Test Forging by Modern Swordsmiths: How Tamahagane Suitability Is Assessed Before Crafting a Blade

Why Do Sword Makers Test Their Materials?

For contemporary sword makers (tōshō / 刀匠), accurately understanding the material suitability of tamahagane (玉鋼) that arrives at their workshop is the starting point of blade-making. Tamahagane supplied by the Nihontō Preservation Society tatara (日刀保たたら) differs in carbon content, impurities, and microstructural uniformity depending on conditions during iron smelting, furnace state, and the origin of the iron sand used. Unlike industrial steel stock, which comes with material specification sheets, sword makers must evaluate their materials through a combination of experience and experimentation.

In the craftsman's world, this "material evaluation" process is called "test forging" (tesuto tanren / テスト鍛錬). The more carefully a maker performs this step—determining the individual character of each piece of tamahagane—the more precise the subsequent blade-making plan becomes. This article explains the concrete procedures and judgment criteria involved.

Initial Assessment of Tamahagane—Looking, Breaking, Striking

When material arrives, the first step is visual assessment. Observation of the tamahagane's fracture surface (the cross-section revealed when steel is broken) reveals the state of its microstructure.

• Crystal grain size: Fine, uniform crystals indicate carbon content is appropriate and the steel is suitable for forging. Coarse grains may signal hypereutectoid conditions (excessive carbon) or segregation.
• Color and luster: A fracture surface with silvery-gray brilliance is ideal. Yellowish tints may indicate concern about sulfur-based impurities.
• Absence of voids and inclusions: The presence of slag (ore dross / 鉱滓) can cause poor forge welding (tanjitsu / 鍛接), so makers check visually for major defects.

The subsequent "striking test" involves gently tapping a small lump in cold condition and observing how it fractures. High-carbon, brittle steel tends to crack cleanly when struck cold, while low-carbon, softer steel tends to deform.

Small Blade Prototyping—A Downsized Test Before Production

Once initial assessment is complete, the maker proceeds to "small blade prototyping" (kogata-na / 小刀). Before forging a full-size production blade, a small blade or test piece is made from the same material to confirm actual forging behavior.

Behavioral Verification Through Folding-and-Forging

The small blade stock is heated in the furnace and subjected to several cycles of folding-and-forging (orikaeshi tanren / 折り返し鍛錬). During this process, the maker observes:

• Ease of forge welding: Whether uniform welding occurs at appropriate temperature. Materials prone to forging cracks (kitae-ware / 鍛え割れ) may contain excessive impurities.
• Spark pattern: The pattern of sparks emitted from heated steel serves as a simple indicator of carbon content and impurities. Higher carbon content produces brighter sparks (following the same principle as spark testing).
• Presence or absence of forging cracks: When cracks appear readily in a specific temperature range, the cause (overheating, impurities, or rapid cooling) must be identified.

Creation of Hardening Test Pieces

After forming the small blade, a quenching (yaki-ire / 焼き入れ) test is performed. The same material is subjected to multiple water temperatures and clay coating thicknesses, and the hardness development and hamon (刃文, blade pattern) tendencies are observed.

Hardness measurement combines traditional "file testing" (yasuri-ate / 鑢当て)—feeling the resistance when a file is drawn across the surface—with modern numerical measurement using a Rockwell hardness tester. For sword-making steel, post-quenching hardness typically targets around HRC 60, though the specific goal varies by individual maker.

Carbon Content Estimation—Combining Craftsman Experience with Modern Analysis

There are two methodological traditions for estimating tamahagane's carbon content: traditional and modern analytical approaches.

The traditional estimation method relies on a combination of "charcoal consumption," "forging behavior," and "quenching hardness" to gauge carbon content trends. Higher carbon content yields greater hardness during quenching but reduces toughness; sword makers balance this through intuitive judgment—adjusting for sensations of "too hard" or "too soft."

As a modern analytical approach, some sword makers collaborating with research institutions and universities employ fluorescence X-ray analysis (XRF) or dedicated carbon-sulfur measurement instruments for elemental analysis. When carbon content (weight %), phosphorus, sulfur, and other impurity levels are quantified numerically, material stability and forging-plan precision improve significantly. However, equipment costs are high, and not all workshops have adopted such instruments.

Material Allocation to Outer Steel and Core Steel

Once test forging and analysis have clarified material properties, the maker decides how to allocate stock to the "outer steel" (kawagane / 皮鉄) and "core steel" (shingane / 心鉄) used in blade-making.

• Outer steel: Material with relatively high carbon content (approximately 0.8–1.2%) and high uniformity, whose quenching hardness is stable and reliable.
• Core steel: Material with low carbon content and high toughness (0.5% or below), providing the blade's resilience (bend-resistance).

Even within a single batch of tamahagane, carbon content varies from piece to piece. As the maker observes each lump, they make individual judgments—"this piece is outer steel, this piece is core steel"—and use them accordingly. The precision of this allocation directly affects the quality of the finished blade.

2026 Trends—Digital Recording of Material Information

A growing number of contemporary sword makers are digitally documenting and database-organizing the test results from each batch of tamahagane. Information such as "this tatara batch number has higher carbon content and is optimized for core steel" accumulates for use in future blade-making plans.

Furthermore, deepening collaboration with the Nihontō Preservation Society tatara allows makers to cross-reference production-process data with post-forging evaluations, feeding improvements back into tatara iron-making itself. The fusion of traditional craftsmanship and data-driven methods is opening new possibilities for the transmission of Japanese sword technology.

Summary—The Technology of "Knowing" Invisible Material

Contemporary sword makers' test forging is a scientific and experiential process for engaging with tamahagane, an inherently non-uniform natural material. From visual assessment through small blade prototyping, quenching tests, and the application of modern analytical equipment, multiple means are combined to ascertain material suitability. The technology of "knowing" invisible material is the point of departure for creating excellent blades and ranks among the most important tacit knowledge carefully transmitted from master to apprentice.

Works created by contemporary sword makers can be viewed online at toukenza.jp. We invite you to discover a single blade in which a craftsman's technique and philosophy are embodied, from material selection through completion.