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

Why Is Tamahagane Not Uniform?

Tatara ironmaking produces tamahagane that is fundamentally different from uniform industrial products. Within the three-day and three-night smelting process, blocks of iron with different carbon content in each section are created, depending on the temperature distribution in the furnace, the timing of iron sand input, and the type and amount of charcoal. The carbon content of tamahagane is distributed across a range of approximately 0.6 to 1.5%, and this range means "material for differentiated use" for the swordsmith.

High-carbon tamahagane (1.0 to 1.5%) possesses hard and brittle properties and is suitable as material for kawagane (edge iron) used in the blade portion. Low-carbon tamahagane (with carbon content roughly 0.2 to 0.5%) is tough and possesses high resilience, and is used for shingane (core iron) that forms the center of the sword blade. Note that 0.6 to 0.9% is a medium to high-carbon range that overlaps with the carbon range of kawagane and is not used for shingane. Appropriately sorting and combining these two types forms the foundation for producing Japanese swords that achieve both sharpness and resistance to breakage.

This distribution of carbon content supports the unique creation process of Japanese swords: sorting tamahagane born from a single smelting furnace rather than artificially mixing iron and steel. The swordsmith discerns the properties of the tamahagane before them and finds the most suitable use for each part — it can be said to be work of distinguishing the differences in properties that materials possess, rather than creating new materials.

Distinguishing Carbon Content — Traditional Techniques

Swordsmiths possess empirical knowledge for distinguishing carbon content through sight, sound, and touch, without using scientific analytical instruments. When looking at the cross-section of broken tamahagane, high-carbon pieces show fine and brilliant cross-sections, while low-carbon pieces have coarse and dark grains. The sound when struck also differs; high-carbon produces a hard metallic ring, while low-carbon produces a muffled sound. Moreover, the carbon content can also be estimated from the pattern of sparks when heated (spark test). High-carbon steel scatters intense sparks, while low-carbon steel produces mild sparks.

These sensory judgments are cultivated through decades of experience and are passed down from master to apprentice through oral instruction and practice. In recent years, some swordsmiths have begun using metal analysis equipment (such as fluorescent X-ray analysis) to quantitatively confirm carbon content, but it is mostly used as a complement to traditional empirical knowledge.

If the carbon content is misjudged, using tough tamahagane for kawagane will fail to achieve the expected hardness, or conversely, using overly hard tamahagane for shingane will result in insufficient toughness in the blade. To avoid such failures, before beginning work on a single sword, the swordsmith invests time in carefully sorting the tamahagane at hand. The accuracy of this judgment significantly affects the quality of the finished sword.

The Practice of Selection — Breaking and Classification

Selection of tamahagane begins by breaking blocks of tamahagane with a metal hammer. Breaking allows observation of the cross-section, and the manner of breaking itself provides hints about carbon content. High-carbon tamahagane breaks cleanly, while low-carbon tamahagane is tough and breaks with difficulty. The fragments of broken tamahagane are sorted by weight and size, classified as "for shingane" or "for kawagane," and stacked in preparation for tsumiwakashi (layered annealing).

The amount of tamahagane required to make a single sword reaches several times the finished weight. This is because impurities are removed during forging and fall away as scale (iron oxide). It is not uncommon for the final amount used to be one-third or less of the initial amount.

The classified shingane and kawagane tamahagane are often not used as a single block alone, but rather multiple pieces with similar properties are combined and stacked. Because a single piece of tamahagane alone tends to retain uneven carbon content, combining fragments with similar properties is a technique to bring the base steel closer to uniformity after forge-welding.

Tsumiwakashi and Forge-welding of Shingane and Kawagane

The sorted shingane and kawagane are each heated in a furnace, and repeated folding and forging (orikaeshi tanren) expels internal impurities and equalizes carbon. There is a principle that shingane undergoes fewer folding forgings (to avoid losing too much carbon), while kawagane undergoes more (to thoroughly remove impurities and increase the purity of the blade edge). For the complete process of folding and forging, please also refer to From Tamahagane to Japanese Sword — The Complete Process of Folding Forging and Its Scientific Meaning.

Finally, forge-welding is performed by selecting structures such as "kohubuse" (wrapping the shingane with kawagane) or "hon-sanmai" (true three-layer). The judgment at this stage — what proportion to combine the two types of iron — is the core of the swordsmith's philosophy and technique, and is the most important choice in determining the grain pattern, hamon, and resilience of the base steel of the finished sword.

The differences in structure such as kohubuse and hon-sanmai appear not only in the cross-sectional structure of the finished sword but also in the grain pattern of the base steel that emerges when the sword is polished. Even starting with the same tamahagane, the combination of sorting and forge-welding creates a unique grain pattern for each sword. This individual variation is also one of the reasons why Japanese swords stand apart from mass-produced industrial products.

Dialogue with Modern Materials Science

From the perspective of modern materials science, the part of a Japanese sword corresponding to kawagane approaches eutectoid to hypereutectoid steel with 0.7 to 1.0% carbon, and forms a martensitic structure through quenching to achieve high hardness. In contrast, shingane corresponds to hypoeutectoid steel and secures resilience through a ferrite-pearlite mixed structure.

The sorting techniques that swordsmiths have accumulated as empirical knowledge over centuries align remarkably with modern steel structure theory, and the precision of the work astounds modern researchers. At the intersection of tradition and science lies the essence of Japanese sword technology.

Even from the perspective of appreciating and collecting Japanese swords, the sorting of shingane and kawagane and forge-welding are crucial processes that determine the appearance of the base steel and hamon. Understanding the background of material selection becomes a clue to more deeply appreciate the character of each sword.

Frequently Asked Questions About Sorting Shingane and Kawagane

• Is all tamahagane used for the blade?: No. As mentioned above, high-carbon tamahagane is used as kawagane for the blade edge, while low-carbon tamahagane is used as shingane for the core of the blade. Even the same tamahagane has different roles depending on the location.
• Does the selection standard differ by swordsmith?: The basic framework of selection—high-carbon for kawagane, low-carbon for shingane—is common, but specific judgments such as how finely to sort and in what proportions to combine them differ depending on the school and each swordsmith's experience and philosophy.
• Are shingane and kawagane still selected from tamahagane in modern times?: Modern swordsmiths also use tamahagane produced by tatara ironmaking and perform similar sorting and folding forging.