Until the Blade Is Clothed in Radiance — The Process and Science of Japanese Sword Polishing

Polishing Is the Work That "Completes" the Sword

While a Japanese sword is given its form by the kaji (smith), it is the togishi (polisher) who draws out its latent beauty. Polishing is not merely the act of "buffing"; it is an advanced technical system that scientifically and artistically reveals the sword's jihada (grain pattern of the steel), hamon (temper line), and kitae-hada (forging grain). Even a skilled togishi requires several weeks to several months to polish a single sword, and the cost not infrequently rivals the value of the sword itself.

There are two purposes to polishing. The first is functional — ensuring sharpness and preserving the edge. The second is aesthetic — drawing out the sword's appreciation value to the fullest. To achieve both simultaneously, the togishi follows a step-by-step, precise process.

Shitaji-togi: Shaping the Sword's True Form

The first stage of polishing is called shitaji-togi, which proceeds by moving step by step from coarser whetstones to finer ones, shaping the body of the blade and removing surface scratches and oxidation.

In shitaji-togi, multiple whetstones are used in stages as follows.

The technical key at this stage is to polish while preserving the blade's sori (curvature), the balance of motohaba and sakihaba, and the clarity of the yokote line. Because even slight unevenness in polishing mars the sword's beautiful silhouette, the togishi advances the work by repeatedly checking with fingertip touch and visual inspection.

Types of Whetstones and Their Science: The Roles of Natural and Synthetic Stones

The whetstones used in polishing are broadly divided into natural whetstones and synthetic whetstones.

Natural whetstones are sandstone and mudstone-type stones quarried from various locations throughout Japan, with grain size and hardness varying greatly depending on the source. Most important among them is the uchigumori-to. Stones from specific quarries in Kyoto Prefecture are currently considered the highest quality, and they give the polished surface a distinctive semi-gloss finish called kasumi (haze). This kasumi effect visually accentuates the jihada pattern and hamon.

No two natural whetstones have the same characteristics, and the togishi must discern the individuality of each stone before using it. In recent years, obtaining high-quality uchigumori-to has become markedly difficult, and among working togishi the common understanding is to "treat each stone as a treasure."

Synthetic whetstones have uniform grain size and allow stable polishing, but the random polishing effect produced by the complex mineral composition of natural stones is difficult to replicate. Most present-day togishi make use of synthetic whetstones for the coarser stages of shitaji-togi, while reserving natural whetstones for the finishing stages.

Shiage-togi and How to Draw Out the Hamon

The true mastery of polishing lies in shiage-togi, also known as keshō-togi. At this stage, two delicate techniques are employed — hazuya and jizuya — each using small pieces of whetstone roughly the size of a thumb that are affixed to the fingers.

In shiage-togi, the following two techniques are used according to purpose.

• Hazuya: The work of polishing only the cutting edge using small pieces of uchigumori-to. This draws out the martensite structure (the hardened layer produced by yaki-ire (quenching)), and the brightly shining hamon appears. The hamon forms along the boundary of the yakiba-tsuchi (clay coating) applied at the time of yaki-ire, and appears as patterns showing the individuality of the tōkō (swordsmith) such as suguha, notare, gunome, and chōji.
• Jizuya: The process of polishing the body of the blade — the part called ji (ground) — other than the edge, revealing the kitae-hada patterns (itame, masame, mokume, etc.). Fine differences in the angle at which the whetstone is applied, the pressure, and the way it is moved directly affect the finished result, and even with the same stone, the "landscape" drawn out differs depending on the togishi.

Viewed from a metallurgical standpoint, polishing the hamon exploits the difference in polishing rates between the martensite structure (hardness HRC 60 or above) and the pearlite/ferrite-based jigane (steel of the blade surface) structure. Structures with differing hardness and Young's modulus reflect light at different rates even when polished under the same conditions, and this difference is visualized as the hamon.

Challenges Facing Modern Togishi

From the perspective of bearers of traditional craft, the number of togishi is by no means large. The NBTHK (Society for the Preservation of Japanese Art Swords) — a public interest incorporated foundation — is said to certify several hundred togishi nationwide, but those with the highest level of skill are even more limited.

The greatest structural challenge is the depletion of natural whetstones. The quarries of uchigumori-to are gradually declining in output, and obtaining high-quality stones grows more difficult year by year. Togishi devise ways to use each single whetstone down to the very last thin sliver, and it is common for them to continue using stones inherited from their masters over a span of several decades.

Furthermore, polishing old celebrated swords comes with the constraint of "not thinning the blade." Because old celebrated swords of high cultural-property value become very slightly thinner with each polishing, today's togishi are required to have the skill of drawing out "maximum effect with minimum polishing." As the original body thickness is also a standard of appraisal, over-polishing can be regarded as damage to a cultural property.

In recent years, research has also been advancing into utilizing CT scanning and 3D scanning for non-destructive inspection as a pre-polishing evaluation, and approaches have been attempted in which the process is designed after ascertaining the condition of internal flaws and rust.

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

Present-day togishi have begun making supplementary use of digital technology. Microstructure observation via microscope and numerical evaluation using surface roughness gauges are drawing attention as means of putting into words and visualizing the transmission of technique from master to apprentice. By supplementing with objective data the tacit knowledge that once could only be "learned through feel," gains in efficiency for training successors are anticipated.

Research and development of ceramic-based synthetic whetstones and nano-particle abrasives is also advancing. However, many togishi point out that "the random polishing effect produced by the complex mineral composition of natural stones cannot be fully reproduced with uniform synthetic stones."

Japanese sword polishing is a complex technical discipline where metallurgical engineering, materials science, and traditional aesthetics intersect. Operating under the philosophy of "drawing out the beauty the sword inherently possesses," togishi integrate scientific knowledge with refined sensory skill as they engage with each and every sword. That practice continues to bridge into the present day the skill and spirit that the tōkō (swordsmith) of centuries past poured into iron.