Until the Blade Shines——The Process and Science of Japanese Sword Polishing

Polishing is the Work that "Completes" the Sword

While Japanese swords are given form by the bladesmith, it is the work of the polishing master (togishi) that draws out their latent beauty. Polishing is not merely a process of "rubbing" but rather a sophisticated technical system that scientifically and artistically reveals the blade's jihada (surface patterns), hamon (edge pattern), and kitaehada (forging texture). Even skilled polishing masters require weeks to months to polish a single blade, and the cost often rivals the value of the sword itself.

The purpose of polishing is twofold. One is functional——ensuring sharpness and preserving the edge. The other is aesthetic——maximizing the sword's appreciation value. To achieve both simultaneously, the polishing master follows a staged and precise process.

Foundational Polishing: Preparing the Blade's True Face

The first stage of polishing, called "shitaji-togi" (foundational polishing), involves progressively moving from coarser to finer whetstones while establishing the blade's shape and removing surface scratches and oxidation.

Initially, "ara-to" (coarse whetstone) or "kaisei-to" (correction whetstone) is used to establish the basic shape of the blade. Next, "bin-sui-to" (Binzen water whetstone) refines the blade's outline and hiraniku (the blade's cross-sectional thickness). This is followed by multiple stages using "nagura-to" class whetstones. Nagura stones are classified by grit into categories such as "chu-nagura" (medium) and "sai-nagura" (fine), removing scratches from previous stages while achieving a uniform surface.

The technical key at this stage is maintaining the blade's "sori" (curvature), balance of "motohaba" and "sakihaba" (widths at hilt and point), and clarity of the "yokote" line while polishing. Even slight polishing inconsistencies damage the blade's graceful silhouette, so the polishing master repeatedly alternates between tactile confirmation with fingertips and visual inspection.

Whetstones and Their Science: The Role of Natural and Artificial Stones

Polishing whetstones are divided into "natural whetstones" and "artificial whetstones."

Natural whetstones are sandstone and mudstone-based stones quarried throughout Japan, with grit and hardness varying significantly by location. Most critical among these is the "uchigumori-to" (inner-clouded whetstone). Currently, stones from specific mining locations in Kyoto Prefecture are considered highest quality, imparting a distinctive semi-gloss finish called "kasumi" (mist) to the polished surface. This kasumi effect makes the jihada patterns and hamon visually prominent.

No two natural whetstones possess identical properties, and the polishing master must discern each stone's individual character. High-quality uchigumori-to has become increasingly difficult to obtain in recent years, and among working polishing masters there is a shared recognition of treating "each stone as a treasure."

Artificial whetstones offer uniform grit and stable polishing, but the complex mineral composition of natural stone's random polishing effects are difficult to replicate. Most contemporary polishing masters employ a dual approach: using artificial whetstones for coarser foundational stages and natural whetstones for finishing work.

Finishing Polish and Revealing the Hamon

The true art of polishing lies in "shiago-togi" (finishing polish), also called "keshō-togi" (cosmetic polish). This stage employs delicate techniques using "hazuya" and "jizuya"——small thumb-sized fragments of whetstone attached to the finger.

Hazuya is the work of polishing only the ha (cutting edge) section using small pieces of uchigumori-to. This reveals the martensite structure (the hardened layer created by quenching), making the white, gleaming hamon appear. The hamon takes form along the boundary line of the quenching paste applied during hardening, manifesting as patterns like "suguha" (straight), "notare" (wavy), "gunome" (scalloped), and "chōji" (clove), displaying the individual character of the swordsmith.

Jizuya is the polishing of the "ji" (the main body of the blade apart from the edge) that reveals the forging texture patterns (itame, masame, mokume, etc.). Minute differences in whetstone angle, pressure, and movement directly affect the finish, and the same stone produces different "scenery" depending on the polishing master's technique.

From a metallurgical perspective, hamon polishing exploits the difference in polishing rates between the martensite structure (hardness HRC 60 or higher) and the pearlite and ferrite-based ground structure. Structures differing in hardness and Young's modulus exhibit different light reflectance rates even under identical polishing conditions, and this difference becomes visualized as the hamon.

Challenges Facing Contemporary Polishing Masters

From the perspective of preserving traditional craftsmanship, the number of polishing masters is relatively small. The Japan Art Sword Preservation Society recognizes several hundred polishing masters nationwide, but those possessing the highest technical skill are even fewer.

The greatest structural challenge is the depletion of natural whetstones. The mining sites for uchigumori-to have seen gradually declining output, making the procurement of high-quality stones increasingly difficult year by year. Polishing masters maximize their use of each whetstone down to the thinnest fragment, and many continue using stones inherited from their masters over decades.

Additionally, polishing ancient masterwork blades carries the constraint of "not thinning the blade." Since culturally significant ancient swords gradually become thinner with each polishing, modern polishing masters must possess the technique to "achieve maximum effect through minimal polishing." Since original blade thickness is an evaluation criterion in appraisal, excessive polishing may be deemed cultural property damage.

Recent research has begun utilizing non-destructive inspection methods such as CT and 3D scanning in pre-polishing assessment, with emerging approaches involving identifying internal flaws and rust conditions before designing the polishing process.

Fusion of Innovation and Tradition——The Future of Polishing Technology

Contemporary polishing masters are beginning to employ digital technology in supplementary roles. Microscopic observation of structures and numerical surface roughness measurement are gaining attention as means of articulating and visualizing the skills traditionally passed from master to apprentice. There is expectation that complementing tacit knowledge previously learned through "sensation alone" with objective data will improve efficiency in successor training.

Research and development in ceramic-based synthetic whetstones and nano-particle polishing materials continues. However, many polishing masters point out that "the random polishing effects produced by the complex mineral composition of natural stones cannot be fully replicated by uniform artificial stones."

Japanese sword polishing is a composite technical discipline where metal engineering, materials science, and traditional aesthetics intersect. Guided by the philosophy of "revealing the beauty the blade inherently possesses," the polishing master engages with each individual blade while integrating scientific knowledge with refined intuition. This endeavor continues to bridge the technique and spirit that swordsmiths embedded in iron centuries ago to the present day.