Features of Japanese Swords
When describing the characteristics of Japanese swords, they are often expressed as "not breaking, not bending, and cutting well." However, this expression contains a contradiction. To say "not breaking" implies that the iron must be soft, as a soft material like rubber can bend without breaking but cannot be said to be hard. Conversely, to say "not bending" implies that the iron must be hard. This creates a contradiction: it suggests making a single sword out of iron that is both soft (flexible) and hard — which is logically impossible.
The way to resolve this contradiction is through the craftsmanship specific to Japanese swords. The method involves encasing relatively soft iron within a harder iron. Essentially, it is like wrapping sweet red bean paste (an in 卯紗, monaka) with a crust. This design means the outer layer (the exterior iron) is hard and resistant to bending, while the softer iron inside absorbs shocks and prevents breaking.
Note: When referring to "soft iron" here, it means iron with toughness (粘り, nebarī).
The outer iron is called "kawagane" (皮鉄, skin iron), and the inner iron is called "shingane" (芯鉄, core iron).
Furthermore, since the edge of the blade could chip if made too hard, it's often alloyed separately with toughness as "hagane" (刃鉄, cutting iron) and the nakagoya (ridge) part is separately forged as "mune gane" (棟鉄, ridge iron). These different types of iron are crafted for each part of the blade according to their functions and then assembled into a single sword.
While in China and other regions, early sword-making techniques involved varying the hardness between the blade and the tang (nakago) to prevent breakage, the practice of forging each part separately and then assembling them into one sword is unique to Japanese swords.
In traditional tatara steelmaking, high-quality steel called "tamahagane" (玉鋼, jewel steel), which contains a uniform amount of carbon suitable for swords, is produced from iron sand. By repeatedly folding and forging this steel, strength and toughness are enhanced. These materials are combined according to the needs of each part, and the finished blade is shaped with a curvature (sori) to allow cutting with less force.
However, an important point to remember is that the method of using tamahagane as the main material, wrapped in core steel (shingane), is a post-Meiji (modern) sword-making technique, and this tradition has been passed down to the present. The methods and materials of swords from the earlier Koto (antique sword) period are not yet fully understood, and the above method of forging is derived from the techniques developed after the Meiji period. It is unclear whether ancient swordsmiths used such processes, but during the Koto period, tamahagane was not available, so ancient swords did not incorporate tamahagane in this way.
Moreover, until the Heian period, some regions used iron ore as the raw material rather than iron sand. These early swords, made from less purified and unevenly carbonized materials, did not have the refined tamahagane of later periods. This impurity and variation could create areas of different hardness within the blade, producing a natural hamon (temper line) not seen in modern swords. Because of their toughness and flexibility, these swords could bend without breaking and were strengthened further through heat treatment.
Since ancient swords were made from relatively soft (tough) iron, there was no need for a core steel, and they were forged without inserting a core (maru forge). Some heavily worn old swords show what might appear to be core steel on the surface, but generally, the use of hard tamahagane for the core is a technique developed by swordsmiths in the modern (post-Koto) era to prevent breakage.
The Act on the Prevention of the Possession, etc., of Swords, firearms, and swords (commonly called the Sword and Gun Control Law) states that "a Japanese sword is a sword that has been tempered and folded using traditional manufacturing methods for martial or ornamental purposes." Swords made during wartime by repurposing leaf springs from vehicles or western iron sheet (such as certain military swords) do not conform to this definition and are not recognized as Japanese swords, nor are they issued registration certificates. No matter how high-quality western iron used or how well-made the weapon, if it does not meet the above criteria, it is simply a "knife" or "sword," not a Japanese sword.
Regarding the master swordsmith Masamine Sumitani, a Living National Treasure, he stated in his book Nihon Tō Shokunin Shokudan (Kōgeisha Publishing, 1971):
*"Especially when we make Japanese swords today,