Carbon Content Analysis of Tamahagane and Sorting Steel for Shingane and Kawagane: The Science of Steel Selection

Why Is Tamahagane Not Uniform?

Tamahagane born from tatara smelting is fundamentally different from uniform industrial products. Within the three-day-three-night smelting process, the temperature distribution in the furnace, the timing of iron sand input, and the types and quantities of charcoal create blocks of iron with varying carbon content in different regions. Tamahagane's carbon content distributes across a range of approximately 0.6–1.5%, and this variation means "selectable materials" for the sword smith.

High-carbon tamahagane (1.0–1.5%) exhibits hard, brittle properties and is suited as the material for kawagane used in the blade edge. Low-carbon tamahagane (0.6–0.9%) is tough and exhibits high resilience, used for shingane that forms the core of the blade. The foundation of producing a Japanese sword that achieves both cutting ability and resistance to breakage lies in properly sorting and combining these two types.

Distinguishing Carbon Content—Traditional Techniques

Sword smiths possess empirical knowledge of distinguishing carbon content through sight, sound, and touch without relying on scientific analytical instruments. When observing the cross-section of broken tamahagane, high-carbon material displays a fine, bright surface, while low-carbon material appears coarse-grained and dull. The sound when struck also differs—high-carbon produces a hard metallic ring, while low-carbon produces a muffled sound. Furthermore, carbon content can be estimated by the way sparks fly when heated (spark test). High-carbon steel scatters vigorous sparks, while low-carbon steel produces gentler sparks.

These sensory judgments are cultivated through decades of experience and transmitted from master to apprentice through oral instruction and practice. In recent years, some sword smiths have appeared who use metal analysis instruments (such as fluorescence X-ray analysis) to quantitatively verify carbon content, but these are typically used primarily as supplements to traditional empirical knowledge.

Selection in Practice—Breaking and Classification

The selection of tamahagane begins by breaking chunks of tamahagane with a hammer. Breaking allows observation of the cross-section while the manner of breaking itself provides a hint of carbon content. High-carbon tamahagane breaks cleanly and sharply, while low-carbon material is tough and breaks reluctantly. Fragments of broken tamahagane are sorted by weight and size into categories for "shingane" and "kawagane," then stacked in preparation for "tsumiawakashi" (flame welding).

The amount of tamahagane needed to produce a single sword reaches several times the weight of the finished blade. This is because impurities are removed and lost as scale (iron oxide) during the forging process. It is not uncommon for the final amount used to drop to less than one-third of the original.

Tsumiawakashi and Forge-Welding of Shingane and Kawagane

The sorted shingane and kawagane are each heated in the furnace, and repeated folding-forging (orikaeshi tanren) is performed to expel internal impurities and homogenize the carbon content. There is a principle that shingane is folded fewer times (to avoid losing too much carbon), while kawagane is folded more times (to thoroughly remove impurities and increase the purity of the blade edge).

Finally, structures such as "koubuse" (wrapping shingane with kawagane) or "honsammai" (true three-ply) are selected and forge-welded. The judgment at this stage—in what proportion the two types of steel are combined—represents the core of the sword smith's philosophy and technique, and becomes the most critical choice determining the jihada grain pattern, hamon blade pattern, and resilience of the finished sword.

Dialogue with Modern Material Science

From the perspective of modern materials science, the portion of a Japanese sword corresponding to kawagane approaches eutectoid to hypereutectoid steel containing 0.7–1.0% carbon, and high hardness is achieved through the formation of martensite microstructure during quenching. Conversely, shingane corresponds to hypoeutectoid steel, with toughness secured through a mixed ferrite-pearlite microstructure.

The selection techniques that sword smiths have accumulated as empirical knowledge over centuries align remarkably with modern steel microstructure theory, and the precision of this knowledge astonishes modern researchers. At the intersection of tradition and science lies the essence of Japanese sword technology.