Polishing, Jitekko, nugui
Polishing, adjustments of the final appearance of the structure and steel particles of Japanese sword blades.
The polishing of Japanese sword blades is divided into two parts. The first part is SHITAJI shaping, the second part is SHIAGE polishing.
The main tool for polishing is grinding stones. Natural or synthetic stones are used. At present, there are synthetic stones replacing all grades for shaping and polishing, except for the final stone UCHIGUMORI and the finger stones HAZUYA chipped from it.
Stones for shaping are ARATO (approx. 180), BINSUI (approx. 220), KAISEI (800-1200),
Stones for polishing are CHU-NAGURA (2000) KOMA-NAGURA (5000) AND UCHIGUMORI (12000)
Also used are the finger stones HAZUYA , which are thin slices chipped from the UCHIGUMORI stone, and JIZUYA, which is chipped from the NARUTAKI stone. JIZUYA is now also available in a synthetic version.
The grinding stone is mounted on a pad and the blade held in the hands is worked by pressure and strokes on it. This requires practice. The side facing the stone is worked, so that the grinding surface is not visible to the grinder as he works.
We start on the Arato stone. It's a stone with a coarse, sharp grain. The purpose of working on arato is to create the most perfect SUGATA, to precisely shape the blade, facets and sharpen the edge. After hardening and shape corrections, the surface of the blade is uneven in places and may be slightly wavy. The blade is blunt when hardened, with a blade width of 2mm. These defects need to be corrected. Work is always done in the order of the spine area, the shinogi-ji area, the blade and ji and shaping the shinogi rib, and finally shaping the tip and yokote edge. The Arato stone is worked slightly oblique at the shinogi-ji surface, perpendicular at the ji and blade. When the work on the Arato is completed, the shape of the blade is clearly defined, the facets are sharp, including the edges forming the tip.The areas forming the spine and the areas above the shinogi are flat. The areas between the shinogi rib and the blade are ground into the shape of a lens. The lens-shaped cut also forms the tip surface. Arato also reveals the difference between the softer, lighter JI surface and the hard, darker, cloudy part of the edge , the HAMON line. The subtle nuances in the hamon line, ashi, nie and other metallurgical effects are usually not very obvious at this stage. The UTSURI line is rarely seen unless it is very pronounced indeed. The structure of HADA steel is not obvious in well worked steel.
This is followed by work on the binsui stone. Here the purpose is to refine the facets and shape. It is worked in the same way as the ARATO stone. The metallurgy is a little more pronounced than on ARATO. The HADA is not distinct.
Working on the KAISEI stone is the last step in shaping the blade. Again, the purpose is to fine-tune the SUGAT. It is worked at an angle of about 45°. The poiuze surface of the BOSHI tip is worked perpendicular to the axis of the blade. When the KAISEI is finished, the shape of the blade must be perfect, the facets and cutting edge perfectly sharpened and the tip perfectly defined, with sharp koshinogi and yokote lines. It is at this stage that the finer metallurgical effects begin to emerge. Niou, ashi, kinsuji, sunagashi and nie. Hada is still not distinct.
Next is the work on the chu nagura stone. It is the first step of the polishing phase. The stones used for the shiage polishing stage do not have a great material removal capability like the stones used in the shitaji shaping stage. However, they have the ability to smooth the surface of the steel and reveal the structure of the overlay and the metallurgy of the steel.
Grinding on the Nagura stone is done by parallel strokes with the axis of the blade. The exception is the boshi tip area. This is still ground perpendicular to the axis. The chu nagura stone begins to reveal the structure of the overlay. The koma nagura stone is similar, only finer in grain. The work on both nagura stones is the same. Komanagura, however, begins to form a shiny mirror-like surface. Usually, after the polishing is complete on komanagura, we can clearly see the texture of the overlay. The glossy surface will somewhat reduce the contrast between the soft and hard parts of the blade, thus negating the hamon and metallurgical effects. However, it is possible to see them clearly when illuminated at an angle.
Uchigumori is the only stone for which there is no synthetic variant. Working on this stone unifies the surface, and reveals the steel particles, the structure of the overlay, and most of the metallurgical effects in steel. Uchigumori is the last stone to be worked on by blade pressure. Next are the finger stones, which in turn are pressed against the blade with the thumb and are ground by a parallel motion with the axis of the blade.
The polishing of Japanese sword blades consists of three parts. The first is the shaping of the blade on arato, binsui, kaisei stones. The purpose of this phase is to achieve perfect shape, sharp facets and the correct shape of the surfaces below and above the shinogi rib, spine and tip. The first three stones are distinguished by their ability to take more steel from the blade being worked.
The second stage is polishing the blade on chu-nagura, koma-nagura, uchigumori and hazuya and jizuya finger stones. These stones already have little ability to remove larger amounts of steel, but they do soften the surface of the steel, gradually revealing its folded structure and the crystalline structures formed during the hardening process.
The hazuya finger stones are thin segments of the uchigumori stone. They may be paper-backed (washi) By applying thumb pressure and strokes parallel to the longitudinal axis of the blade, they unify the quality of the polished surface. The choice of the appropriate stone type whitens the hamon line and brings out the texture of the steel. After the use of the hazuya stone, the surface of the steel is milky.
The jizuya finger stone removes the milky discolouration of the steel, brightens the steel and significantly highlights the individual particles of the steel. It has the ability to "open up" the structure of the steel being reloaded. As with all polishing stones, the second stage of polishing depends on the characteristics of the steel of the particular blade. Sometimes it is enough to work with finger stones only briefly and the structure of the steel is clearly visible. Sometimes highlighting hada (the structure of the overlay) and hataraki (the metallurgical activity) is a matter of several hours of work.
The final step to finish the steel surface is to use jitekko or nugui paste. These pastes are made up of oil and iron oxides. Some nugui are composed of multiple items. Polishers also mix their own nugui and often keep their recipes secret. For example, you can buy a jitekko and three types of nugui at a polishing store. Jitekko brings out the subtle texture of the overlay and steel particles. It does not significantly affect the colour of the steel. It is used for traditional sashikomi style polishing. It is said to show the true beauty and quality of the steel. The result of working with nugui is not as challenging as after using nugui. The contrasts of the metallurgical effects and the snake are more subtle. When jitekko is applied for a long time, the texture of the steel being reloaded is "hollowed out", creating an aged hada effect. It is up to the taste of each polisher how distinctive the hada is. In any case, the application of jitekko can change the resulting face of the steel very significantly. To increase the effect of working with jitekko, the blade is heated before its application by using a cotton swab with which to spread the paste under pressure in simultaneous strokes. It is heated by pouring hot water over it.
The basic is Congo (iron oxide and oil). When applying them, the result is different. Some more accentuate the hada (joints between steel particles, layers), some more the color differences of steel particles depending on the carbon content (light, dark). Again, very much depends on the type of nugui and length of application. And of course the quality and type of steel. For swords with a hada formed by combining steels (Soshu, Gassan) the result of highlighting the color differences of the steel particles is of course faster and more pronounced than for blades with a single type, semi-fine structure.
Of course, the preparation of the steel by the stones before the actual application of the paste is also essential for the final form of the blade. However, the method and length of application of the paste has the ability to radically affect the final form of the blade. A blade which, after a short application of paste, exhibits a hada like nashi ji, or ko itame, may, after repeated and longer applications, emerge as mokume, o-mokume, itame, etc. I usually use a mix of jitekko and kongo paste, and then another nugui from the Namikawa heibei store to bring out the color particles. Longer application can affect the contrast of the utsuri line. This sometimes only comes out significantly after longer and repeated applications of the paste. This also applies to chikei effects.
In the photos I tried to capture the changes after repeated application of the paste. (mix nugui, jitekko) Each application lasted about 5min. Before application, the blade is after using hazuya stone (without jizuya). The hada is closed, not very clear. The surface of the steel is milky colored. Each successive application more accentuates the structure of the folding. In the last application, the color differences of the individual steel particles and some chikei also stand out.