# Why are katana blades curved

## The curvature of the Japanese blade

### Why are (most) Japanese sword blades curved?

This question came up when I was dealing with the bokken, the wooden image of a Japanese steel sword.

The curve wasn't there from the start. Before the 10th century the Japanese swords - like their Chinese models - were straight. There was then a transition phase in which the blade was only locally curved, the rest of the blade stayed straight. E.g. a curve just after the tang, with the rest of the blade remaining straight. Eventually the continuously curved blade became "in".harris

Technically, the curvature is the result of the differential hardening the blade. Differential means that the blade is not hardened evenly, but that certain parts are hardened more than others. In the case of the sword, of course, this is the cutting edge. The cutting edge becomes very hard, but also brittle, whereas the back of the blade remains elastic. This is done by covering the entire blade with a layer of clay, but removing most of it at the cutting edge. This also explains the cutting pattern ("hamon"). It clearly shows the transition from hard, brittle to elastic steel. When hardening in water (with approx. 40 ° C)after the blade in the clay coat once more to glow (at approx. 850 ° C; steel melts at approx. 1500 ° C) was brought, the cutting edge now cools faster than the rest of the blade, which lies under a thick layer of clay. Therefore the cutting edge becomes harder and one speaks of differential hardening.

The curvature is not the result of any thermal expansion, but a change in volume. This is due to the fact that austenite is converted into martensite in the strongly quenched cutting edge. The more slowly cooling area of ​​the rest of the blade body transforms and contracts differently. Considerable internal tensile stresses occur here, which are able to bend the cutting edge, which is small in terms of volume. The curvature can be severe, and it is just a distortion, not a deformation.herbert

### There are several ways to counteract this severe distortion

1. As already indicated above, the blade is completely hardened. However, since a Japanese katana is a single-edged sword, the blade will still bend. This is because the back of the blade is much thicker than the cutting edge and, due to this fact alone, takes longer to cool down, so the steel structure in the back of the blade has more time to change.
2. You make a double-edged sword. That can then also be hardened differentially.
3. The best and by far the most difficult method would be to "counter-curve" the blade before hardening it. The blade is shaped so that it is first curved in the opposite direction (i.e. towards the cutting edge). Hardening then causes it to curve in the other direction. If you've done everything right, the two curves cancel each other out. The result would be a straight blade. And to achieve that result would be damn difficult!

### Then why not do it that way?

1. If the blade were to be hardened through, the (Japanese) sword would lose its excellent properties for which it is so famous. The differentially hardened sword combines elasticity with sharpness. In the fully hardened state, the blade would be a great compromise between these two properties.
2. No, you don't want a katana double-edged. (See below "different reasons".)
However, hardening a double-edged blade is also an art, as the blade warps in both directions.
This can be counteracted by not hardening the blade as much overall (50 Rockwell as opposed to 60-70).Blade question
3. This method not only sounds theoretically complicated, it is even more difficult in practice! Only great masters of blacksmithing could do something like that.

### Maybe there was still different reasonswhy people didn't try to make swords in particular?

It is noticeable in this context that the curvature of the Japanese sword did not decrease over time, but on the contrary practically increased steadily.

It wasn't just that the blade was originally straight and only later became crooked, as described above. But that the curvature got stronger and stronger.harris

In addition to the sharpness of the blade, this suggests that the curvature was not only due to the hardening process, but that it also has (te) technical combat functions:

At least from the end of the 8th century armor (and until the beginning of the 17th century Bajutsu) the horse was an important tactical moment in Japanese military history. However, if a sword is used from the horse, curved blades have a certain advantage over straight weapons, as they are more effective as a cutting weapon. As evidence for this I would like to state that all equestrian peoples I know use such curved swords, be it e.g. the Mongols or the Arabs.

However, I have to admit that the sword was by far not the most important weapon, at least for the Japanese on horseback, because of its relatively short range. Rather, it was first the bow and then the spear. bajutsu

However, the use of the sword was forced with the invasion of the Mongols in 1274. nut

### Why is Japanese armor so much lighter than European?

Japan had deliberately isolated itself from the continent for a long time, and thus also withdrawn from the exchange of combat technology. The Japanese did not have to defend themselves against ever newer weapons, as was the case in Europe. There was a kind of conservation or a strong slowdown in development in the military sector.

Also the heavily ritualized warfare fair play made an upgrade unnecessary and hardly useful.