We all dread the fatal flaw. 

These tend to be hidden on rusty blades, and revealed by polish. 

Depending on who made the blade and when, a fatal flaw will send the value to zero. Sometimes however, the balance of positives in a sword allow it to be appreciated and even paper to the top levels, with a so-called “fatal flaw” present.

Cosmetic and functional

Flaws come in two general types, cosmetic and functional. The functional flaws are the more concerning set of flaws, though the cosmetic flaws are those that normally disturb buyers.

There is no reason for anyone to have an expectation of an 800 year old sword that fought for a couple of centuries to be in flawless condition any more than you would expect your grandma to look like a 17 year old.

Old blades are old. This means they were used, polished down, they were handmade by people who learned their craft by repetition, experiment, trial and error, but had no knowledge of material science or atomic structure and how steel “works.”

As such we need to give older blades some fond relaxation of standards when it comes to flaws.

Victory is often gained by a sword whose edge is broken. It is not right to admire the ancient sword having no scar. It should show its marks of service. In the case of modern work, it is different. It should be flawless.

— The Complete Manual of the Old Sword (1793)

What the anonymous author of this work is trying to point out is that old swords will show their signs of use, and new swords are held to different standards. New swords, the presence of flaws on the surface indicate a lack of skill or poor material being used to make the sword.

Old swords, it’s just part of being old, and being well used.

Obviously, fewer cosmetic flaws will indicate better health and more skill regardless of age, but it quickly becomes unrealistic to hold up an old sword to the standards of a freshly made one.

Functional flaws

The primary concern with flaws should be in regards to functional flaws. For us today, actually, this doesn’t mean anything at all. We are applying the warrior’s standards when we talk about functional flaws.

hagire is a crack in the edge of a blade, and this is a scary flaw because it can cause the sword to break in half. As the book above points out though, many victories were had with a blade with a broken edge. So, the author is imploring us to not dismiss a great old blade just because it took on one of these problems… it’s part of the life cycle of an ancient artifact.

Still, this is the nature of the “fatal flaw” … it is fatal not because it is unattractive, but it is fatal because it interferes with the sword’s use as a weapon.

Similar to this, when the hamon is lost through fire, we have a blade that will no longer take an edge. Going through fire essentially reverses the process of putting the hamon on the blade. A new hamon can be added again, but this is never the smith’s original concept.

The sugata may end up different from intended, and the activities will never be as good as original. So in general when a blade is retempered (“saiha”) in this manner it is considered a full loss of value. Putting on a new hamon will allow it to take an edge again but the work is now considered (at best) joint work between the original maker and the artist who repaired the sword. It’s just that we know the repair can only ever be a shadow of the original, so for this case, it’s mostly considered a full loss of value. 

The other primary functional flaw that we will see is an interrupted hamon. This happens as a result of polish over centuries, or aggressive polish required to remove deep chips. Some chips are left in the blade because it is considered that losing too much material elsewhere won’t be a good tradeoff for removing a chip. 

As the manual says above, we shouldn’t regard these chips as anything to worry about as they are signs of use and part of the life cycle of a sword. That the sword went, and fought, and returned, is far more important than it going on a shelf for 800 years and never proving itself but being in mint condition. They are both in the end states that we as collectors should like to have. But, I cannot tell you how many times a potential buyer looked at my photos that were magnified 4x life size and then told me that there was a 0.1 mm chip in the blade and he was very concerned as a result.

For me, I just put my head in my hands. Spotting an extremely minor edge defect that comes from using a tool at high magnification and sub millimeter damage, is nothing that anyone should be getting their underwear tied in knots over.

I will again point to the old words above to set people straight.

If the polish though interrupts the hamon, we have two issues that come from that. The first is the obvious case that this area of the blade will no longer hold a good edge, but the less obvious case is that the whole blade is under tension created at the time of yaki-ire. This tension is released when the hamon is broken, so now we have variable tension throughout the blade and it will not properly handle impact. Such a blade is more likely to bend or fail at the point where the hamon is lost.

A subset of this problem is the lost boshi. Japanese experts consider the kissaki to be the head of the sword, and the boshi is its face. We can easily look this over because this part of the sword is small, and also difficult to polish so often times the polish is lackluster here. But healthy kissaki and boshi is a key element in passing to the topmost levels.

The oldest blades were often made with a small kissaki and thin hamon for the boshi and as a result, they wore out quickly and lost the hamon in this area. If you attempt to put a hamon back into this but without redoing the hamon everywhere, you will put an interruption in the hamon somewhere around the monouchi which is a lot worse problem. Still sometimes this type of repair is done and then the interrupted hamon in the monouchi is hidden with skillfull polishing.

As well, sometimes when a boshi is gone, a polisher will just add on an extremely fine line approximating the last few atoms of a boshi existing. Experts will know what the situation is and understand that this is done almost out of a sense of giving some dignity to the sword. When you see an oshigata showing a razor fine boshi then you need to interpret that as being in the gray zone, that the boshi is effectively gone and what remains is quasi-speculation. It shouldn’t matter much between completely gone and a few atoms left.

Age forgiveness

These issues are not acceptable on blades from the Shinto and Shinshinto period, what were considered modern when the Manual of the Old Sword was written.

The major difference is that Shinto and Shinshinto swords for the most part did not see fighting. This was a peaceful time in Japan and aside from testing their swords on peasants and the occasional scrap, there was no big reason for a sword to go into battle for the most part of the Tokugawa rule in Japan. As such, these blades have a good reason to have simply been preserved and handed down. 

Muromachi blades and older saw serious fighting and as a result of that, serious repair. On top of this, the more centuries means more chance of neglect and repair from neglect. 

These fatal flaws from above do not apply if a sword is past a certain age and is made with a certain level of skill.

You need to care about this for two reasons:

  1. so you don’t throw away something that has value because someone told you it has a fatal flaw
  2. when you buy something papered, you need to understand where the rules bend, in case you wanted to avoid a fatal flaw and you thought the papers would guide you out


In general, a fatal flaw on a Muromachi to modern blade will prevent it from papering. So you are pretty safe when you look at the papers and want to interpret it as a stamp of no such flaws on the blade.


In the Nanbokucho period, we sometimes see blades from Sadamune that underwent yaki-naoshi by Yasutsugu passing through to Juyo in spite of being saiha. Such blades will have this indicated in the Juyo paper in a very obvious way (look for 再刃 on the example below). They are few but they do exist and you need to be aware of it. This kind of blade is considered so special that it is accepted at high levels in spite of the hamon being remade. Some of the Kyoho Meibutsu Cho blades are also saiha as they went through various fires in the Edo period.  

Saiha Meibutsu Shishi Sadamune

There are also saiha works of Soshu Yukimitsu (Meibutsu Fudo Yukimitsu), Shintogo Kunimitsu, and Awataguchi Yoshimitsu that have passed Juyo. You can see the common thread here is that these are ultimate level smiths, and in all cases the work is special and so given a pass. In the case of Yoshimitsu, it has gone as high as Tokubetsu Juyo and Juyo Bunkazai while having a “fatal flaw” and in the case of Sadamune and Samonji, also to Juyo Bunkazai. 

Saiha Awataguchi Yoshimitsu (Naruse Heirloom)

This Yoshimitsu is a good example to consider when we are dealing with exceptional state plus fatal flaw.

There are only two works of Yoshimitsu known that are not tanto. The other is a tachi also burned and yaki-naoshi by Yasutsugu. That tachi is the Meibutsu Ichigo Hitofuri Toshiro, the “Once in a Lifetime Sword.”

Anyway, this blade is unusual in that it is a koshigatana or an uchigatana made by Yoshimitsu. The sword is nicely signed and the position of the signature makes it clear the blade should be worn edge up. The quality of manufacturing is outstanding and given the rarity of any long work of Yoshimitsu, all of the positives so highly outweigh the saiha state that it has passed to the highest level.

This illustrates to us that no flaw is truly fatal if there are enough outstanding counterpoints.

We do see however examples in the Kamakura period and earlier where blades will get papers with the “fatal” flaw more serving as a cap on how high the blade can progress.

Rai Kunitoshi, a very high level master from the Kamakura period, I have seen his blades at Tokubetsu Hozon with no boshi. Such a blade will not pass Juyo, but the balance of quality of the work, age of the blade, and importance of the maker has the NBTHK saying that this fatal flaw is more of a major issue with the blade. Not sufficient to stop it from passing to Tokubetsu Hozon but Juyo will be out of the question. If the smith were lesser caliber, the work more poor elsewhere, or the blade simply younger, it would not pass even to Hozon in this situation.

So those are the keys to focus on:

  1. age 
  2. skill level of the smith
  3. in spite of the flaw being present, the rest of the blade is outstanding


When we get to the Heian period, the expectations for boshi to have made it through 800 years intact further recedes. A blade with an impacted or a completely not present boshi can pass through to Juyo but not likely to Tokuju in this situation.

Ko-Bizen, Ko-Hoki, Ko-Yamashiro, Ko-Senjuin, all of these can suffer from age related boshi reduction. It’s nicer when the boshi is perfect but it’s only rarely going to happen at this point due to age.

When it comes to hagire these are often dealt with similarly on Kamakura and older blades, in that the hagire is knocked out and so made obvious. The resulting blade is not likely to pass Juyo but if the rest is significantly good as per the above three requirements, the blade will pass Tokubetsu Hozon in this condition. 


Fatal flaws are just not always fatal… this is a rule that can be bent for works of significantly high caliber and age. If your sword is neither, then this flaw will end the commercial and papering life of your sword, but it may not make it less beautiful or admirable to you. 

All flaws need to be taken with a sense of appropriate balance vs. the rest of the blade. That is the most important thing to realize when it comes to dealing with the thought of flaws.

Unfortunately the fact that we live in an age of computerized mass produced manufacturing, where we expect perfection in aesthetics and scratch-free arrival of our $200 purchase from Amazon, has impacted our ability to properly assess ancient artifacts and their condition.

Especially in our case, with swords, where we restore them as well as possible instead of simply preserving them, this has resulted in less tolerance than there should be for flaws.

Japanese dealers usually solve the problem of customers being flawphobic by presenting them Shinshinto and Shinsakuto blades that will be on their first polish. There is no point in trying to argue with the customer in their assessment, rather, flawphobia can be used to support the market for newer blades. This is an entirely reasonable point of view.

Some of those customers will graduate through study and on hands learning to get over their worries about flaws. As they grow as students, in almost all cases appreciation for the oldest work rises and they will naturally return on their own with less concern and more understanding in this area. The day someone can buy a Heian sword and be emotionally accepting of the fact it is not mint condition, is the day that they really made a leap in terms of their knowledge and understanding of swords.