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

What Is Yaki-Nao-shi? — The Technique of "Bringing the Sword Back to Life"

Yaki-nao-shi (焼き直し) refers to the technique of applying a second quenching to an already-completed Japanese sword. Since the hamon is determined during the quenching process, yaki-nao-shi is also an act of re-forming the hamon. It has been performed as a means of restoration when a sword has broken, recovery when the hamon has faded, or as a final resort for correcting defects.

While the term "yaki-nao-shi" is used in everyday language to mean "redoing" or "reconsideration," in the context of Japanese swords it refers to a specific and specialized technical practice. On the other hand, in the world of sword appraisal, it is also a negative signal indicating "a sword that once had some problem," and the presence or absence of yaki-nao-shi becomes an important item to verify in appraisal. Since yaki-nao-shi is information directly related to the history of the sword itself, it becomes an inevitable point of discussion in situations involving purchase or appraisal.

Situations in Which Yaki-Nao-shi Is Performed

Situations in which yaki-nao-shi is performed can be broadly classified into three types.

• ①Loss of Hamon (Yaki-Ochi): When exposed to severe fire or heat, the martensite structure that comprises the hamon transforms at high temperatures and reverts to a homogeneous structure, resulting in the loss of the hamon. During the great fires of the Edo period (such as the Great Fire of Meireki), many swords that were burned along with their storage buildings lost their hamon and records remain of them being re-quenched. Among existing famous ko-katana, there are multiple examples where yaki-nao-shi history is confirmed from historical records.
• ②Restoration of Broken Swords: When a sword is broken, after processing the fractured surface, it is sometimes reshaped shorter and then re-quenched. It often becomes shorter than its original form, and the restored sword undergoes a process called "suri-age."
• ③Hamon Modification by Commission: While rare, there are cases recorded in Edo period historical documents of samurai commissioning yaki-nao-shi to change the hamon design of their swords. In modern times, sword smiths sometimes perform it to correct the hamon of their own creations.

What is common to all these situations is that yaki-nao-shi has been chosen as a means of "continuing to use the original sword" rather than "abandoning the original sword." The fact that the hamon can be re-formed while preserving the jihada and form of the sword body itself is also the reason why yaki-nao-shi has continued to be selected as a means of restoration.

The Technical Process of Yaki-Nao-shi

In principle, yaki-nao-shi must be performed by the sword smith who originally created the sword or a sword maker of equal or greater skill. In modern times, it is considered a dedicated task of the sword smith.

1. Tsuchi-Otoshi (Removing Clay): Removing dirt and traces of previous quenching from the sword's surface.
2. Tsuchi-Oki (Applying Clay): Applying clay before quenching — designing the hamon pattern. Whether to reproduce the original hamon or create a new design is at the sword maker's discretion.
3. Yaki-Ire (Quenching): A process similar to the original manufacturing, but steel that has already been quenched once may have accumulated internal structural fatigue, and the results may not be exactly the same as the first quenching. In particular, the steel of ko-katana that have undergone extensive folding and forging carries the risk of intergranular corrosion from re-heating.
4. Togishi (Polishing): Finishing work is necessary.

Since the process proceeds through a series of steps from removing clay to polishing, yaki-nao-shi is not a task that can be completed in a short time, but rather a labor-intensive restoration that depends on the skill of both the sword smith and the polisher.

Clues for Distinguishing Whether Yaki-Nao-shi Has Been Performed

It is said that there are several clues that experts look for in sword bodies on which yaki-nao-shi has been performed. A representative example is the contrast between the condition of rust remaining on the tang (nakago) and the condition of the hamon. The tang is protected by being covered with clay during quenching, so in principle it should accumulate rust from the same number of years as the blade body itself, but if the impression of the hamon and the aged appearance of the rust on the tang appear inconsistent, this becomes one piece of evidence suggesting yaki-nao-shi.

Additionally, irregularities in the boushi (quenching on the point section) and unnatural divisions at the boundary between the jihada and hamon can also be cited as factors for judgment. However, evaluating such characteristics requires specialized knowledge and experience, and cannot be determined solely by amateur observation. Only connoisseurs who have seen numerous swords and public appraisal institutions like NBTHK can make definitive judgments about whether yaki-nao-shi has been performed.

Limitations of Yaki-Nao-shi — Issues of Appraisal and Technique

Yaki-nao-shi has significant limitations in both technical and appraisal aspects.

• Reduction in Appraisal Value: In NBTHK examinations, if a sword is confirmed to have undergone yaki-nao-shi, its rating may be significantly reduced because it becomes difficult to evaluate the work style of the original period and sword maker. Particularly when ko-katana from the Kamakura and Nanboku-cho periods have been re-quenched, the aging changes in the jihada are combined with the re-quenching, resulting in something different from the "original hamon of ko-katana." Swords noted in the appraisal certificate as "yaki-nao-shi present" tend to experience a significant decline in market value.
• Risk of Physical Deterioration: When re-heating the blade, there is a risk of minute cracks forming around the hamachi and monouchi areas. Blades with a history of use and storage have accumulated internal stress, and the behavior during rapid cooling becomes unpredictable.
• Difficulty in Reproducing Hamon: The complex hamon of ko-katana (such as midare-chooji, inazuma kinkei, and tobi-yaki) are the product of the original sword maker's technique in applying clay and their quenching intuition, and it is practically impossible for later sword makers to reproduce the identical hamon. It is not uncommon for the hamon after yaki-nao-shi to be judged as "inferior to the original."

What to Verify When Considering Purchase or Ownership

When considering a sword with a history of yaki-nao-shi, the basic first step is to check the contents of the appraisal certificate. If the appraisal certificate has passed the screening of an appraisal institution, it will reflect whether yaki-nao-shi was performed and the evaluation based on that fact, making it an important factor for decision-making before purchase. Additionally, if documentation remains that explains the sword's provenance and history of transmission (such as who owned it and when, and how it came to be re-quenched), it can be helpful in understanding the sword's value.

If you suspect yaki-nao-shi has been performed on a sword you already own, it is best to consult with a trustworthy appraisal institution or specialist before making any changes based on your own judgment. Improper handling could place additional stress on the blade.

The Significance of Yaki-Nao-shi in Modern Times

While yaki-nao-shi often receives negative evaluation, it is also an essential technique in the context of sword restoration. When historically significant masterpiece swords have lost their hamon due to fire or war damage, choosing partial recovery through yaki-nao-shi over complete loss can be a rational decision from the standpoint of cultural property preservation.

Among renowned contemporary Living National Treasure sword smiths, there are examples of performing yaki-nao-shi as a form of "revival" rather than "repair" on damaged swords by senior sword smiths. Out of respect for the original sword maker, it is sometimes expressed as "restoration," but technically it is the same process.

Yaki-nao-shi is simultaneously "imperfect recovery" and "an act of continuing the life of the sword." From the perspective of viewing Japanese swords not merely as objects but as cultural entities possessing life, there is room for positive evaluation as the best choice within limitations. TOUKENZA handles swords with detailed provenance information including the presence or absence of yaki-nao-shi history, and you can carefully review this online while considering your purchase.