Japanese swords: helpless against armor? An overview.
Based on several interactions I had over the years, I wanted to share a bit of a rambling post on Japanese swords, and their use in historical battles.
This topic was brought up again in a recent Matt Easton video. As for the subject of the video, which discussed which swords will be useless against plate armor, and to some extent armor in general, I find the choice of including "all Japanese swords" odd, and the arguments presented very lacking. The main reason for this post is to address once again the eternal, everlasting misconceptions people have on Japanese swords, their use and their durability.
Matt's argument rests on two main points. One, is the "material" issue. He claims Japanese swords are brittle at the edge, and soft at the spine which translates as the tip, and edges more likely to break off, and the sword to bend when used to attempt bypassing armor gaps. This implies that Japanese swords overall were unsuitable to sustain clashes with armor, or any other hard surface (including other swords).
This is a recurring idea that shows up systematically every time Japanese swords are discussed, despite the amount of literature, scientific and historical, to dismiss it.
The claim originates from the hardness level of Japanese swords. In material science, high hardness, in modern steel, correlates with brittle mode of failure. It also correlates with high strength, and low toughness. In layman terms, a very high hardness, high strength but low toughness material will resist to wear of the surface, localized plastic deformation, does require a lot of force (for a given cross section) to deform, and when it does it absorbs little energy and cracks (low elongation).
Now, the hardness value alone is not sufficient to prove all of the above, especially considering the pre-modern context of historical steels.
There are some interesting findings from a Japanese sword made 500 years ago. In the paper published by Kwak et al, they performed hardness, micro-tensile and micro fracture tests on two different antique swords. In the oldest of the two, they found that even with high hardness, the micromechanical testing study indicates that in the sharp-edge regions, strength and fracture toughness are simultaneously enhanced by the presence of fine pearlite.
Further, none of these settings are considering the effect of geometry. Edge geometry plays a fundamental role in terms of failure, and reinforced tips/edges with appleseed geometry are much more resilient against impact compared to finer ones. Here is a video showing impact testing on a modern, 1084 knife blade, hardened and tempered at 64HRc vs one hardened and tempered at 57 HRc-62 HRC. Using different practical impact tests, despite the anecdotal evidence, you can see that the harder edge performs much better than the softer one. And that is mostly governed by edge geometry and high strength, in fact at more acute edge angles, the aforementioned relationship is valid: the harder edge cracks while the softer one rolls.
As for the softer spine, we are again not considering the effect of geometry. Most Japanese swords, as a matter of fact, are very thick and rigid. Ignoring the fact that with pre-modern steel, you will not have a completely through hardened sword, and that most historical blades have ductile cores of wrought iron/low carbon steel, Japanese swords are much stiffer on average than many European blades such as rapiers or longsword designs. This alone should reframe the whole argument because no, Japanese swords are not easier to bend, quite the opposite, and when it happens they deform rather than break. I struggle to see how this can be a bad design for dealing with armor.
Finally, the second point rests on blade geometry and the shape of the tip. Ironically, in his video he is showing an elongated, slender tip geometry known as ōkissaki. With a regular, short Japanese style tip, I would agree that it will be very hard to successfully bypass mail. However, this is an entirely different geometry. We know some of those tips were reinforced, and the slender acute point definitely gives them a good chance to burst into mail rings. I think discarding their effectiveness without testing is not a great argument, especially since those tips were used against armor in various other contexts.
Overall, while Japanese swords did not transition into estocs, there was a change in blade designs from the 14th century which accounted for dealing with armor. Leaving aside the topic of the video, and extending the context as a general "swords use against armor", there is a text known as Kenpō Ryakki which talks about fighting in armor with swords.
It talks about striking with full force with a sword against hands and limbs to break bones. In a Chinese text written by Chen Zongyou, Exposition of the Original Shaolin Staff Method, it is written that
>Japanese swords, and the tie bian, they are the same
the tie bian being a mace. And the main strategy suggested is to aim for the gap and to thrust with your blade. That is also found in other later period manuals; from the Kōdōkan Jūdō Official Bulletin "Yūkō no Katsudō," Volume 6, Number 6: Jūdō Anecdotes from the Atsukaishin-ryū, based on the Noda Family Documents, we can read that:
>When the body is firmly protected by armor, it cannot be cut, so thrusting is preferable.
Most of the gaps identified in the Japanese armor system are also quite universal: the armpit, the face, through the gap in the skirt armor to the lower abdomen, the feet, the hands.
Some of these places, such as the groin and the armpit, could be shielded by mail patches, and other flexible armor elements. The tip of these swords was in a sense expected to deal with such armor. There is definitely a correlation between extension of flexible armor to cover the gaps, and ōkissaki/shobu zukuri geometries.
To conclude, I do not think that Japanese katana and Japanese swords in general are completely useless against heavy armor. The arguments on steel durability are exaggerated and overly emphasised on Japanese swords, while the development of dedicated geometries is often ignored.
