What we commonly refer to as a Samurai sword, is an absolutely amazing piece of craftsmanship. For the time it was made, the sword smiths way of working with steel seems to be way ahead of their time in what we refer to as today's standards. Below is some more detailed information that is interesting to read and it explains how the swords were made and folded to have as many as 215 (approx. 65000) layers of steel, to sometimes up to 220 (approx. 1million) layers. This resulted in an exceptionally strong, light, aesthetically beautiful and durable blade
Historically katana (s) were one of the traditionally made Japanese swords (nihont) that were used by the samurai of feudal Japan. Japanese sword blades were often forged with different profiles, different blade thicknesses, and varying amounts of grind. (The grind of a blade refers to the shape of the cross-section of the blade).
The forging of a Japanese blade typically took many days or weeks, and was considered a sacred art, traditionally accompanied by a large panoply of Shinto religious rituals. As with many complex endeavors, rather than a single craftsman, several artists were involved. There was a smith to forge the rough shape, often a second smith (apprentice) to fold the metal, a specialist polisher, and even a specialist for the edge itself. Often, there were sheath, hilt, and tsuba specialists as well.
The steel bloom, or kera, that is produced in the tatara contains steel that varies greatly in carbon content, ranging from wrought iron to pig iron. Three types of steel are chosen for the blade; a very low carbon steel called hocho-tetsu is used for the core of the blade, called the shingane. The high carbon steel, called tamahagane, and the remelted pig iron, called nabe-gane, are combined to form the outer skin of the blade, called kawagane. Only about 1/3 of the kera produces steel that is suitable for sword production.
The best known part of the manufacturing process is the folding of the steel, where the swords are made by repeatedly heating, hammering and folding the metal. The process of folding metal to improve strength and remove impurities is frequently attributed to specific Japanese smiths in legend.
In traditional Japanese sword making, the low carbon hocho-tetsu is folded several times by itself, to purify it. This produces the soft metal, called shingane, to be used for the core of the blade. The high carbon tamahagane and the higher carbon nabe-gane are then forged in alternating layers. The nabe-gane is heated, quenched in water, and then broken into small pieces to help free it from slag. The tamahagane is then forged into a single plate, and the pieces of nabe-gane are piled on top, and the whole thing is forge welded into a single block, which is called the age-kitae process. The block is then elongated, cut, folded, and forge welded again. The steel can be folded transversely, (from front to back), or longitudinally, (from side to side). Often both folding directions are used to produce the desired grain pattern. This process, called the shita-kitae, is repeated from 8 to as many as 16 times. After 20 foldings, (220, or about a million individual layers), there is too much diffusion in the carbon content, the steel becomes almost homogeneous in this respect, and the act of folding no longer gives any benefit to the steel. Depending on the amount of carbon introduced, this process forms either the very hard steel for the edge, called hagane, or the slightly less hardenable spring steel, called kawagane, which is often used for the sides and the back.
During the last few foldings, the steel may be forged into several thin plates, stacked, and forge welded into a brick. The grain of the steel is carefully positioned between adjacent layers, with the exact configuration dependent on the part of the blade for which the steel will be used.
Between each heating and folding, the steel is coated in a mixture of clay, water and straw-ash to protect it from oxidation and carburization. This clay provides a highly reducing environment. At around 1,650F (900C), the heat and water from the clay promote the formation of a wustite layer, which is a type of iron oxide formed in the absence of oxygen. In this reducing environment, the silicon in the clay reacts with wustite to form fayalite and, at around 2,190F (1,200C), the fayalite will become a liquid. This liquid acts as a flux, attracting impurities, and pulls out the impurities as it is squeezed from between the layers. This leaves a very pure surface which, in turn, helps facilitate the forge-welding process. Due to the loss of impurities, slag, and iron in the form of sparks during the hammering, by the end of forging the steel may be reduced to as little as 1/10 of its initial weight. This practice became popular due to the use of highly impure metals, stemming from the low temperature yielded in the smelting at that time and place. The folding did several things:
- It provided alternating layers of differing hardenability. During quenching, the high carbon layers achieve greater hardness than the medium carbon layers. The hardness of the high carbon steels combine with the ductility of the low carbon steels to form the property of toughness.
- It eliminated any voids in the metal.
- It homogenized the metal, spreading the elements (such as carbon) evenly throughout - increasing the effective strength by decreasing the number of potential weak points.
- It burned off many impurities, helping to overcome the poor quality of the raw Japanese steel.
- It created up to 65000 layers, by continuously decarburizing the surface and bringing it into the blade's interior, which gives the swords their grain (for comparison see pattern welding).
Generally, swords were created with the grain of the blade (called hada) running down the blade like the grain on a plank of wood. Straight grains were called masame-hada, wood-like grain itame, wood-burl grain mokume, and concentric wavy grain (an uncommon feature seen almost exclusively in the Gassan school) ayasugi-hada. The difference between the first three grains is that of cutting a tree along the grain, at an angle, and perpendicular to its direction of growth (mokume-gane) respectively, the angle causing the "stretched" pattern. The blades that were considered the most robust, reliable, and of highest quality were those made in the Mino tradition, especially those of Magoroku Kanemoto. Bizen tradition, which specialized in mokume, and some schools of Yamato tradition were also considered strong warrior's weapons.[