Magnetization and Demagnetization of Nihontō: The Reality of Magnetic Issues Arising During Forging and Storage, and How to Address Them

As nihontō are iron and steel products, they can become magnetized. There are situations in a tōshō's workshop where magnets are used, and depending on the storage environment, a sword may become magnetized without anyone realizing it. The topic of "sword magnetization" rarely surfaces in public discussion, yet it is an important theme in the world of nihontō as a practical issue affecting appraisal, storage, and use. This article explains the causes of magnetization in nihontō and the practical aspects of demagnetization treatment.

Why Nihontō Become Magnetized

The primary component of nihontō is iron (Fe), and iron belongs to the category of ferromagnetic materials. When an external magnetic field is applied to a ferromagnetic material, it becomes magnetized, and under certain conditions a "remanent magnetization" persists.

There are several main causes of magnetization in nihontō.

• Effects during the forging process: When steel is heated and hammered, the magnetic domain structure of the iron changes. Normally, magnetism disappears at high temperatures (above approximately 770°C, the Curie point, ferromagnetism is lost). However, if an external magnetic field is present during the cooling process, there is a possibility that the magnetization becomes fixed in that direction. Historically, swords cooled under the influence of the geomagnetic field (the Earth's magnetic field) may acquire a weak remanent magnetization aligned with that field.
• Contact with tools and equipment: In a modern tōshō's workshop, there are devices containing power tools, electric motors, and electromagnets. Prolonged exposure to the strong magnetic fields of these devices, or direct contact with them, can cause a sword to become magnetized. Cases are cited in which a togishi working near an electric motor used to rotate a whetstone inadvertently brought the sword too close.
• Magnetic fields in the storage environment: Long-term storage near everyday sources of magnetism—speakers, electromagnetic clamps, display stands with built-in magnets, refrigerator doors—can also cause progressive magnetization. When museums or private collectors use magnets in the stands that hold swords, the effect can sometimes be non-negligible.

Problems Caused by Magnetization

There are three main perspectives from which to consider what problems arise when a sword is magnetized.

• Effects on appraisal: Appraisal of nihontō involves scrutinizing a wide range of elements—hamon, jigane, shape, and more—but a strongly magnetized sword attracts iron particles, making it easy for fine iron filings to adhere to the jigane surface. This adhering material can alter the appearance of the jihada and hinder accurate appraisal. Immediately after togi in particular, the togi powder contains fine iron particles, and such adhesion becomes conspicuous on a magnetized sword.
• Risk of accelerated rusting: On a magnetized iron surface, fine iron particles are more easily attracted electrostatically, and these can act as nuclei for rust. Particularly in storage environments where humidity control is inadequate, the chain of magnetization → iron particle adhesion → localized oxidation → rust progression can occur.
• Effects on togi work: When a togishi polishes a magnetized sword, the togi powder from the whetstone (fine whetstone particles plus detached iron filings) tends to stick to the blade surface. This can cause togi scratches or uneven luster. It is said that skilled togishi are sensitive to such changes and will sometimes check the magnetization state of a sword before beginning togi.

Methods and Practice of Demagnetization

There are several methods for demagnetizing a magnetized nihontō.

• AC demagnetization (kōryū datsuji): This is the most common method. By applying an alternating magnetic field while gradually reducing the field strength, the magnetic domains are rearranged in random directions and the remanent magnetization is removed. A device called a shōjiki (demagnetizer) or degausser is used. The basic procedure is either to slowly pass the sword through the coil of the shōjiki, or to start the shōjiki near the sword and gradually move it away while keeping it running.
• Demagnetization by heating: When steel is heated above the Curie point (approximately 770°C), its magnetism is lost, and if no external magnetic field is present during cooling, the magnetization does not return. However, because this method applies heat to the sword itself, there is a risk of changes to the hamon and distortion, and it is not normally used on swords of cultural property value. It is understood as a naturally occurring phenomenon during the process of a tōshō making a new sword, but intentional application to a finished sword is avoided.
• Reverse magnetic field using permanent magnets: In theory, applying a strong reverse magnetic field can achieve demagnetization, but excessive reverse magnetization also carries the risk of producing remanent magnetization in the reverse direction. This method requires difficult precision control and is considered unsuitable for general sword management.

In practical terms, AC demagnetization using a shōjiki is the safest and most reliable method. While it is a routine procedure in the manufacture of precision machinery and medical equipment, it is not necessarily widely recognized in the world of nihontō. Some sword dealers and togishi do keep a shōjiki on hand and may perform a magnetization check as a routine step before restoration or togi.

Everyday Magnetization Prevention and Checking

For sword collectors and iai-dō practitioners, the everyday preventive measure to be aware of is the choice of storage location. The basic guideline is to store swords at a distance of at least 30 cm from speakers, refrigerators, computers, magnetic storage accessories, and similar items.

For a simple check of whether a sword is magnetized, a compass can be used. When a compass is brought near a magnetized sword, the needle will be seen to move toward the sword. This check is a convenient method that requires no special equipment.

Nihontō are simultaneously refined works of metal craftsmanship and actual tools that are fully subject to the physical properties of iron and steel. Magnetization is one of those physical properties, and understanding it correctly and managing it appropriately is what makes it possible to pass swords on to the next generation in good condition.