March 28, 2008

Japanese Sword Sharpness

Stupid Question ™
Aug. 8, 2002
By John Ruch
© 2002

Q: Just how sharp are katana swords? Are they sharper than Western swords?

A: The katana, or standard samurai sword, has earned a mystique over the past 600 years, though all types of Japanese swords and polearms before and after it were made in the same general way.

Before we generalize, note that there was no single “brand” of katana. Over the centuries, there have been different types and makers, and long stretches of inferior, mass-produced blades.

It’s also unknown exactly how sharp historical blades were kept, because surviving ones were regularly re-polished and sharpened.

In general, katana were known for being very sharp (one legend involved a sword being stuck into a stream and neatly slicing leaves that floated against its edge), and modern versions are razor-sharp. But this was not really the katana’s claim to fame, and doesn’t mean much anyway. If sharpness was all that counted, swords would be made out of broken glass.

The katana was famous for keeping its edge and for not breaking in battle (at least, not as often as enemies’ blades). Steel strong enough to hold an edge is usually brittle; steel flexible enough not to break is usually too soft to hold an edge. The katana combined both qualities.

The thin, slightly curved katana blade consists of a softer, shock-absorbing core and a hard steel jacket. The iron is hammered out and folded back on itself, then hammered out again, over and over, creating thousands of strong layers. The core has perhaps a few thousand layers, the jacket up to 60,000.

After being forged, the blade is coated with clay that is thinner on the edge and thicker on the back. When the blade is tempered in fire and water, the clay causes different parts of the blade to harden differently. The rapidly cooling edge becomes very hard steel. The slowly cooling back becomes a softer form of steel. Voila! Both hardness and resilience.

(Note that today’s mail-order katana are mostly showpieces made of inferior metal that can easily break.)

The average Western sword was surely inferior in both strength and edge. One exception might be the Viking sword, which also used a probably inferior version of a layered core and hardened edges.

The katana sharpness legend dates primarily to the Tokugawa or Edo period of c. 1600 to 1868, a prolonged peacetime in which samurai had little to do except practice non-lethal martial arts and write essays romanticizing themselves.

Since blades weren’t battle-tested, professional tameshigiri, or “cutting tests,” became popular. The favorite test was to hack apart the corpse of an executed criminal—or sometimes a still-living one. It was common for a sword to slice a body in two through the hips with one stroke.

Tameshigiri were called sharpness tests, but really they demonstrated the strength of the blade (and the skill of the wielder—amateurs often fail to cut a reed mat with a katana). Katana have been known to cleave musket barrels, iron helmets, half-inch iron rods and steel elevator cable.

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