What Is Kizu? 疵 — Sword Flaws & Blemishes Explained

Kizu (疵) is the general Japanese term for any flaw, blemish, or defect in a sword blade — from harmless surface marks to fatal cracks that destroy a blade's value and function. In nihonto collecting, kizu covers everything from a shallow scratch to a hagire crack through the hardened edge, and knowing which flaws are cosmetic and which are catastrophic is the single most important skill a buyer can have.

Every antique Japanese sword has a story written in its surface, and part of that story is its condition. Some kizu are minor and expected on a blade centuries old; others render an otherwise fine sword worthless. This guide explains the main types of kizu, how serious each one is, and how flaws shape the price and desirability of a blade.

What counts as a kizu

Kizu (疵, also written 傷) means "flaw" or "wound." On a nihonto it refers to any imperfection in the steel or the finish, whether it originated during forging, during use, or from later mishandling and neglect. Collectors sort kizu into two broad categories that determine everything about a blade's value:

  • Forging flaws — defects born in the smithy, from imperfect folding, welding, or hardening of the tamahagane steel. These are permanent and cannot be polished out.
  • Acquired flaws — damage from use, corrosion, or bad handling: chips, bends, rust, and scratches. Some are correctable by a skilled polisher; some are not.

The critical distinction is between flaws that merely mar appearance and flaws that compromise the blade's structural integrity or its hardened edge. A blade can carry cosmetic kizu and still be a treasure; a single fatal kizu can reduce a masterwork to a study piece.

The main types of kizu

These are the flaws a collector inspects for during kantei (appraisal). Learn to recognize them by name:

  • Hagire (刃切れ) — a crack crossing the hardened edge and the hamon. This is the single most serious flaw a blade can have: it is fatal, it kills both value and function, and it cannot be repaired. See our full guide to hagire.
  • Ware (割れ) — an open split or gap in the steel skin (jigane), from a burst blister or an opened forging seam. Severity depends entirely on location and depth; read our page on ware.
  • Sabi (錆) — rust or corrosion. On the blade it is a flaw to be removed by a professional; on the tang it is prized patina. See sabi for this vital distinction.
  • Fukure (膨れ) — a blister, a subsurface pocket of trapped gas or slag from forge-welding. Harmless while closed, but it becomes a ware if it bursts under polishing.
  • Kirikomi (切込) — a nick or notch in the edge from blade-to-blade contact in combat. On an old blade these are often left as honorable battle scars.
  • Hakobore (刃毀れ) — chipping of the cutting edge, from hard use or impact. Small chips can be polished out; large ones require significant metal loss.
  • Shinae (撓え) — a fold or "wrinkle" in the steel, a stress line from the blade being bent and straightened. A weakness that can develop into a crack.

How kizu affects value — the buyer's angle

For a buyer, condition is money. Two blades by the same smith, of the same era and quality, can differ enormously in price based solely on their kizu. Understand the hierarchy of severity:

  • Fatal flaws — hagire above all, and deep ware that reach the edge. These blades cannot pass the higher levels of NBTHK shinsa and are sold as study pieces at a fraction of their potential value.
  • Serious flaws — significant fukure near the edge, deep rust pitting, large chips. These depress value substantially and may limit which papers a blade can earn.
  • Minor flaws — small closed fukure high on the blade, faint hada openings, light surface scratches. Expected on old blades; a good togishi (polisher) can address most, and they have modest impact on value.

Two warnings for buyers. First, never buy a blade "in polish" without inspecting for hagire under good light — a fresh polish can visually distract from a fatal crack. Second, some flaws are only revealed by polishing: a closed fukure can burst into a ware on the stones, so a blade sold "as found" carries risk. When in doubt, buy papered blades and have an expert inspect condition first.

Frequently asked questions

What is a kizu on a Japanese sword?

A kizu (疵) is any flaw or defect in a sword blade — a catch-all term covering forging faults, cracks, rust, chips, and blisters. Kizu range from harmless cosmetic marks to fatal structural cracks, so the word alone does not tell you how serious a problem is.

Which sword flaws are fatal?

The fatal flaw is hagire (刃切れ), a crack crossing the hardened edge, which destroys both value and function and cannot be repaired. Deep ware (splits) reaching the cutting edge, and cracks running through the blade, are also considered fatal or near-fatal.

Can kizu be repaired?

It depends on the flaw. A skilled polisher can remove light rust, minor scratches, and small chips by carefully reducing metal, but forging flaws such as hagire, deep ware, and blisters are permanent and cannot be polished out. Repairing acquired damage always costs some of the blade's steel.

Do all antique swords have kizu?

Nearly all centuries-old blades carry some minor kizu, and modest cosmetic flaws are expected and accepted on antiques. What matters is whether the flaws are cosmetic or fatal: a blade with small, high-placed imperfections can still be a fine collectible, while one with a single hagire is severely compromised.

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