard but brittle microstructure) and pearlite-ferrite (soft but tough microstructure) produced by yaki-ire (quenching), enables the blade to reconcile the opposing properties of hardness and toughness.

What Changed Across kotō, shintō, and guntō

• kotō (before Keichō): tamahagane derived from sand iron and tatara (traditional smelting) specific to each production region was used, and the kawagane-shingane structure was built in through the tanren (forging) methods unique to each school.
• shintō and Shinshintō (Edo period onward): while the homogenization of commercially available steel progressed, innovations in tanren methods and yaki-ire were passed down by individual tōkō (swordsmiths).
• Shōwa-era guntō: in response to rapidly increasing demand, machine production and mass manufacturing advanced, and many examples used not only traditional tamahagane but also Western steel and steel derived from modern steelmaking as represented by the Mantetsu sword, as well as oil quenching.

The differences among these three categories should be discussed first and foremost as differences in materials and manufacturing methods. In the Meiji era there were also modern attempts such as the improved sword associated with Murata Tsuneyoshi — the so-called Murata sword — which mixed Swedish steel with Japanese steel, and the blending of traditional techniques and modern metallurgy had already begun in the period leading up to the guntō.

The Reality of "Destructive Testing" — What Can Be Measured and What Cannot

Data comparing the strength of kotō, shintō, and guntō side by side under identical conditions through destructive testing does not, in practice, exist. This is because most surviving kotō and shintō are cultural properties or artworks of equivalent standing, and subjecting them to tests involving destruction is essentially impermissible. The bulk of scientific knowledge to date — including Tawara Kuniichi's research — has been accumulated through cross-sectional observations obtained during the polishing process, examination of broken or damaged blades, and hardness measurements (such as Vickers hardness testing) and metallographic observation conducted on reproduction swords and test pieces made by modern tōshō (swordsmiths).

Shōwa guntō, on the other hand, include examples where mass-produced pieces regarded as having relatively low artistic value have been subjected to research and disassembly surveys, and it has been reported that some pieces show a more uniform microstructure than traditionally made tamahagane hand-forged swords, and exhibit a hardness distribution characteristic of oil quenching. This should not be taken as leading to the simple conclusion that guntō are weak, but rather regarded as circumstantial evidence that manufacturing choices prioritizing mass-production efficiency produced strength characteristics different from those of the traditional composite structure.

1. kotō and shintō achieved a balance of strength and toughness by building in the kawagane-shingane structure through regional tamahagane and the tanren methods of each school.
2. In guntō, pieces have been reported that, as a result of manufacturing choices that prioritized supply volume, exhibit hardness distributions and microstructures different from those of traditional swords.

The Mantetsu Sword: An Attempt to "Make a Japanese Sword with Modern Steel"

The best-known representative attempt at modern steelmaking during the Shōwa era is the Mantetsu sword, said to have involved the technical staff of the South Manchuria Railway. Research and manufacturing proceeded under the concept of whether modern steelmaking methods using iron ore produced in northeastern China as raw material could mass-produce Japanese swords without relying on traditional tatara (traditional smelting) and tamahagane. This was not mere production of a substitute but also a kind of engineering challenge to see how far the metallurgical technology of the day could reproduce the performance of a Japanese sword.

Research comparing such modern steel swords with traditional tamahagane swords shows differences in material composition and microstructure, but sufficient data systematically comparing sharpness and durability in actual combat has not been preserved. The reality is that much guntō research remains limited to observation of surviving pieces and the accumulation of partial scientific analyses, and it is necessary to read the various reports with an awareness of this limitation.

Non-Destructive Analysis as a Practical Option

The techniques that answer the need to understand the internal structure of a blade without compromising its value as a work of art are non-destructive methods such as X-ray, CT, and X-ray fluorescence analysis. These can visualize, without damaging the blade, the density distribution and compositional makeup of the internal steel, and in some cases even the condition of the boundary between kawagane and shingane. Although they do not yield the direct strength data that destructive testing provides, they are a technology considered likely to become the mainstay of future comparative research, in that multiple pieces can be examined across the board while preserving their value as cultural properties.

Exhibition facilities and research institutions for swords are also gradually increasing the opportunities to publish such analysis results, and this is becoming the foundation for reframing the kotō-shintō-guntō distinction not as "which is superior" but as "what design philosophy each is a product of."

What Should Be Expected from Comparative Research

What matters when discussing differences in strength among kotō, shintō, and guntō is not a simple question of which era is stronger, but understanding what manufacturing methods were chosen and for what purpose. kotō and shintō were made to provide versatility that could withstand both combat and ceremonial use, while guntō were made to meet the different demand of large-scale supply in a short period. The means of knowing the strength of surviving swords without damaging them are limited, but non-destructive X-ray, CT, and X-ray fluorescence analysis and reproduction experiments by modern tōshō (swordsmiths) are fields that are gradually filling this gap. When appreciating a sword, if one reflects not only on its shape and hamon (temper line) but also on what manufacturing philosophy it was forged under, even a single blade can come to look quite different.