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Swords, Cutting and Military History

Hidden dangers: inspecting swords for flaws in practical cutting

Hidden dangers: inspecting swords for flaws in practical cutting

photo by Byronv2

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Most cutting groups have a standard safety inspection of blades prior to the actual start of cutting.

Swords are inspected by the owner or a qualified individual to ensure they possess no weak points or flaws which could cause the blade to break during cutting.  Some flaws are glaringly obvious, such as a large crack, but some are extremely subtle and very hard to spot.

Flaws include those caused during forging / manufacture,  those caused by usage, and those caused through poor sharpening,  polishing or re-tempering.

Today we’ll take a look the the general topic of sword flaws, drawing on the highly detailed descriptions of flaws  (kizu) used by experts who appraise Japanese art swords.  This is a process known as “kantei“.

When I speak of art swords, I refer to antique nihonto (Japanese swords, e.g. katana, wakizashi / shoto, tanto, etc.) which are collected for their age, craftsmanship, artistic beauty, and the fame of the smith (or lineage of smiths) who made them. All of those factors, and more, are taken into consideration for the appraisal of an sword as a work of art.

The “kizu” (flaws) described  in the kantei process are quite well defined, as you might expect from such a well-establish and rigorous system of appraisal.  The references and terminology of kizu are quite extensive, defining exactly certain specific kinds of flaws in very specific areas.  As this level of description is much too involved for the inspection of practical cutting blades, I’ve used only the major kizu classifications for the purposes of this article.  An example of just a few kizu can be seen here:


from KizuThe Japanese Sword Index

from Kizu The Japanese Sword Index


It goes without saying those tasked with inspecting blades  would do well to err on the side of caution when confronted with any potential defect.  The following list of flaws is only a suggestion; never discount your own personal experience with live blades.  If something about a blade gives you a bad feeling, even if there are no visible fatal flaws, I would strongly suggest going with your gut instinct.

For those who wish more information on the topic, and to see photographs of actual kizu on blades, please check the links listed in the Sources section found at the end of the article.


Classes of Kizu

Generally speaking, kizu are divided into two groups:  Fatal and Non-fatal flaws.  From the art sword point of view, non-fatal flaws decrease the value of a blade;  fatal flaws ruin it.   For a practical cutting blade most non-fatal flaws should be taken as signal the blade need care, sharpening or polishing;  in many cases those inspecting the blade may wish to  disallow the use of blades exhibiting some of these defects based on their size, depth or placement.  Blades with fatal flaws should absolutely not be used for practical cutting, and in most cases are not repairable.


How to inspect a blade

While there is no set procedure, in general an inspector will check that:

  • The hilt fits tightly onto the tang of the sword,  and any retaining screws or pegs are in place and fully set. Some cutting organizations require that two sets of retaining screws or pegs be used for cutting blades. A single peg may slip or break, releasing the blade from the hilt.  On the other hand, a single peg also suggests the tang does not run deeply enough into the hilt to supply proper support for the blade while cutting.
  • There should be no movement in the cross-guard and supporting elements.
  • If the sword is shaken, there should be no slippage or movement of the tang within the hilt.
  • The entire cutting edge is inspected for cracks or chips.  A bright light source is recommended for this inspection, as edge cracks can be almost invisible
  • The sides of the sword are inspected for pits, breaks, lamination issues, blisters, cracks and chips
  • The spine (or other edge) is inspected for the above

The following video shows, very broadly and in Japanese fashion, the way a sword can be inspected before use.  Note the Sensei makes sure to hold the blade up to the light, so any tiny cracks or imperfections can be noticed.



Non-fatal Flaws

Blisters (fukure):  A sign of poor folding during forging, it appears as a raised / domed blister on the side of a blade, caused by poor lamination (layering) of the folds.  Common in cheaper cutting blades, in most cases it is on the surface layer only and should not cause an issue.

Chips in the edge (ha-kobore):  With use and wear, it’s inevitable that blades will develop small chips in the hardened edge.  These can also be caused by striking hard objects.  Blades with chips which do not extend beyond the the very edge can temporarily be used for cutting, but the owner should be warned that the blade needs sharpening / polishing if they wish to continue cutting with it.  Larger chips, or breaks which extend past the tempered edge should not be allowed in cutting, and are likely not repairable without an expert re-tempering the blade.

Faded / Missing tempering line (nioi-gire):  For blades with a defined hamon (temper line), this appears as an area where the line fades out, or dips right to the cutting edge or the inner edge of the temper line, so as to disappear.  This indicates the possibility of poor tempering / hardening, but could also be caused by poor polishing of the edge (the temper line exists, it is just not visible).  Inspectors may wish to disallow such a blade, but they can also conduct a scratch hardness test.  These are small pieces of metal with points of a known hardness which you lightly press into the metal being tested, to make a tiny mark or scratch.  If a test strip can make a mark in one of the faded temper zones, and not on the normal temper line — or if one scratch is noticeably deeper than the other — you will know there is an issue with the edge temper / hardness.  Scratch hardness testers are available at many hardware and most industrial supply stores.

Open layer (kitae-ware):  Found in folded steel swords, these are tiny surface breaks or gaps which indicate poor folding technique during manufacture.  Common in cheaper cutting blades,  these should not cause an issue.

Over-hardened steel:  Where the temper line (hamon) of a blade is exceptionally bright and shiny — looking almost like a mirror finish — there can be some concern that the edge had been over-hardened, creating the potential for the edge to crack, chip or shatter.  While not a fatal flaw, use of a blade suspected of being over-hardened should be carefully monitored and re-checked during the cutting session

Patched steel (umegane):  A spot where  steel has been inlaid into a pit or break in an attempt by the manufacturer to “repair” a flawed blade.  There is no way of knowing how deep the filled flaw may be, nor how it will react to the stress of cutting.  The inlaid metal does not add to the strength of the blade, so due concern should be shown where patched steel is noted.

Rust pits (kuchikomi):  A spot on the blade where rust has eaten a pit, and the rust is still visible.  Indication of poor blade care, and possibly of poor sharpening / polishing.  In folded steel blades, if the rust has eaten past the hard surface steel into the core it creates a weak point where stress could cause the blade to break.  In mono-steel blades, only very deep rust pits should cause concern.

Wrinkles (mukade-shinae):  If a blade has been bent and straightened, it’s possible the surface  of the blade which was at the acute angle of the bend will develop what appear to be very minor wrinkles.  These are not cracks, but simple surface “stretch marks” where the steel stretched slightly during the bend, then was pushed back as the blade was straightened .  So long as no actual cracks into the blade’s surface are found, it should not affect the blade in cutting.


from Kizu visual glossary for beginners - discussion thread, The Japanese Sword Forum

from Kizu visual glossary for beginners – discussion thread, The Japanese Sword Forum
Click to enlarge

Fatal Flaws

Cracked or broken tip: The very end tip of a blade is broken off.  Provided the break does not extend past the temper line, the blade is safe to use.  If it does, it is a fatal flaw.  Cracks on the tip follow the next set of rules.

Cracks in the temper line / edge (yaki-ware):  An extremely serious flaw, indicating the sword may break off during cutting, they can be very hard to spot.  These cracks can be caused by the stress of cutting;  stress cracks tend to run from the blade edge straight across the blade, or be at a slightly oblique angle.  They can also be caused by poor tempering / manufacture, in which case the crack tends to follow the line of the temper;  in use, these cracks could literally cause a section of the tempered steel to shatter, rather than crack across the width of the blade.

Cracks on the spine (karasaguchi): For single edged blades, cracks noted on the spine (back) of the blade are extremely serious, indicating a stress fracture.

Many fine cracks (shinae):  Very poor quality steel, or steel which includes slag or other major inclusions, will show areas where the surface of the blade looks rough, with many fine-to-medium sized cracks.  These can occur anywhere on a blade.  Absolutely should not be used for cutting.  Note this is different from the Non-fatal flaw of wrinkles (mukade-shinae)

Tired blade (tsukare): Caused by poor or over-sharpening or polishing in folded steel blades.  An area of darkness on the steel shows the surface hardened steel case has been worn through, exposing the softer core steel.  This creates a weak spot in the sword which can easily cause the blade to bend, crack or even shatter.



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