Yaki-naoshi: The Art and Limits of Re-tempering Japanese Swords

What Is Yaki-Naoishi? — The Technique to "Give a Sword New Life"

Yaki-naoishi (焼き直し), or re-quenching, refers to the technique of performing heat treatment once again on an already completed Japanese sword. Since the hamon of a Japanese sword is determined during the quenching process, yaki-naoishi is also an act of reshaping the hamon. It has been performed as a final resort for repairing a broken sword, recovering a hamon that has disappeared, or correcting certain defects.

The term "yaki-naoishi" is used colloquially to mean "doing over" or "reconsidering," but in the context of Japanese swords it refers to a very specific and specialized technical procedure. On the other hand, in the world of sword appraisal, yaki-naoishi serves as a negative signal indicating "a sword that once had some problem," and its presence or absence is an important consideration in authentication.

Main Circumstances in Which Yaki-Naoishi Is Performed

The circumstances under which yaki-naoishi is carried out can be classified into three main categories.

① Disappearance of Hamon (Yaki-Ochi): When a Japanese sword is exposed to intense fire or heat, the hamon can disappear. This occurs because the martensite structure that makes up the hamon undergoes phase transformation at high temperatures and reverts to a uniform structure. Many swords stored in warehouses that burned down in the great fires of the Edo period (such as the Meireki fire) lost their hamon and were subsequently re-quenched, as documented in historical records. Among the extant famous old swords, there are several whose yaki-naoishi history is confirmed by historical sources.

② Repair of Broken Swords (Setssusonmen): When a sword breaks, the break surface is treated, the broken section is reshaped to create a shorter sword, and yaki-naoishi is performed. In such cases the sword is often shorter than the original form, and the repaired sword undergoes processes such as "ori kaeshi" and "suriage" (polishing down).

③ Design Changes by Request: Though rare, historical records from the Edo period document instances where samurai commissioned re-quenching to change the hamon design of their swords to match their preferences. In modern times, master swordsmiths themselves occasionally perform yaki-naoishi to correct a hamon on their own work that they are unsatisfied with.

The Technical Process of Yaki-Naoishi

As a principle, yaki-naoishi must be performed by the original swordsmith who made the blade or by a craftsman of equal or superior skill. Subcontracting the work to a polisher or metalworker presents both technical and ethical problems, and in modern times it is considered the exclusive domain of the master swordsmith.

The process begins with tsuchi-otoshi (soil removal), in which dirt and traces of previous quenching are removed from the blade surface. Next, tsuchi-oki (soil application) — the application of clay before quenching — is performed to design the hamon pattern. This step offers some freedom in designing a new hamon, but whether to replicate the original pattern or create a new design is the swordsmith's decision.

Yaki-ire (quenching) is carried out using the same process as the original manufacture, but steel that has undergone quenching once may have accumulated structural "fatigue," and the result will not necessarily be identical to the first quenching. In particular, the steel of old swords subjected to extensive folding during forging carries a risk of intergranular corrosion from reheating.

The Limitations of Yaki-Naoishi — Problems in Authentication and Technique

Yaki-naoishi faces significant limitations in both technical and appraisal contexts.

The decline in appraisal value is the primary concern. When yaki-naoishi is confirmed by NBTHK review, evaluation of the original period and swordsmith's workmanship becomes difficult, often resulting in a substantial downgrade in rating. In particular, when old swords from the Kamakura or Nanboku-chō periods have been re-quenched, the original jihada structure has undergone aging changes over time, and the hamon after re-quenching becomes qualitatively different from the hamon original to old swords. Authentication certificates frequently include notation stating "yaki-naoishi present," and swords with this marking tend to see a decline in market value.

Physical deterioration risks are also significant. When reheating the blade, there is a risk of microscopic cracks forming near the hada-machi (edge boundary) and monouchi (striking zone). The thermal shock from rapid cooling during quenching is similar to that of the first quenching, but a blade with a history of use and storage has accumulated internal stresses, and the behavior of these stresses when released during rapid cooling becomes unpredictable.

The difficulty of reproducing the hamon is also a practical limitation. Complex old sword hamons in particular—such as midareba-chōji, inazuma-kinsuji, and tobi-yaki—are the product of the original swordsmith's accumulated tsuchi-oki technique and years of quenching intuition, and it is practically impossible for later swordsmiths to recreate precisely identical hamons. It is not uncommon for the hamon after yaki-naoishi to be judged as "inferior to the original."

The Significance of Yaki-Naoishi in Modern Times

While yaki-naoishi often receives negative evaluation, it remains an indispensable technique in sword restoration. When a historically significant famous sword loses its hamon due to fire or war damage, choosing partial recovery through yaki-naoishi rather than complete loss can be a rational decision from the perspective of cultural property preservation.

Among contemporary renowned swordsmiths designated as Living National Treasures, there are examples of re-quenching damaged swords made by senior swordsmiths from before the war as "restoration," treating them as a form of "regeneration." In such cases, the work may be referred to as "re-conditioning" rather than "yaki-naoishi" out of respect for the original craftsman, though technically the process is identical.

Yaki-naoishi is at once an "imperfect recovery" and an "act of continuing the sword's life." From a perspective that views Japanese swords not as mere objects but as cultural living entities, yaki-naoishi deserves positive evaluation as the best choice within limitations.