The steel for a Japanese sword is produced from the black, sand-like iron oxide (Fe2O3) called satetsu. To make steel out of satetsu, the oxygen has to be removed, and carbon introduced into the iron. This process is called smelting. In the traditional Japanese smelter, tatara, the iron is in relatively low temperature to produce the raw steel called tamahagane. Tamahagane can vary in terms of texture and colour because it depends on if the original iron has any impurities, which can vary depending on where the iron came from.
The smith carefully goes through the pieces of tamahagane sorting them according to their carbon content. He then hammers them to plates and breaks the plates to smaller pieces. From these pieces he collects a rectangular block about 7,5 to 12,5 cm a side, and weighing 2-3,5 kilos. (It should be noted that the finished blade weighs only about a half of this. Much material is lost in the making.) The smith wraps the block in rice paper to hold it 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 "woodgrain" look of the finished blade is the result of the folding. This patterning is called hadame. The pattern on the blade changes depending ion if the smith fold his steel sidewise or length wise or may be a bit 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 blades built from 5 segments (the center, 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 outer surface steel of the blade is normally folded thirteen to twenty times to produce a much higher carbon content. The core is normally folded ten times to produce a lower carbon content this centre is called shingane. If the edge is made from a separate piece, steel called hochogane (or hagane) is used. It is constructed from tamahagane this is old iron from pots and pans.), 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 is to limit the damage to short area, if the hardened edge begins to chip. One peculiarity of the yaki-ire is, that the blade bends because of the differential heating near the front and the back of the blade. The smith has to anticipate this bending, and initially make the blade less curved than intended for the finished blade. Often the curvature has to be adjusted after the hardening. The finished product is polished this is a great skill and requires years of training - then the blade is fitted with koshirae (the scabbard, handle, habaki, etc.)