Construction of a Katana

The forging of a Japanese blade typically took hours or days, and was considered a sacred art. 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 most famous part of the manufacturing process was the folding of the steel. Japanese swords and other edged weapons are manufactured by the Chinese method of repeatedly heating, folding and hammering the metal. 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 eliminated any bubbles 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 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 ); however, the belief that the layered structure provides enhanced mechanical properties of the steel is false, as layers act as weld points which can only serve to weaken the integrity of the blade.

Contrary to popular belief, continued folding will not create a "super-strong" blade; once impurities are burnt off and the carbon content homogenized, further folding offers little benefit and will gradually burn out the carbon, leading eventually to a softer steel less able to hold an edge. More than about a dozen folds are uncommon and swords never have more than two dozen folds. A blade folded 12 times will have more than 4,000 'layers' underneath the initial blade to begin with. Even before this point, more layers does not equal a better sword; an even and clean composition is obtained early in the folding process, and control of carbon content has a much greater effect on the blade's functionality. Thus, the best results were usually obtained at 8-10 folds.

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.

One of the core philosophies of the Japanese sword is that it has a single edge. This means that the rear of the sword can be used to reinforce the edge, and the Japanese took full advantage of this. When finished, the steel is not quenched or tempered in the conventional European fashion. Steel's exact flex and strength vary dramatically with heat variation. If steel cools quickly, from a hot temperature, it becomes martensite , which is very hard but brittle. Slower, from a lower temperature, and it becomes pearlite , which has significantly more flex but does not hold an edge. To control the cooling, the sword is heated and painted with layers of sticky clay. A thin layer on the edge of the sword ensures quick cooling for a hard edge, with a thicker layer of mud on the rest of the blade causing slower cooling and softer, more flexible steel to give the blade the required flex. When the application is finished, the sword is quenched and hardens correctly. This process also makes the edge of the blade contract less than the back when cooling down, something that aids the smith in establishing the curvature of the blade.

Eventually the Japanese began to experiment with using different types of steel in different parts of the sword.

The vast majority of good katana and wakazashi are of wariba-gitae type. The makuri-gitae is made using two steels, one folded more times than the other, or of a lesser carbon content. When both sections have been folded adequately, they are bent into a 'U' shape and the softer piece is inserted into the harder piece, at which point they are hammered out into a long blade shape. By the end of the process, the two pieces of steel are fused together, but retain their differences in hardness.

More complex models allow for parrying without fear of damaging the side of the blade. To make han-sanmai-awase-gitae or shiho-zume-gitae, pieces of hard steel are then added to the outside of the blade in a similar fashion. The last generally accepted model, the shiho-zume-gitae , is quite rare, but added a rear support.

Anatomy of the katana

Each blade has a unique profile, mostly dependent on the smith and the construction method. The most prominent is the middle ridge, or shinogi . In the earlier picture, the examples were flat to the shinogi, then tapering to the blade. However, swords could narrow down to the shinogi, then narrow further to the blade, or even expand outward towards the shinogi then shrink to the blade (producing a trapezoidal shape). A flat or narrowing shinogi is called shinogi-hikushi , whereas a fat blade is called a shinogi-takushi .

The shinogi can be placed near the back of the blade for a longer, sharper, more fragile tip or a more moderate shinogi near the center of the blade.

The sword also has an exact tip shape, which is considered an extremely important characteristic: the tip can be long ( ōkissaki ), medium ( chūkissaki ), short ( kokissaki ), or even hooked backwards ( ikuri-ōkissaki ). In addition, whether the front edge of the tip is more curved ( fukura-tsuku ) or (relatively) straight ( fukura-kareru ) is also important.

The kissaki (point) is not a "chisel-like" point, nor is the Western knife interpretation of a "tanto point" found on true Japanese swords; a straight, linearly-sloped point has the advantage of being easy to grind, but it bears only a superficial similarity to traditional Japanese kissaki. Kissaki have a curved profile, and smooth three-dimensional curvature across their surface towards the edge - though they are bounded by a straight line called the yokote and have crisp definition at all their edges.

A hole is drilled into the tang ( nakago ), called a mekugi-ana . It is used to anchor the blade using a mekugi , a small bamboo pin that is inserted into another cavity in the tsuka and through the mekugi-ana, thus restricting the blade from slipping out. To remove the tsuka one removes the mekugi. The swordsmith's signature ( mei ) is placed on the nakago.

Decoration

Almost all blades are decorated, although not all blades are decorated on the visible part of the blade. Once the blade is cool, and the mud is scraped off, the blade has designs and grooves cut into it. One of the most important markings on the sword is performed here: the file markings. These are cut into the tang, or the hilt-section of the blade, where they will be covered by a hilt later. The tang is never supposed to be cleaned: doing this can cut the value of the sword in half or more. The purpose is to show how well the blade steel ages. A number of different types of file markings are used, including horizontal, slanted, and checked, known as ichi-monji , kosuji-chigai , suji-chigai, o-suji-chigai , katte-agari , shinogi-kiri-suji-chigai , taka-no-ha , and gyaku-taka-no-ha . A grid of marks, from raking the file diagonally both ways across the tang, is called higaki , whereas specialized "full dress" file marks are called kesho-yasuri . Lastly, if the blade is very old, it may have been shaved instead of filed. This is called sensuki . While ornamental, these file marks also serve the purpose of providing an uneven surface which bites well into the tsuka , or the hilt which fits over it and is made from wood. It is this pressure fit for the most part that holds the tsuka in place during the strike, while the mekugi pin serves as a secondary method and a safety.

Some other marks on the blade are aesthetic: signatures and dedications written in kanji and engravings depicting gods, dragons, or other acceptable beings, called horimono . Some are more practical, grooves for lightening and extra flex (as well as an intimidating sound, called tachikaze , when swung with force). Grooves come in wide ( bo-hi ), twin narrow ( futasuji-hi ), twin wide and narrow ( bo-hi ni tsure-hi ), short ( koshi-hi ), twin short ( gomabushi ), twin long with joined tips ( shobu-hi ), twin long with irregular breaks ( kuichigai-hi ), and halberd-style ( naginata-hi ).

The presence of a so-called "blood groove" or fuller does not in actuality allow blood to flow more freely from cuts made with the sword. It leads to no demonstrable difference in ease withdrawing a blade nor reduce the sucking sound that many people believe was the reason for including such a feature in commando knives in World War II. The grooves are analogous in structure to an I beam , lessening the weight of the sword yet keeping structural integrity and strength. Nowadays, they may be included for decorative purposes or as advertising gimmicks rather than for structural reasons.

Furthermore the grooves (always done on both sides of the blade) make a whistling sound when the sword is swung (the tachikaze ). If the swordsman hears one whistle when swinging a grooved katana then that means that just one groove is making the whistle. Two whistles means that both the edge of the blade and a groove are making a whistle, and three whistles together (the blade edge and both grooves) would tell the swordsman that his blade is perfectly angled with the direction of the cut.

Because of the distinctive whistling sound of the grooves, it is said that ninja did not carry swords with grooves (at least while going on stealth missions), because the distinctive whistling sound that comes with them would betray the ninja's presence.

Polishing

When the rough blade is completed, the swordsmith turns the blade over to a polisher called a togishi , whose job it is to polish the steel of the blade to a glittering shine and sharpen the edge for battle. This takes hours for every inch of blade, and is painstaking work with different kinds of very fine stone. Early polishers used three types of stone, whereas a modern polisher generally uses seven. The modern high level of polish was not normally done before around 1600. It almost always takes longer than actually crafting the blade does, and a good polishing makes a blade look better, while a bad polishing makes the best of blades look like guntō . More importantly, an unschooled polisher can permanently ruin the blade geometry or wear the steel down to its core steel, both of which effectively destroy the sword's monetary, historic, artistic, and functional value.

There are two general styles of polishing: hadori and sashikomi . Hadori is modern (invented in the last 100 years) and involves a whitened hamon roughly following the actual hamon , and a darkened body to make the whitened hamon stand out. Hadori is likened to "makeup" that enhances the beauty of a face. Sashikomi is closer to the old styles of polish, and does not attempt to outline the hamon. It more readily reveals details of the actual hamon and crystalline structure compared to hadori.

One of the ways in which blades can be judged is by what this polishing reveals: the crystalline structure of the blade becomes quite visible, and the hamon (incorrectly known in English as the temper line, where the hard edge fades into the softer steel of the body) shows the unique nature of the sword. Each blade is distinct in its hamon and the grain ( hada ) of its steel. The hamon , which is determined primarily by how the clay is applied, is often used as a kind of signature of the smith, above and beyond his own signature, and each tradition of swordsmiths often has a particular style of hamon it prefers over all others. Hamon vary from straight to wavy to shaped like crabs or zigzags, and in their wandering they reveal important facts about the blade itself. A good polishing reveals what speed the edge was cooled at, from what temperature, and what the carbon content of the steel is. It does this by displaying either predominantly nioi , which is a mix of extremely fine martensite with troostite (another type of tempered steel), or the larger martensite crystals called nie , which look like individual dot-like mirrors.