Polishing Techniques for Shinogiji and Hiraji: Whetstone Stages and Luster Control
* Image is for illustrative purposes only.Table of Contents
Two Faces of Japanese Sword Polishing—Shinogiji and Hiraji
When viewing the cross-section of a Japanese sword blade, multiple "surfaces" exist running from the edge (ha) toward the spine (mune). The two most important of these are "hiraji" (the flat surface area) and "shinogiji" (the ridge-side surface).
Hiraji refers to the broad flat plane between the edge-side surface where the hamon runs and the shinogisuj (ridge line). Most of the beauty of a Japanese sword resides in this hiraji, where the forge patterns (itame, masame, mokume, etc.) of the jigane (base metal) appear most clearly.
Shinogiji refers to the surface running from the shinogisuj to the mune, a sloped surface narrower than the hiraji. In swords with shinogizukuri (ridge-style construction), the way this surface is finished greatly affects the three-dimensional presence of the blade and its visual appeal when examined.
Polishers (togishi) polish these two surfaces using completely different approaches. The types of whetstones used, the pressure applied during polishing, and the final quality of luster aimed for—all differ. Understanding this difference is key to understanding the technical framework of Japanese sword polishing.
Stages of Polishing—From Rough Whetstones to Finishing Whetstones
Japanese sword polishing is a staged process that progresses sequentially from coarser whetstones to finer ones. Broadly speaking, it is divided into two major phases: "foundational polishing (shitaji-togi)" and "finishing polishing (shiage-togi)." The rough polishing stage performed by the swordsmith himself is explained in final inspection and the swordsmith's quality judgment, but the process after the sword passes into the polisher's hands proceeds as follows.
Stone Stages in Foundational Polishing
- Arato (rough whetstone) (#80–#240 equivalent) is the coarse whetstone used in the initial polishing, shaping the blade from its rough-finished state as received from the swordsmith. This is an important stage where the sword's "appearance (nari)" is finalized, as it corrects the height, position, and thickness of the shinogi according to design specifications.
- Bisuito (water-stone) (#400–#600 equivalent) is used after the blade's shape is refined, to work surface scratches down to a medium finish. It is at this stage that the work of clarifying the ridge line of the boundary (shinogisuj) between hiraji and shinogiji is performed.
- Nakanagura whetstone (#800–#1200 equivalent) is used at a stage close to the final phase of foundational polishing. It imparts a uniform polish finish to the entire blade and prepares for the transition to the finishing whetstone.
Stone Stages in Finishing Polishing
- Uchigumori whetstone is a specialized finishing whetstone used in Japanese sword polishing, with natural stones from Kyoto being particularly prized. A hiraji polished with this whetstone takes on a surface with a distinctive cloudy finish, as if shrouded in mist (kasumi). This "uchigumori finish" is the essence of the aesthetics of Japanese sword polishing, bringing forth the forged patterns of the jigane in their most beautiful appearance.
- Hatorito (blade-edge whetstone) is a tool specialized for polishing the edge of the hamon. It employs a tool called "hatori" (blade-edge polisher), where the whetstone is attached in small, brush-like strips. It is delicate work that clarifies the contour of the hamon and brings out the nie and nioi.
- Nuguigishi (wipe/buff polish) is a fine abrasive used in the final stage of polishing, with the effect of further clarifying the forged texture (kitaeme) of the jigane.
Differential Polishing of Shinogiji and Hiraji—Why the Same Whetstone Cannot Be Used
The primary reason why different approaches are necessary for hiraji and shinogiji is that the finish quality aimed for is fundamentally different.
| Surface | Target Finish | Characteristics |
|---|---|---|
| Hiraji polishing | Cloudiness (kumoriage) | A hiraji polished with an uchigumori whetstone does not possess a glossy, mirror-like luster. Rather, a "cloudy finish" that softly diffuses light is considered beautiful. Within this cloudiness, the itame, masame, and mokume of the jigane emerge into view, and the swordsmith's forging technique reaches the eye of the viewer. A cloudy finish is not a defect but an expression of "beauty" intentionally created by Japanese sword polishing. |
| Shinogiji polishing | High luster (mirror-like finish) | Though called mirror polishing, it is not a perfect mirror finish like modern stainless steel polishing, but rather a luster with "sufficient brightness to see through the layers of jigane." When this luster is contrasted with the cloudiness of the hiraji, the line of the shinogisuj stands out visually, and depth and three-dimensionality are born in the blade. |
This contrast of "cloudy hiraji and lustrous shinogiji" is the aesthetic core of Japanese sword polishing, embodying an aesthetic fundamentally different from Western sword polishing (which polishes the entire surface uniformly).
The Rarity of Whetstones and Contemporary Challenges
The quality of Japanese sword polishing depends directly on the quality of whetstones, but the finest natural whetstones—particularly rough uchigumori whetstones—are rapidly becoming depleted. Kyoto, the Kameyama region, and mountainous areas of Tochigi Prefecture were the primary sources, but many quarries have already closed or become unprofitable, making it extremely difficult to obtain high-quality natural whetstones.
Among polishers, there is a widespread understanding that "quality natural whetstones are inherited from one's master," and many craftspeople carefully maintain and continue using a single piece of fine whetstone for decades. There exists a market where a single whetstone trades for hundreds of thousands of yen or more, and the rarity of these materials supports the transmission of polishing techniques among togishi.
On the other hand, the quality of synthetic whetstones has improved significantly in recent years, and some are now appearing that perform no worse than natural whetstones in the foundational polishing stage. However, in the final stage of finishing polishing—particularly in the finishing of hiraji using uchigumori whetstones—it is currently considered difficult to completely reproduce the unique "polish finish" created by the complex mineral composition of natural stone using artificial whetstones.
The Transmission of Polishing Technique—The Polisher Master-Apprentice System
Mastering the differential polishing of shinogiji and hiraji requires years of master-apprentice training. The cultivation of polishers also follows an apprenticeship system similar to that for swordsmiths, with at least 8–10 years typically required from apprenticeship to independence.
Developing the "eye" for polishing is said to be the greatest challenge. How much pressure to apply with the whetstone, what angle to use, how to read the surface condition as indicated by the light—these are difficult to convey through words and must be learned through years of watching one's master's hands.
In modern times, the number of polishers is also declining, and just as there is a shortage of swordsmiths, a shortage of polishers has become a major issue in the transmission of Japanese sword culture. The fact that the existence of first-rate polishers capable of realizing the beautiful contrast between shinogiji and hiraji determines the "completion" of a Japanese sword demonstrates that polishers are craftspeople equally important to swordsmiths.
