The Japanese katana design is a samurai sword that maximizes cutting power. The blade is traditionally made from tamahagane steel, forged from iron sand and folded metal. The shape of the blade is a wedge-shaped semi-circular curve, and it has two differentially hardened sides, one convex and one concave. The sword is designed to be swung in a “pounding” motion, both to increase the speed of blade and the momentum of attacks. The pounding motion is common in Japanese martial arts such as kendo, which has many similar forms. So, how does the Japanese katana’s design optimize its cutting power?
The pointed curve
The traditional japanese katana is a chopping weapon rather than a thrusting one. However, it would not be very effective as a chopping weapon if its edge was straight. Because the Japanese sword’s edge is curved, the Japanese katana can be used as a chopper. However, these curved edges make them difficult to use for thrusting motions. When cutting an object with a curved edge, one must swing into the object in order to gain momentum while turning the blade; this requires extreme speed and skill.
Traditional Japanese katana’s blades have differential hardness on two sides. The two distinct sides of a Japanese katana create a powerful edge that can easily penetrate thick or hard objects such as armor and wood. Differential hardening techniques were used by blacksmiths up until the end of the Edo period, about 250 years ago; therefore, these techniques cannot be attributed to the katana’s creation.
The “hamon” or tempering pattern
The hamon or tempering pattern of the Japanese katana is a result from the differential hardening in the steel. This pattern is considered by many people to be one of the most important aspects of Japanese sword aesthetics. Although, the hamon does not increase the cutting power of a sword, it does make it visually pleasing to many sword collectors and enthusiasts.
The “tune” of the blade
The curve of the blade, described in point 1, can be adjusted to many different curvatures; therefore, it can be finely tuned to cut certain objects better. This tuning is done through the “tune” or tempering process. The amount of heat to which a metal is subjected during this process determines how much it will bend when put under extreme stress. Thus, sword blades can be tuned to have a more efficient cutting edge in specific sword fighting styles.
From the ambient vibration of the blade
It is known that sound waves traveling through steel disturb its crystalline structure. This can cause a sword to break, or bend when struck by incoming shock waves. For this reason, Japanese smiths often performed “tempering” by slightly bending the blade back and forth before it cools to create an optimum crystalline structure for toughness. The ability of a katana’s sword to maintain structural integrity is essential for being able to cut through objects without breaking or bending.