Polishing Techniques for Shinogiji and Hiraji: Whetstone Stages and Luster Control

The Two Faces of Japanese Sword Polishing—Shinogi-ji and Hira-ji

When examining the cross-section of a Japanese sword blade, multiple "planes" exist from the edge to the spine. The two most important of these are the "hira-ji" and the "shinogi-ji."

Hira-ji refers to the broad flat surface between the plane where the hamon runs on the edge side and the shinogi-suji (ridge line). The majority of a Japanese sword's beauty resides in this hira-ji, where the forging patterns of the jigane (base steel)—including itame, masame, and mokume—appear most clearly.

Shinogi-ji refers to the plane from the shinogi-suji to the spine, a narrower sloped surface compared to hira-ji. In shinogi-zukuri swords, how this surface is finished greatly influences the three-dimensionality of the blade and its visual appeal.

The togishi (polisher) polishes these two planes using completely different approaches. The type of whetstone used, the pressure applied during polishing, and the quality of the final luster—all differ. Understanding this distinction is key to understanding the technical system of Japanese sword polishing.

Stages of Polishing—From Ara-to to Final Finishing

Japanese sword polishing is a staged process that progresses sequentially from coarse to fine whetstones. It is broadly divided into two major phases: "shitaji-togi" (undercoat polishing) and "shiage-togi" (finishing polishing).

Whetstone Stages of Undercoat Polishing

Ara-to (equivalent to #80–#240) is the coarse whetstone used in initial polishing to shape the rough-finished blade received from the swordsmith. The height, position, and thickness of the shinogi and edge are corrected according to specification in this process, which is a critical stage where the sword's "form" is finalized.

Bisuito (equivalent to #400–#600) is used after the blade shape is refined to reduce surface scratches to a moderate fineness. The work of clarifying the ridge line between hira-ji and shinogi-ji (the shinogi-suji) also occurs at this stage.

Nakanagura whetstone (equivalent to #800–#1200) is used near the finishing stage of undercoat polishing. It applies uniform polish marks across the entire blade and prepares the transition to finishing whetstones.

Whetstone Stages of Finishing Polishing

Uchigumori whetstone is a specialized finishing stone in Japanese sword polishing, with naturally occurring Kyoto stone being highly prized. The hira-ji polished with this whetstone develops a distinctive hazy surface, like mist. This "kumori skin" is the core of Japanese sword polishing aesthetics, bringing out the forging patterns of the jigane most beautifully.

Hatoru-to (edge-taking whetstone) is a tool specialized for polishing the edge of the hamon, using a tool called "hatoru" (edge-taker) made by attaching whetstone in a small brush-like form. This delicate work clarifies the contour of the hamon and accentuates the nie and nioi.

Nugui is a fine polishing agent used in the final stage of polishing, with the effect of making the forging marks (kitae-me) of the steel surface even clearer.

Differentiating the Polishing of Shinogi-ji and Hira-ji—Why They Cannot Be Polished with the Same Whetstone

The primary reason different approaches are necessary for hira-ji and shinogi-ji is that "the intended finish quality is fundamentally different."

In hira-ji polishing, deliberately creating "kumori" (a hazy finish) is essential. Hira-ji polished with uchigumori whetstone does not have a shiny mirror-like luster. Rather, the "kumori skin" that softly diffuses light is considered beautiful. Within this haze, the itame, masame, and mokume of the jigane emerge, allowing the swordsmith's forging technique to reach the viewer's eye. The kumori skin is not a flaw but an expression of "beauty" intentionally created by Japanese sword polishing.

In shinogi-ji polishing, conversely, producing a high luster (finishing close to a mirror polish) is traditionally required. However, even when referred to as mirror polish, it is not a perfect mirror finish like modern stainless steel polishing, but rather a luster with "brightness that allows the layers of jigane to show through." This luster contrasts with the haze of the hira-ji, making the shinogi line visually prominent and creating depth and three-dimensionality in the blade.

This contrast—"hira-ji with kumori, shinogi-ji with luster"—is the aesthetic core of Japanese sword polishing and embodies a fundamentally different aesthetic from Western sword polishing (uniformly polishing the entire surface).

Rarity of Whetstones and Modern Challenges

The quality of Japanese sword polishing depends directly on the quality of whetstones, but the finest natural whetstones—especially raw uchigumori stone—have been rapidly depleting in recent years. While the Kyoto-Kameoka region and mountain areas of Tochigi Prefecture were major production sites, many quarries have already closed or become unprofitable, making it extremely difficult to obtain high-quality natural whetstones.

Among togishi (polishers), there is a recognition that "good natural whetstones are passed down from one's master," and many craftsmen carefully manage and continue using a single piece of fine whetstone for decades. A market also exists where a single whetstone is traded for hundreds of thousands of yen or more, with the rarity of the material supporting the transmission of togishi technique.

On the other hand, the quality of synthetic whetstones has improved dramatically in recent years, and some now perform comparably to natural whetstones at the undercoat polishing stage. However, in the final stage of finishing polishing—particularly the finishing of hira-ji using uchigumori whetstone—it is currently considered difficult to fully replicate with synthetic whetstones the distinctive "polish finish" created by the complex mineral composition of natural stone.

Transmission of Polishing Technique—The Togishi Apprenticeship System

Mastering the differentiated polishing of shinogi-ji and hira-ji requires long years of master-apprentice training. Togishi are trained through an apprenticeship system similar to that of swordsmiths, with a minimum of 8–10 years typically required from apprenticeship to independence.

Developing the "eye" for polishing is considered the greatest challenge. How much pressure to apply with the whetstone, at what angle, and how to read the surface condition shown by the quality of light—these are difficult to convey in words and must be mastered through years of observing the master's hands.

In modern times, the number of togishi has been declining, and like the shortage of swordsmiths, a shortage of togishi has become a major challenge in the transmission of Japanese sword culture. The fact that the existence of top-tier togishi capable of realizing a beautiful contrast between shinogi-ji and hira-ji influences the "completion" of a Japanese sword demonstrates that togishi are craftspeople equally important as swordsmiths.