What Is Rust? — The Phenomenon of Iron Oxidation

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

However, in the world of Japanese swords, the evaluation of "rust" is not uniform. Certain rust that develops on old swords does not diminish the sword's value; rather, it is highly valued as "proof of the sword's provenance." Conversely, rust that develops from improper storage can render the blade irreversibly damaged. To understand this distinction, one must grasp the chemical reality of rust.

Ubu-zumi (Patina Rust) — The Stable Layer Built by Time

"Ubu-zumi" (patina rust, or initial rust) refers to the dark brown to gray-black rust layer that develops on the nakago of old swords left unmaintained for extended periods. While there is no question that it is rust to the eye, in sword appraisal this rust is protected as "an important information source that demonstrates the sword's provenance."

From a chemical perspective, the primary component of ubu-zumi is magnetite (Fe₃O₄), which forms a stable oxide layer. Because the magnetite layer is dense and fine-grained, it functions as a "passivation layer" that protects the interior iron from further corrosion. In other words, ubu-zumi is a "self-defense layer" formed by iron over a long period of time.

The reason ubu-zumi is valued in sword appraisal lies in its stability. Ubu-zumi formed over hundreds of years cannot be reproduced by artificial means in the short term. By observing the ubu-zumi on the nakago, one can infer the time the sword has experienced, its storage environment, and whether it has undergone later modifications such as "ateageito." This is why polishers avoid polishing the nakago and protect the ubu-zumi.

Aka-zumi (Red Rust) — The Primary Culprit in Iron Corrosion

The orange to reddish-brown rust commonly associated with the word "rust" is "aka-zumi." Its chemical primary components are hydrous iron oxide (FeOOH/Goethite, or γ-FeOOH/Lepidocrocite), which 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, and corrosion accelerates. This is the phenomenon known as "rust calling forth rust."

The primary conditions under which red rust develops on Japanese swords are as follows:

• High humidity environments (relative humidity 70% or higher): Moisture condenses on the surface and electrochemical corrosion progresses
• Skin oils and salt from hands: Chloride ions in fingerprints act as catalysts for rust
• Depletion of rust-preventive oils: The oil film created by clove oil and camellia oil inhibits oxidation, but when the oil evaporates, corrosion accelerates rapidly
• Acidic atmosphere: Acidic substances such as perspiration and wood gases (formic acid, acetic acid) function as catalysts

When red rust develops on the blade surface or ji-tetsu, even after rust removal, a rough surface called "sabi-hada" remains, significantly impeding appreciation of the ji-hada and hamon. Severe red rust requires removal through polishing, but with each polishing the blade grows thinner, making red rust the greatest enemy of the sword's "lifespan."

Kuro-zumi (Black Rust) — Managed Protective Oxidation Layer

While possessing the same origins as red rust as an iron corrosion product, "kuro-zumi" (black rust) has entirely different properties. The primary component of black rust is the aforementioned magnetite (Fe₃O₄), which forms a dense layer with high adhesion.

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

The "yasuri-me no kurome" (blackening of file marks) intentionally applied to the nakago of swords is a traditional rust-prevention technique applying this principle. Additionally, a technique called "tsukuri-zumi" (created rust), in which the nakago is intentionally rusted to conceal the original grain of the base metal, is also practiced in the world of antiques.

Rust Prevention Techniques by Polishers and Conservators — Practical Rust-Prevention Technology

The techniques practiced by togishi (polishers) and conservators in rust prevention and rust treatment of Japanese swords can be organized as follows.

Fundamentals of rust prevention — Oil application: Small amounts (thin film) of clove oil (choji-abura) or white oil (haku-yu) are applied to sever contact with oxygen and moisture. Excessive oil conversely attracts dirt and promotes corrosion, so the appropriate amount is required. Regular replacement is recommended (every 2–3 months in summer, every 6 months in winter as a guideline).

Early red rust treatment — Flannel wiping and uchiko: Light red rust (spot rust, early-stage rust) can sometimes be removed by gently wiping with medical cotton or flannel fabric. Polishing with uchiko (polishing powder) removes oil completely and temporarily dries 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 ji-tetsu has no fundamental solution other than polishing by a togishi. Because polishing reduces the blade's thickness, avoiding unnecessary polishing contributes to the sword's longevity. The ethical principle of "minimal necessary polishing" is valued in the world of polishers.

Storage environment management: A stable environment with relative humidity of 50–60% and temperature of 10–25°C is considered ideal. Storage in a to-bukuro (sword bag) or katana-bako (sword box) is effective for isolation from outside air, but excessive sealing can cause wood gas to accumulate and create 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" a sword's storage history. If red rust appears only in certain areas of the nakago, one can infer that those areas were exposed to moisture (such as condensation forming within the saya). If the entire blade surface is evenly rusted, prolonged neglect (oil depletion + high humidity) is suspected.

Scientific rust analysis — X-ray diffraction (XRD) and infrared spectroscopy (IR) — has enabled researchers to estimate the generation period of rust based on its composition. At present, the level of distinguishing "rust from several hundred years ago versus several decades ago" has not been reached, but compositional analysis of rust through chemical analysis is being utilized as supplementary information in authentication.

Rust, at first glance a "negative" phenomenon, in the world of Japanese swords embodies positive information: "provenance," "time," and "preservation condition." This paradoxical evaluation reflects the depth of Japanese sword culture, which regards the Japanese sword not as a mere weapon or tool but as a "living entity containing time."