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.

• Abrasion
• Rolled Edge
• Flattened Edge
• Chipped Edge
• Corrosion
• Self Mutilation

Abrasion 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 dulling.
Rolled Edge 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.
Flattened Edge 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.
Chipped Edge 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 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).
Self Mutilation 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 fix.