The Science of Rust—Formation Mechanisms of Ubu-sabi, Aka-sabi, and Kuro-sabi on Japanese Swords and Conservation Treatment Techniques

What is Rust? — The Phenomenon of Iron Oxidation

Metal corrosion is a phenomenon in which iron (Fe) reacts with oxygen (O₂) or water (H₂O) in the air to generate oxides and hydroxides. This chemical change is also called "corrosion," and is generally perceived as a harmful phenomenon that degrades metals.

However, in the world of Japanese swords, the evaluation of "rust" is not uniform. Certain types of rust that appear on old swords do not diminish the sword's value; rather, they are highly valued as "evidence of provenance." On the other hand, rust caused by improper storage can render the blade irreversibly damaged. To understand this distinction, one must understand the chemical nature of rust.

Ubuzumi (Patina Rust) — A Stable Layer Built by Time

"Ubuzumi" (patina rust, original rust) refers to the black-brown to gray-black rust layer that develops on the nakago (tang) of ancient swords that have been left unattended for long periods. While it is undoubtedly rust to the naked eye, this rust is something that should be protected as "important information indicating the sword's provenance" in sword appraisal.

From a chemical perspective, the primary component of ubuzumi is magnetite (Fe₃O₄), which forms a stable oxide layer. The magnetite layer is dense and compact, functioning as a "passivation layer" that protects the underlying iron from further corrosion. In other words, ubuzumi is an "iron self-defense layer" formed over a long period of time.

The reason ubuzumi is valued in sword appraisal is because of its stability. Ubuzumi formed over centuries cannot be reproduced by artificial means in the short term. By observing the ubuzumi on the tang, one can infer the time the sword has endured, its storage environment, and whether post-period alterations such as "resharpening" have been performed. This is why polishing craftsmen avoid polishing the tang and protect the ubuzumi.

Akasabi (Red Rust) — The Primary Culprit of Iron-Eating Corrosion

"Red rust" is the orange to reddish-brown rust that most people associate with the word "rust." Chemically, iron(III) oxide (Fe₂O₃, hematite) is the representative component of red rust, and FeOOH (hydrated iron hydroxide such as goethite) is also included as a component of red rust. These form rapidly in the presence of moisture.

Red rust has a porous and brittle structure with poor adhesion to the iron surface. Because of this, red rust does not function as a protective film; external oxygen and moisture continue to reach the interior, accelerating corrosion. This is the so-called "rust begets rust" state.

The main conditions under which red rust develops on Japanese swords are as follows.

• High-humidity environments (relative humidity of 70% or higher): Moisture condenses on the surface, and electrochemical corrosion progresses
• Hand oils and salts: Chloride ions contained in fingerprints act as catalysts for rust
• Loss of rust-preventative oil: Oil films from clove oil or camellia oil inhibit oxidation, but when the oil evaporates, corrosion accelerates rapidly
• Acidic atmosphere: Acidic substances such as sweat and wood gases (formic acid, acetic acid) function as catalysts

When red rust develops on the blade surface or ground steel, even after the rust is removed, a rough surface called "rushed texture" (sabihada) remains, significantly impairing appreciation of the ground finish and blade pattern. Severe red rust requires removal by polishing, but the blade becomes thinner with each polishing, making red rust the greatest enemy that "shortens the sword's life."

Kurozumi (Black Rust) — A Managed Oxide Protective Layer

While being a corrosion product of iron just like red rust, "black rust" (kurozumi) has completely different properties. The primary component of black rust is the aforementioned magnetite (Fe₃O₄), forming a dense layer with high adhesion.

Under natural conditions, the formation of a magnetite layer is an extremely slow process (tens to hundreds of years), but artificially it can be intentionally generated through "black finishing" (kurozume) or "black burning" (kuroyaki). By applying specific chemical treatments (phosphate treatment, high-temperature steam treatment) to iron products, an artificial black rust layer of magnetite is formed on the surface, preventing subsequent corrosion—a technique widely employed in modern industry.

The "black finish on file marks" (yasurieme no kurome) intentionally applied to the tang (nakago) of swords is a traditional rust-prevention treatment applying this principle. Additionally, a technique called "artificial rust" (tsukurisabi), where the tang is intentionally allowed to rust to some degree to conceal the original ground finish, is also practiced in the world of fine art and antiques.

Rust Prevention by Polishing Craftsmen and Preservation Specialists — Practical Rust-Prevention Techniques

The techniques practiced by polishing craftsmen (togishi) and preservation specialists in rust prevention and rust treatment of Japanese swords can be organized as follows.

Fundamentals of rust prevention — Oil application: A thin film of clove oil (chojiaabura) or white oil (hakuyu) is applied to cut off contact with oxygen and moisture. Excessive oil conversely attracts dirt and promotes corrosion, so an appropriate amount is required. Regular replacement is recommended (every 2-3 months in summer, every 6 months in winter).

Early red rust treatment — Brushing with flannel and polishing powder: Minor red rust (spot rust, early-stage rust) can sometimes be removed by gently wiping with medical cotton or flannel. Polishing with uchiko (polishing powder) has the effect of completely removing oil and temporarily drying the blade, enhancing the adhesion of subsequently applied oil.

Treatment of severe rust — Polishing: Red rust that has penetrated the blade surface or corrosion of the ground steel has no fundamental solution other than polishing by a craftsman. Since polishing reduces the thickness of the blade, avoiding unnecessary polishing contributes to the sword's longevity. In the world of polishing craftsmen, the ethical principle of "minimum necessary polishing" is highly valued.

Storage environment management: A stable environment with relative humidity of 50-60% and temperature of 10-25°C is considered ideal. Storing in a sword bag (toubukuro) or sword box (katanabaako) is effective in blocking outside air, but over-sealing can allow wood gases to accumulate, creating an acidic atmosphere, so caution is necessary.

Reading a Sword's Provenance from Rust

The distribution pattern of rust can serve as a clue to "read" the storage history of a sword. If red rust appears only on a specific part of the tang, one can infer a situation where that part was exposed to moisture (condensation forming inside the scabbard, etc.). If the entire blade surface is uniformly rusted, long-term neglect (loss of oil protection plus high humidity) is suspected.

Scientific rust analysis—X-ray diffraction (XRD) and infrared spectroscopy (IR)—research is also progressing to estimate the generation period from rust composition. Currently, it has not reached the level of distinguishing "rust from several hundred years ago versus rust from several decades ago," but rust composition analysis through chemical analysis is utilized as auxiliary information in authentication appraisals.

Rust, seemingly a "negative" phenomenon, encompasses positive information—"provenance," "time," and "preservation condition"—in the world of the Japanese sword. This paradoxical perspective of evaluation demonstrates the depth of Japanese sword culture, which regards the Japanese sword not merely as a weapon or tool, but as a "living entity imbued with time."

Differences Between Ubuzumi, Akasabi, and Kurozumi — Quick Reference Table

Frequently Asked Questions

Is it okay to remove rust from a Japanese sword myself?

Using sandpaper, abrasive powders, or commercial rust removers to remove red rust from the blade surface is strictly forbidden. This will ruin the ground finish and blade pattern, significantly damaging the sword's artistic value. Treatment should be limited to very gently wiping away only the earliest stage of spot rust with flannel fabric; anything beyond this should be left to consultation with a polishing craftsman. For details on the cost and process of sword restoration and repair, please refer to Explanation of Japanese Sword Restoration and Restoration Business.

Should rust on the tang (nakago) be removed?

It must absolutely not be removed. The ubuzumi on the tang is itself proof of provenance created by centuries of time, and is first-rate material for dating and authentication appraisals. A sword that has been worked on at the tang suffers a significant drop in appraised value.

How often should rust-prevention maintenance be performed?

Rust-preventative oil (such as clove oil) should be reapplied as the basic rule, with a target of every 2-3 months in summer and every 6 months in winter. The intervals should be shortened in high-humidity seasons. For long-term storage, maintaining a stable environment of 50-60% relative humidity is more important than oil application.

Is black rust intentionally applied?

On the tang only, it is intentionally applied as a traditional rust-prevention treatment (black finish) to protect the file marks. Black rust is not applied to blade surfaces polished to a mirror finish.