The Mechanism of Utsuri — The Science of Tanren and Yaki-ire That Produces the Faint Shadow Floating in the Jigane

What Is Utsuri?

Utsuri refers to a pale, shadow-like pattern that floats in the jigane of a Japanese sword, distinct from the hamon. Whereas the hamon appears as a distinct line near the edge, utsuri manifests in the ji, often in the region closer to the mune than the hamon, taking the form of a hazy cloudiness with blurred outlines. Because its appearance changes markedly depending on the direction and angle of the light, determining the presence and type of utsuri requires the trained eye of an experienced togishi or appraiser. It is known to appear prominently in kotō of Bizen-den, especially in works of Ko-Bizen and the Ichimonji School, while it is rarely seen in swords of Sōshū-den — a regional and school-based difference. Because it is less conspicuous than the hamon, it is easy for those who first encounter a Japanese sword to overlook, but it is also a feature whose presence becomes more keenly felt as one accumulates experience in appreciation.

The Metallurgical Conditions That Give Rise to Utsuri

It is widely understood that differences in the microstructure of the steel that makes up the jigane are involved in the mechanism by which utsuri arises. The jigane of a Japanese sword, through repeated orikaeshi tanren of tamahagane, develops a structure in which thin layers with slight variations in carbon content are built up. During yaki-ire of the blade, the area near the edge, where the tsuchioki is thin, is quenched rapidly and transforms into a hard microstructure, while the area near the mune, where the clay is applied thickly, cools more slowly and retains a different microstructure from the edge. The difference in microstructure that arises near this boundary is thought to manifest as a faint contrast of light and shadow on the surface of the polished blade — this is utsuri. In other words, utsuri is a phenomenon that arises only when two factors coincide: the layered structure of the jigane produced by tanren and the difference in cooling rates during yaki-ire; neither factor alone is sufficient to produce it. Because the properties of tamahagane and its carbon distribution differ by region and era, it is thought that even when the same tsuchioki technique is used, the way utsuri appears reflects the distinctive character of each place of production.

Types of Utsuri

Utsuri has several types, each with its own name according to how it appears.

• midare-utsuri — utsuri that appears in irregular waves, corresponding to the irregular hamon
• suguha-chō utsuri — utsuri seen in works with a hamon close to suguha, appearing as a relatively straight band
• jifu-utsuri — utsuri that appears across the entire jigane in a mottled, spotted pattern
• bō-utsuri — utsuri that appears as a straight bar parallel to the hamon regardless of its shape (common in Bizen swords from Ōei Bizen onward)

All of these types subtly vary in appearance depending on the combination of the jigane's kitae-hada (itame, mokume, masame, etc.) and the tsuchioki during yaki-ire, so even though they are grouped under the same term "utsuri," the appearance differs from one blade to the next. Even among works of the same school, individual differences in the depth of utsuri arise due to variations in forging habits and the thickness of the tsuchioki.

The Difference Between Utsuri and Hamon

Both utsuri and hamon are phenomena that derive from yaki-ire, but their natures differ. The hamon is the boundary of the hard microstructure (martensite) itself, which forms when the edge is rapidly quenched, and can be seen through togi as granular nie and nioi. Utsuri, by contrast, is not the hardened microstructure of the hamon itself, but a phenomenon visible as a difference in the reflection of light produced by microstructural changes on the jigane side. For this reason, utsuri has more diffuse outlines than the hamon, and tends to appear and disappear depending on the viewing angle and light source. When a togishi employs the techniques of hadori and nugui at the finishing stage, it is not only to make the hamon appear crisp, but also to draw out the faint expression of utsuri without suppressing it. The appearance of utsuri can change greatly depending on the skill of the togi, and whether the latent beauty of the jigane can be drawn out depends greatly on the togishi's level of expertise.

Why Utsuri Is Valued and Its Role in Appraisal

Utsuri does not directly affect the practical performance of a Japanese sword, but it has been held in high regard from both the appraisal and appreciation perspectives. From the appraisal perspective, the presence or absence and type of utsuri serves as one clue for estimating the period and school. While utsuri is frequently found in Kamakura-period Bizen swords such as those of Ko-Bizen and the Ichimonji School, works from after the Nanbokuchō period within the same Bizen-den, as well as works of Sōshū-den and Mino Tradition, tend to show little utsuri — and it is treated as an indicator reflecting differences in jigane forging methods and yaki-ire practices. From the appreciation perspective, utsuri has been valued by enthusiasts as an element that evokes the quiet depth of the jigane, in contrast to the dynamic beauty of the hamon. Discovering the faint shadow that suddenly floats up in a place apart from the hamon when a blade is held up to the light is considered one of the pleasures of appreciating Japanese swords. Contemporary research continues to attempt to elucidate the conditions under which utsuri arises from a metallographic perspective, and efforts are advancing to understand this phenomenon — which has long been passed down as experiential knowledge — with scientific backing. Utsuri is said to have once declined as a technique after the shintō period, and the very existence of groups of shintō tōkō such as the Ishidō School who attempted to reproduce the utsuri of Ko-Bizen and Ichimonji attests to how sensitive the manifestation of utsuri was to the delicate conditions of tanren and yaki-ire. Precisely because it is a phenomenon so difficult to reproduce intentionally, the beauty of utsuri continues to be held in high esteem as a distinctive value inherent to the jigane of kotō.