The Complete Guide to Blade Patterns (Hamon) — A Thorough Explanation of Temper Patterns, the Very Soul of Japanese Swords

What Is Blade Pattern (Hamon) — The Boundary Where the Sword's Soul Dwells

When appreciating Japanese swords, one of the most important elements is the "blade pattern (hamon)." Blade pattern refers to the white band-like markings that appear near the edge of the sword blade, indicating the boundary line between the hardened and flexible portions created by hardening (heat treatment). In English, it is called "hamon" or "temper line" and has become established as an international technical term.

Blade pattern is not merely decorative, but emerges as the result of the most delicate and important process in Japanese sword making—hardening. It is an element that influences all aspects of the sword's sharpness, toughness, and beauty, and reading the blade pattern directly connects to discerning the skill level, era, and school of the swordsmith who created it. In the appraisals of the Japanese Art Sword Preservation Society (NBTHK), blade pattern is considered the most important evaluation item.

How Blade Patterns Are Formed — The Science of Hardening

To understand blade patterns, it is essential to know the hardening (yaki-ire) process.

After completing the blade, the swordsmith performs a process called "clay application (tsuchi-oki)." A fireproof clay is applied in varying thicknesses—thinly on the edge and thickly on the spine and ridge. This difference in clay thickness creates the difference in hardness after hardening.

The clay-coated blade is heated to approximately 750-800°C in a furnace, and then rapidly cooled (quenched) in water or oil once it reaches the desired temperature. Through this rapid cooling, the edge portion transforms into "martensite," an extremely hard iron crystal structure, while the spine portion is preserved as "troostite" or "sorbite," remaining relatively soft.

This boundary line between hardness and softness appears on the blade surface as the blade pattern. Since the shape of the blade pattern is determined by how the clay is applied and the technique used during hardening, the swordsmith's individual characteristics and school traditions are clearly reflected.

Main Types of Blade Patterns and Their Characteristics

Straight Blade (Suguha)

The straight blade is the simplest blade pattern, characterized by a straight-line edge boundary. Because it conveys an elegant and composed impression, it has been favored by master craftsmen since ancient times. It is frequently seen in Yamashiro school (Kyoto-based) smiths' works and is typical of pieces from the Awatauchi and Rai schools.

Some straight blades contain what is called "straight blade style (suguha-chō)," which includes slight irregularities, and despite appearing monotonous at first glance, they actually have profound depth. The straight blades of the Awatauchi school are particularly called "fine straight blade (hoso-suguha)," distinguished by a narrow, finely honed linear beauty.

While the straight blade appears simple at first glance, it demands precision in clay application that allows no irregularities; experienced smiths fully appreciate its difficulty. For viewers, it is considered a blade pattern whose depth becomes apparent only after careful, sustained observation.

Undulating Wave (Notare)

The undulating wave is a blade pattern that flows in gentle, wave-like undulations, characterized by continuous large wave formations. It is particularly common in the Soshu school (Kamakura-Odawara tradition), with masterpieces by Masamune and Sadamune as representative examples. Depending on the size and density of the waves, it may be classified into categories such as "large undulating wave" and "small undulating wave."

Clove Bud (Chōji)

The clove bud is a blade pattern with continuously repeating forms resembling the buds of clove flowers, and is particularly prevalent in the Bizen school (Okayama-based tradition). It is typical of works from the Ichimonji and Osafune schools, and stands out among blade patterns for its visual elegance. There are abundant variations depending on the era and swordsmith, including "large clove bud," "reverse clove bud," and "triple clove bud."

Alternating Diamond (Gunome)

The alternating diamond is a blade pattern with wave-like formations that repeat regularly, and takes its name from a continuous series of rounded wave forms resembling go stones arranged in a row (the term "alternating diamond" evolved from "go stone pattern"). It originated in the Bizen school and eventually spread throughout the various schools. Diverse variations exist, including "alternating diamond clove bud" and "alternating diamond irregular pattern."

Irregular Pattern (Midaré)

The irregular pattern is a general term for blade patterns formed when the basic patterns above combine complexly to create varied, disorderly designs. It has diverse variations such as "large irregular," "small irregular," and "all-over hardening," and was mastered by smiths of the Soshu and Mino schools (Gifu-based tradition).

All-Over Hardening (Hitatsura)

All-over hardening is when hardening penetrates not only the edge portion but also the body of the blade, forming such a complex blade pattern that the entire blade appears white. It is frequently seen in the late Soshu school from the Southern and Northern Courts period through the Muromachi period, and has an extremely strong visual impact.

Details of Blade Patterns — Nie and Nioi

Essential concepts for appreciating blade patterns are "nie" (boiling) and "nioi" (fragrance).

Nie is the granular brilliance that appears within the blade pattern; when magnified, sand-like particles are visible. This is because the coarse structure of martensite crystals formed by rapid cooling reflects light, and it is commonly seen in old swords from the Kamakura through Muromachi periods. Swords with strong nie are called "boiling finish (kawaradeki)" and convey a sense of natural vigor and strength.

Nioi is when the blade pattern appears soft and hazy, appearing when martensite crystals are distributed finely and uniformly. It is frequently seen in the new and newer swords of the Edo period and creates an impression of refinement and delicacy. It is also called "fragrance finish (nioi-deki)."

When appreciating blade patterns, the "focused observation method"—observing gradually while tilting the lighting at an angle—is effective. First, grasp the overall form (the broad flow of shapes), then confirm the shape of the edge boundary (straight blade, clove bud, alternating diamond, etc.), subsequently discern the texture of nie and nioi, and finally focus attention on the workings of fine details such as "gold streak" and "sand flow." By observing in this sequence, one can develop a three-dimensional understanding of the entire blade pattern's structure and individuality.

Even with the same blade pattern shape, the impression and appraisal differ greatly depending on whether nie or nioi is present. The Soshu school with strong nie and the Bizen school with soft nioi represent opposite poles in terms of the blade pattern's "texture" and beauty.