From the Chinese Dadao and Liuyedao to the Nepalese Kukri, Indian Talwar, Persian Shamshir, Turkish Kiliij, and Greek Kopis, curved swords are the most elegant (and deadliest) bladed weapons of antiquity. However, no other sword is as culturally rich and mystically aesthetic as the Japanese katana.
For centuries, the katana was the subject of global admiration, with its elegant curve complementing the many fine details put into each sword. It still is.
But why are katanas curved? Did traditional Japanese swordsmiths envision the katana to have this distinct appearance? Does it offer any advantages over a straight-edged sword?
Join us as we unravel the secrets behind the katana curvature and answer other riddles you might have about this legendary Japanese sword.
The Real Reason Behind the Katana’s Curvature
Many believe traditional Heian Period swordsmiths deliberately forged Japanese swords in a slightly curved form. They suggest the change in sword-making philosophy reflected the changing feudal war landscape, where more Samurai warriors went to battle on horseback.
While this might be true, the “curving” of the sword’s blade was not purposeful or intentional. One can say it’s the “unexpected” result of a revolutionary method for producing stronger, faster, and more precise swords.
You see, the pre-katana Japanese sword was the Chokuto, which had a straight-edged blade. Early Japanese warriors used the Chokuto between the 6th and 8th centuries, mainly in ancient Japan’s Tohoku Region. There was also the Tsurugi, a double-edged straight-edged sword similar to most Western bladed weapons.
The first katana
By 700 AD, the Katana-kaji Amakuni utilized a unique “folded steel forging” technique borrowed from China, producing Japan’s first single-edged, curved sword – the Tachi.
Over decades, the Tachi came in standard-length and short versions, with the latter called Uchigatana. While the Japanese called the short Tachi Uchigatana, the rest of the world called it “katana.”
Curving the katana
Before the Tachi, Japanese swords had a straight edge. Amakuni’s blade forging technique created a curved sword, albeit inadvertently. How?
The Tachi or katana had a straight edge, but only up to a certain point in the sword-making process.
Katana swordsmiths cover the blade’s length with special clay during the tempering process, aptly called “clay tempering.” The blade’s cutting edge (Ha) has a thinner clay coating than the Mune or spine. Heating and quenching the clay-covered blade results in a temperature differential.
The thinly-clay-covered Ha cools quicker than the thickly-clay-coated Mune, allowing the former to harden (due to martensite crystal formation. Meanwhile, the Mune remains “flexible and soft” (secondary to ferrite and pearlite formation.
Katana swordsmiths perform this heating-quenching-hammering cycle several hundred times. The resulting contraction and expansion causes the katana to bend on its backside, forming the now-iconic Katana curvature (Sori).
So, why are katanas curved? These legendary Japanese swords have the characteristic curve because of temperature differentials during the multi-folding forging process, specifically clay tempering.
Benefits of a Curved Katana
We know that Amakuni and succeeding generations of katana-smiths did not intend for the sword to bend or curve. But it did anyway. So, are there any curved katana benefits we don’t know about?
Some say a straight-edged sword is as easy to unsheathe as a curved blade, often depending on the wielder’s sword-handling skills. Too bad we no longer have medieval Samurai who can enlighten us with their first-hand knowledge about this issue.
Nonetheless, master Iaidoka (Iaido practitioners) attest to the katana’s remarkable drawing or unsheathing ease.
Drawing a curved katana from its Saya follows the natural arc-like trajectory of the human arm. It facilitates a swift and fluid motion, eliminating unnecessary movements and ensuring a quicker strike against a foe.
The Iaido martial art teaches students the ways of the legendary Samurai sword, including unsheathing and cutting a target in a single fluid motion.
Legend says a highly skilled Samurai can defeat an enemy with one lighting-quick draw the opponent won’t know he’s dead.
Although the katana curve makes it possible to cut a target swiftly and precisely, experts say the katana’s cutting prowess is more a function of its Bohi or
This indentation below the Mune allows the katana to shave off some of its weight without undermining its durability and cutting abilities. It also delivers an exceptional Tachi Kaze, that distinct whirlwind sound when we swing the katana. The sonic feedback allows users to determine the katana’s angle, enabling them to make more precise cuts.
While Bohi influences the katana’s cutting prowess, the blade’s curvature also matters. The curved blade cuts targets, not pierces or bites them. It doesn’t get stuck in an opponent’s flesh or bone, allowing the Samurai to swing the katana for another strike.
As mentioned, the Bohi in the curved katana blade makes the sword 20 to 30 times lighter than conventional Bohi-less Japanese swords. The curvature also shifts the blade’s center of balance a bit more forward, enhancing sword control and precision.
A Word about the Sori
Although the katana has a curved blade, its curvature (Sori) varies across swordsmiths.
Controlling how the steel expands and contracts to give the katana its characteristic curvature is not an exact science, especially among traditional Japanese swordsmiths. They often rely on decades of training and actual katana-making to achieve the iconic katana curve.
However, most swords have a katana curvature of 1.5 centimeters or about 0.6 inches, drawn from an imaginary line between the Habaki (collar) and the Kissaki (tip).
Interestingly, the katana’s predecessor, the Tachi, has a 2.7-centimeter (about 1.06 inches) Sori, making it more curved than the katana.
It’s not surprising, given the Tachi blade’s length spanning 70 to 80 centimeters or about 27-9/16 to 31.5 inches. On the other hand, the katana blade only measures 60 to 80 centimeters or 23.62 to 31.5 inches.
The Bottom Line
Why are katanas curved? The katana curvature isn’t a purposeful attribute but a consequence of a unique forging process that bends the blade a few centimeters from the spine. It gives the katana one of its most iconic attributes, while allowing wielders to enhance their sword-fighting abilities. Learn more about katanas here.