Is my sword sharp?
This section discusses what makes a sword dull and how you can test the edge. Everyone always asks if their katana is sharp enough. There are
plenty of questionable tests like cutting paper, shaving the hair off your arm,
or scraping your thumb on the edge with a sage expression on your face. If
you are asking the question, you probably already know the answer. It is
important to understand what makes your katana dull. We have found six
things you should watch out for. These are Abrasion, Rolled Edge,
Flattened Edge, Chipped Edge, Corrosion, and Self Mutilation. We cover
each of these in their own section. If you are wondering about a new
sword, you should also check out the edge geometry section. A good
sharpening should last 6 months to a year, but every sword is different.
You can simply wear down the edge of your sword by cutting materials
containing abrasive particles. We don't think many people cut sand paper,
but it is surprising how abrasive some targets are. Simple paper targets
can quickly dull your sword. Used tatami targets can be filled with
abrasive sand and dust. People cut every type of material and some are
quite abrasive. The edge of the sword is simply worn away. The good
old thumb scraping test works pretty good for this type of problem. Just
make sure you scrape your thumb across the edge and not down the edge. No
need to get all that blood on your sword. The sage expression is optional,
but recommended. Swords made from some types of steel and swords without
traditional differential hardening are much more susceptible to abrasive
The carpenters out there know all about this kind of edge. A cabinet
scraper is sharpened and then the edge is purposely rolled over. While
this works great for putting a smooth finish on wood, it works pretty badly for
your katana. Turning your sword in a hard or medium density target can
roll the edge over. This is more likely for softer steels or extremely
sharp edge geometries. There is a really simple test to detect this type
of problem. Slide your thumb from the shinogi (ridge line) to the ha
(edge). Don't slide your thumb in the other direction! If you feel a
slight bur, the edge has rolled over. You need to check both sides of the
blade along the whole length.
This is quite common for people who cut hard targets. The edge has
simply been beaten down till it is flat. It can still feel sharp to the
standard thumb test. The best way to see a flattened edge is using
reflected light. When looking straight towards the edge, if you can see
the edge, you are looking at a flattened surface.
Large chips are easy to see, but many swords end up with hundreds of very
small chips along the edge. Micro chips are common on swords with very
hard edges and extremely sharp geometry. If you have sharp eyes you might
be able to see them by backlighting the sword. You might need a magnifying
glass if you are vision impaired like the rest of us. If your sword
exhibits micro chips after cutting soft or medium density targets, you may need
a different geometry edge on your sword.
Corrosion can dull the edge of your sword and cause the sword to bind up in
the target. Using a chemical polishing compound will remove the corrosion
but will still leave surface pitted. Using a chemical acid to bring out
the hamon can also dull the edge. Soaked targets like tatami and beach
mats leave corrosive residue on your sword. They can also leave pieces of
the target stuck in the saya. Slight discoloration is okay for high carbon
steel, but swords should be cleaned ASAP after being used. Debris can be
removed from the saya by lightly tapping the koiguchi (saya mouth) on a hard
surface (just don't break the horn koiguchi).
This category is reserved for all the things people do to mess up their
swords. Having the edge feel sharp is not as important as maintaining the
proper edge and surface geometry. A sword is not sharpened like a knife.
What works for a kitchen or pocket knife can make your sword ineffective.
While we encourage people to learn how to sharpen a sword to better understand
the sword, most people will only make their sword worse. Many find that
the sword works better temporarily, but quickly dulls. After repeated
sharpening they find the sword can no longer be made to cut well. The
geometry of the edge has been lost and it will cost more for a professional to