Hitachi Metals, Ltd.

To create a Japanese sword, you start with tama-hagane as the source material and run it through the lapel forging process (shita-kitae). This involves pounding the tama-hagane with a hammer into a flat shape (the mizu-herashi process). The pieces are then piled up, forge welded, and formed into a single block. Next, it is folded lengthwise and forged, and also elongated, cut, lapeled, and then forged again (the shita-kitae process). The iron slag inside is squeezed out during this process, while the remaining slag is broken up into small pieces and spread out. Simultaneously, organic impurities such as phosphorous and sulfur are also extracted.

Next is the age-kitae (side steel-forging) stage.
The tama-hagane, the re-melted pig iron, and the re-melted hocho-iron are folded lengthwise and forged, and also elongated, cut, lapeled, and then forged again to create masses in which the three materials are stacked in grouped and parallel layers (tansatsu-kitae), or in which they are stacked up in alternating and parallel layers (kashigi-kitae), or in which they are stacked in groups and at right angles to each other (konoha-kitae). The metal will be folded dozens of times to go with the shita-kitae process. This produces the side steel, into which the shin-gane(body steel, made up of hocho iron) will be fitted. There are numerous ways of going about this fitting in process as well.

Next, the blacksmith performs the drawing-out process and adjusts the shape. Quenching soil is then applied to the blade based on the kind of temper pattern (hamon) desired, and the blade is heat treated. If the blade is insufficiently curved, the blacksmith will work it to give it more of a curve. The blade is then given a rough polish, the tang (nakago) is forged, the temper pattern and other elements are checked, the registration (signature) is engraved, and the sword is passed on to the polisher. All of the foregoing processes constitutes one cycle of the manufacturing process.

When you consider this process, you can understand just how extremely important it is for the iron to be receptive to forge welding. Because Japanese steel includes almost no chemical elements other than iron and carbon, it can be forge welded without the use of flux, which is not the case with steel as traditionally made in the West.

The forge-welded sections produced through shita-kitae and age-kitae form complex patterns and produce a variety of changes in the unprocessed metal. The tsukuri-komi process involves treating the Nihonto as a composite material. It is done to bring out well the features of the sword, such as its resistance to breakage and bending.

When using Japanese steel the limit lines on those areas to which the heat has reached or not reached certain sections is apparent when the steel goes through heat treating. The temper pattern produced through the adjustment process and by the nioi at those limit areas can be easily and cleanly produced. The minute and soft inclusions scattered throughout the unprocessed metal make it easy to polish, and it is said that it glides across the whetstone extremely well when it is polished. Beyond this, the extremely small sectional differences in the quantity of carbon present produce changes in the pattern, described through such evocative terms as kin-suji (lit., “gold stripes”), inazuma (“lightning”) and jikei (“landscape”) and heighten the sword's beauty as a work of art.