Japanese sword Forging techniques part 2
together and covers in clay and burned rice straws. The block is heated and hammered to fuse it together.
The fused piece of steel is heated, folded and hammered repeatedly to drive out all impurities and air bubbles that might compromise the strength of the finished blade. The folding also results in the typical "woodgrain" look of the finished blade. This patterning is called hadame. The pattern depends on whether the steel has been folded lengthwise or breadthwise, or combination of both. A craft full smith often combines steel from different sources to produce even more pronounced effect. The terms often used when talking about the steel of the blade are jihada meaning the patterning of the steel and jitetsu the quality and texture of the steel.
There are different schools on how the blade is constructed. Some smiths do it from two pieces, the surface and the core. This construction is called makuritae or kobuse. It is also possible to make the blade from four (the core, sides, and the cutting edge) pieces. This construction is called honsanmai. There are even blades constructed from five pieces (the core, the sides, the edge and the back of the blade). This is called shihozume. It takes great skill from the smith to seamlessly fuse these pieces together. Any opening or crack between the pieces would result in inferior and weak blade. This work is called tsukurikomi.
The steel intended for the hard outer surface of the blade, kawagane is made of steel of high carbon content and is commonly folded 13-20 times. The steel intended for the core shingane has much lower carbon content and is folded about 10 times. If the edge is made from a separate piece, steel called hochogane (or hagane) is used. It is made from tamahagane and zukuoroshigane (old iron from pots etc.), and folded around 18 times. The steel for the back of the sword (mune) is called munegane and is quite hard.
When the different pieces of the blade have been worked together, the smith works the metal into the shape of the blade. First the blade is given it's general shape, and then in sequence kissaki (the point), monouchi (the part of blade used for cutting) and nakago (the tang). The shape is finished with a series of planes and files.
The truly critical phase of making a blade is the hardening, yaki-ire. The blade is covered with a paste made out of clay, charcoal powder and pulverized sandstone. The paste is applied thicker near the back of the blade than on the cutting edge. Thus the cutting edge will be hardened much harder than the rest of the blade. The pattern between the thick and thin layers of paste will produce the hamon, the wavy hardening pattern on the blade. Often the smith makes thin stripes of thick paste all the way to the cutting edge. These produce series of thin sectors of soft steel called ashi. Their function