Forge Cracks in Nihonto: Causes, Prevention, and Swordsmith Countermeasures for Steel Cracking During Tanren

What is Forge Crack—The Greatest Defect Threatening a Sword Blade

"Forge crack" (kitae-ware) is a collective term for cracks, breaks, and delamination that occur inside or on the surface of a sword blade during the forging (toughening) process of a Japanese sword. In English, it is expressed as "forge crack" or "delamination." Forge crack is not merely a surface defect; it can fundamentally compromise the strength of the blade, making it one of the most carefully guarded defects among sword smiths.

Forge cracks can be broadly classified into two types. The first is "surface crack," which are fine cracks that occur on the blade surface during or after forging. Minor ones can sometimes be removed through polishing, but deep ones necessitate the disposal of the blade. The second is "internal delamination (interlayer crack)," which is a type where layers of steel accumulated through fold-forging separate from one another. This is difficult to see by appearance but is the most dangerous defect, manifesting when the blade is used (through bending stress and impact). Polishing specialists often discover it during polishing work, and it is one of the main issues pointed out as a defect in authentication (examination).

Chemical Causes—Uneven Carbon Distribution and Oxidation

The occurrence of forge cracks involves the complex interplay of multiple chemical and physical factors. From a chemical perspective, when the distribution of carbon (C) contained in tamahagane is uneven, regions of high and low carbon content exist adjacent to each other locally. High-carbon steel has low toughness (resilience/flexibility) and high brittleness, making it prone to cracking under repeated impact during forging.

The issue of oxidation is also important. When steel is exposed to air (oxygen) during forging, iron oxide (scale) forms on the surface. If this scale is not properly removed, it gets pressed into the interior of the steel and remains as a non-metallic inclusion (inclusion). Inclusions become stress concentration points, making cracks likely to propagate from them. When steel is removed from the furnace and struck during forging, "iron sparks" (tetsu-ka) fly off; it is important to sufficiently disperse the oxide scale at this time, and sword smiths use a sophisticated technique of deliberately blowing off the scale with the first strike of the hammer.

Physical Causes—Temperature Management Failure and Over-Striking

From a physical perspective, failure to manage forging temperature is the primary cause of cracking. If steel is forged at too high a temperature (overheating range: 1300°C or above), "over-burning" (sugoyake) occurs, and crystal grains become coarse, weakening grain boundaries. Over-burned steel has significantly reduced toughness and cracks easily during folding and hammering. Conversely, forging at too low a temperature becomes "cold-striking" (tsumetataki), causing "low-temperature crack" where steel with reduced plastic deformation capacity cracks.

"Over-striking" (uchisugiru) is also a cause of cracking. If fold-forging is repeated too many times (roughly over 15 to 20 times), carbon content decreases excessively, and the internal structure of the steel becomes overly refined, paradoxically increasing brittleness in some cases. The appropriate number of forging iterations varies depending on the quality of tamahagane and the intended use of the sword, but the sword smith's individually accumulated "sense of forging" includes the judgment of this appropriate number of iterations, and it is often carefully transmitted in oral teachings (kuden) to apprentices.

Distinguishing from Similar Defects—Differentiating from Edge Cracks and Blisters

Forge cracks are easily confused with other defects of similar appearance. "Edge crack" (hagire) is a crack that runs across the hamon (edge pattern), and in most cases is said to occur from impact or accidental causes during use after completion, differing in origin from forge cracks that result from the forging process itself. "Blister" (fukure) is a swelling or bubble-like defect caused by air or foreign matter being trapped inside the steel during forge-welding (tansetsu) while forging; it is distinguished from forge cracks in that when polishing progresses and the surface ruptures, internal voids become exposed. Since all of these are defects that significantly affect the evaluation of a blade, careful differentiation is performed in authentication.

Practical Prevention Measures—From Material Selection to Process Management

Practical measures to prevent forge cracks begin at the material selection stage. The appropriate carbon content in tamahagane is approximately 0.6–0.9% for the outer iron (kawagane) and 0.2–0.4% for the core iron (shingane), and sword smiths roughly judge carbon content by striking the material and assessing the sound, feel, and color of the fracture surface. Tamahagane with low homogeneity (uneven carbon distribution) has high risk of forge cracks, so eliminating high-risk materials at the material selection stage is the first prevention measure.

Regarding the hardening and tempering processes, research as of 2026 has clarified that the precision of temperature adjustment is extremely important for crack prevention. Some modern sword smiths combine scientific management using thermometers with traditional "eye" (discernment), producing more reliable swords.

When summarizing the key points for preventing forge cracks, they are as follows.

• Ascertain the carbon distribution in tamahagane beforehand and avoid materials with low homogeneity
• Maintain forging temperature within an appropriate range, avoiding both overheating and overcooling
• Sufficiently remove oxidation scale with each strike and prevent the introduction of inclusions
• Discern the number of fold-forging iterations according to material and intended use, avoiding over-striking
• Carefully inspect the blade after forging and during polishing to detect fine cracks early

It is important to note that these are not independent processes but function in coordination as a series of management from material selection through forging to hardening. If judgment is wrong at any single stage, there is a danger that all the careful work up to that point will be wasted, so sword smiths must maintain tension throughout the entire process.

Points to Confirm When Purchasing or Authenticating

When purchasing or authenticating a sword, confirming the presence or absence of forge cracks is essential. Professional polishing specialists and authenticators hold the blade up to light from various angles and carefully confirm that there are no irregularities in the jihada (surface grain) or linear shadows. Particularly the shinogi-ji (ridge area) and the portion near the edge are said to be areas where cracks are likely to occur and are inspected thoroughly. Additionally, in the authentication process by institutions such as NBTHK (Japan Art Sword Preservation Association), the soundness of the blade is considered one of the important elements of evaluation, and it is believed that if structural defects are confirmed, the authentication results may be affected. Selecting a sword with a trustworthy authentication certificate can be said to be one of effective means of avoiding such risks.

Note that there are also defects like internal delamination that are difficult to distinguish by appearance, so it is desirable to confirm the actual product by a specialist and carefully review the content of any already-issued authentication certificates before purchase.