NIKU by Keith Larman
Niku literally translates into "meat". It's a term used to describe the tendency of a traditional Japanese blade to have a bulging cross section on the side surface. Another way to think about niku is the degree of "roundness" of the ji surface. Niku is distinct from thickness of a blade, or the kasane. A thick blade with a flat ji surface has no niku. A blade could be vastly thinner and lighter but if it has a lot of roundness to the ji it will have niku. So you have to separate niku from kasane.
Depending on the era, katana were often made with considerable niku. A common method for determining the amount a blade has been polished in its lifetime is to look at the amount of niku. If the blade is "hiraniku ari", or with a lot of niku, it's probably not been polished a whole lot or else the polishers were extremely careful to preserve the blade. If the surface of a blade that started out life with considerable niku is flat, well, it's either been polished a lot or been polished by someone along the way who didn't take the time nor care to preserve the niku. Consider the following drawing (Figure 1). The solid lines show the blade when first made and polished. The dashed lines show what happens to the ji surface over time. (Obviously all surfaces will be reduced during repeated polishing, but I only wanted to illustrate the effect on niku.)
What you will see is a gradual flattening of the ji surface. This is inevitable as the polisher normally will be concerned with preserving the edge material as much as is possible. So if material has to be removed due to chips or rust, generally it has to be removed all over the blade, but it's better to sacrifice bits of the side over losing hard edge. And no matter how good you are, given enough time it will simply get flatter. There's only so much you can to without turning the sword into a toothpick!
This shows us why niku will tend to gradually lessen over repeated polishing. This brings us to the topic of this post. Why niku at all? Consider the physics of a cut. The katana is a slicing weapon delivered with a fairly significant amount of force. And anyone who has trained with a katana knows that two of the most difficult parts of learning to use the sword is getting a proper slicing action and at the same time maintaining a true path (hasuji) through the cut. A cut properly delivered to an appropriate target feels almost effortless, almost like you're not doing anything but letting the sword cut through the target. However, if you don't slice enough, or rotate the blade just a hair, suddenly the cut becomes vastly more difficult. That difficult feeling is part of the reason for niku.
What you're feeling when this happens are a variety of forces on the ji surface or on the edge itself. If you're not slicing the cut enough, the downward force of your cut is now concentrating into a smaller surface area of the edge. The brittle edge. And if you rotate the blade just a little, all the force is now no longer passing straight back through the entire blade (where it's strongest), but out at an angle meaning the ji is being pressed into the target.
Blade A, a shinogi zukuri with niku has a fairly wide angle where you could strike the target and still have support behind the edge. So blade A is most likely to not be damaged with an off-angle cut. There's more steel supporting the edge. This extra steel provides a shock absorber of sorts to the edge itself. This type of extreme niku turned out to be optimal for dealing with more difficult targets like armor and did more than just slice but also cleaved to an extent.
But because of the wider angle of the very edge on blade A, the edge won't get as sharp as Blade B or C. But it still won't dull nearly as fast as B or C because it's simply stronger with more material. The edge on B, a shinogi zukuri without any niku, is thinner even though the blades have the same kasane at the shinogi. This edge actually can be sharpened a little sharper, but it's also weaker overall. The angle at which force can attack the edge where the edge will retain support behind it is much smaller. So the edge is weaker, more likely to roll, more likely to be thinner, hence also more easily chipped. Sword C, a hira zukuri, is even thinner on the edge. So it can get laser sharp, but the edge is just a little more fragile still.
At this point in the discussion I think it's important to briefly consider the metallurgy of the traditional Japanese sword. They are an absolute testament to the creativity of the smiths of old. They took a relatively "dirty" source of steel and devised methods to create amazingly resilient swords of unparalleled beauty. But, contrary to movie, cartoon, and other popular culture depictions, they cannot cut boulders, won't cut machine gun barrels, and won't cleave a marble column in half. They broke. And broke fairly often historically. And they chipped and they bent. They were never indestructible. So when the samurai of old had to face opponents wearing more than just cotton clothes having a stronger edge became critical. Armor and various other means of self-protection were used all throughout history. If you put yourself into the typical samurai's tabi, having your blade snap in two in the middle of a crowded battlefield is just not a happy event. And if people are wearing various types of armor you want your sword to survive even the bad cuts. Tamahagane based steel is beautiful to look at and can be quite tough when forged into a beautiful sword by a master smith . But it's not indestructible. So a balancing act was maintained with the traditional blade. Niku was introduced to ensure a balance between a strong supported edge and a razor sharp cutting edge. So back to the shapes.
What happens when each of these blades passes through a target? Referring back to Figure 2.
Notice that sword A has a wider cross section closer to the edge. B is of the same width but it's higher on the blade due to the flattening. Then C, the hira, is thinner overall. What effect does this have on cutting? Well, on a soft thin target blade C will cut it like a hot knife through butter. Think about it for a second. When a blade passes through a target it has to "lift and separate" as it passes through. The hira has the thinnest blade of them all. The mass is distributed totally differently essentially "flattening out" a shinogi zukuri design. So the mass is spread out on a thinner but wider surface area (taller edge to back). And since it's so thin it only has to separate the target "just enough" to allow the blade to pass through. Also, since the blade is essentially a large flat wing, as it enters it will start to keep itself in line, much like a wing on an airplane. So the cut stays truer with less chance of scooping the cut. And to top it all off, the hira has such a thin edge that it can be razor sharp. And as long as the target is soft and won't roll the edge, that sharp fine edge can work wonders on the target. So it sounds like the hira is the ultimate cutting sword.
But it's not. Not all targets are the same. If we consider a similarly soft target, but get it progressively thicker the equation starts to change. Ever tried to cut a thick piece of cheese with a wide knife? It gets hung up. Cut the same piece of cheese with a cheese knife (the type with holes in the blade) or a cheese wire. It's simple. The difference isn't sharpness, its drag through the medium. So we introduce a new consideration. While the thinner blades are very sharp and still only have to separate the target a little bit, if the target is sufficiently thick they now has to deal with friction of the surface of the ji passing through the target. As the target gets thicker the taller hira blade now starts to reduce it's own effectiveness. By distributing the mass along a flatter plane, it also makes for greater surface area. And as that passes through the target the target is pressing on the face. So the wider and heavier the target and the larger the ji surface, the more friction the blade experiences as it passes through the target. The swordsman gets a feeling of "drag" during the cut. So let's return to niku again.
In Figure 3, the niku of sword A now shows its advantage. On a sufficiently thick target the very shape of the blade creates a wedge that lifts and separates the target a bit more. This combined with the shinogi-ji being either flat or pulling back toward the mune means that much less blade surface is in contact with the target. So less surface area, less friction. On a thick target the thicker shinogi zukuri blade with niku might actually feel easier during the cut even though the hira will displace the target less and is sharper!
Then if we talk about harder targets, well, the advantage of a sharper edge diminishes as the strength of the edge becomes more of an issue. A really hard target might just chip or break the hira blade. But the slightly duller but vastly stronger edge on a shinogi zukuri blade with sufficient niku actually might cut better.
History Lesson - what can we learn?
I think it's important to note that hira zukuri is rather rare historically in katana lengths. Given the strength of the old steel, the metallurgical knowledge of the day, and the fact that not all smiths stress relieved their swords after quenching hira zukuri wasn't exactly a great choice in that length. If the sword is always used perfectly with perfect form, there probably is no issue. But just a little rotation on a hard target and the lack of thickness in the blade (kasane) would likely mean a broken blade or at a minimum a chipped or cracked edge. Not a good thing if your life depends on it.
Niku was "par for the course" for shinogi zukuri blades which themselves were the overwhelmingly favorite design. The reason being that it added edge support, improved edge life, and allowed the sword to survive even harder targets even though the brittle edge was often relatively fragile. The thickness was enhanced while weight was controlled through the use of the distal taper. All in all a truly elegant design and a balance of many factors.
Fast Forward to Today
So do we know have answers to all our questions? Shinogi zukuri with niku is the "right" way to go, right? Well, "yes they likely were" and "no, maybe it's not so simple today".
There are modern western smiths like Howard Clark (http://www.mvforge.com) who are making swords out of homogeneous steel and are using all the combined knowledge of the past along with modern metallurgical science to create blades that are stronger, more resilient, and sometimes near indestructible. Howard Clark has a blade made by a highly technical duplex heat treatment that yields a simple notare hamon, martensitic edge, and a bainite body (as compared to the traditional pearlite). The duplex L6 from Howard Clark can actually take a severe flex without breaker or even taking a "set". It may not have the beauty, hada, and jigane of a Koto blade, but it will certainly outperform them by tremendous margin regardless of tsurikomi.
Hira zukuri was not a common form in katana lengths. And for good reason. The steel of the day couldn't stand the stresses of cutting at katana lengths. But a hira zukuri sword made today by a quality smith with top notch heat treating can likely stand more abuse than most antique shinogi zukuri blades of old. The newer steels and the newer heat-treating methodologies of today make for dramatic improvements in many areas. So a flat hira design made by a top-notch smith with a top-notch heat treat might be just as resilient to sets and breaks as any historical katana. If not significantly more.
But other things haven't changed. No matter the strength of the blade, you still have to get through the target. And the big honking sailboat sized hira blade that some use today in competition still doesn't lift through targets. And they have a rather considerable surface area that must pass through the entire target. So while it might seem like "cheating" on soft thin targets, the hira blades start to be a liability on vastly thicker, heavier targets. The surface area of the blade passing through all that material adds a lot of drag. So it's a give and take. Like most topics of the sword, it's complex and too varied to make too many generalizations.
What about High Shinogi vs. Low Shinogi vs. Bo-hi, etc.?
Okay, this is quite a tangent, but I think it's important and will invariably end up discussed. A high shinogi is defined in terms of the slope of the shinogi-ji, not the distance between shinogi on each side of the blade. So angling the shinogi-ji surfaces back towards the mune creates a high shinogi. This can have a number of effects. One is that having that high shinogi actually creates a small turbulent area behind the blade creating a subtle "whooshing" sound when the sword is swung cleanly. This should appeal to Iai practitioners. This can also lighten the blade considerably while minimizing structural compromise.
So if the desire is to lighten the blade, tapering the shinogi-ji back to create a high shinogi can do the trick. It will also have another effect - reducing the surface area of the face of the sword that comes into contact with a target during a cut (see, it is related to the topic) . So it might also have the slight effect of making cuts on thick targets easier. Less drag yet again. But it still occurs at the cost of mass (which if it's too heavy is a good thing, but if it's too light is bad because you lose inertia during the cut). A low shinogi means generally more weight that might be a good thing if you need more momentum during a cut, but a bad thing if you're worried about minimizing drag during the cut.
Then there's bo-hi. I'm not a fan from a performance standpoint . Think about what it's doing. If the blade has a low shinogi and a bo-hi, you're reducing weight creating an edge on the top of the bo-hi to grab target as the blade passes through. Then if the bo-hi is grooved deeply rather than smoothly and shallow , you've removed significant material from the cross section so a poorly angled cut should snap the blade. It reduces weight and reduces strength. On the other hand, tapering the shinogi ji to create a high shinogi can reduce weight similarly but without such a serious compromise of the lateral strength of the blade. And the high shinogi also reduces surface area at the same time. It seems to be a vastly better solution.
And remember that bo-hi were usually done historically for a variety of reasons. The first was to have a way of cutting out weld flaws or other "boo-boos" that appeared on the surface. This could save a blade from the trash heap. The second reason for cutting in grooves was to "correct" an old poorly balanced sword. Both of these methods work given the motivation, but ultimately both reasons are fixing something after the fact. The final reason was purely for aesthetics. Some just like 'em. This is a personal taste issue -- but it can become a safety issue if the blade is being made for tameshigiri.
My personal opinion
To state the obvious, if we're talking about antique blades or blades made in the traditional fashion, they should have niku appropriate to the smith, school, or tradition the blade came from. If for no other reason because if it doesn't, it's most certainly not a traditional blade shape. It's part of the traditional shaping so it should be there. It makes sense given the history of how the swords were used, the metallurgy of the blade, and how that material was heat-treated.
If we're talking about a production blade, factory produced, well, I'd want to see some niku in my blade and I likely wouldn't want bo-hi. Here the concern is safety. Factory production *generally * (there are exceptions of course) isn't conducive to high performance. The heat-treating in a factory is usually geared towards reliability, repeatability, and "good enough" results. So if the hamon forms, the blade isn't overly brittle or soft, well, there ya go. Customers of blades at this level rarely ask about heat treating details, rarely ask about destructive testing, rarely ask about how much abuse a sword can take -- I know, I've dealt with this segment myself for quite some time . They will ask about whether it has a cool hamon and whether the yokote is real. But most smiths will tell you that while you can form a beautiful hamon and get reliable results, that really doesn't mean you've maximized the potential of the steel. A talented "heat-treater" will push it to the limits. This is at odds with the "as many as possible survive heat treat" philosophy that a factory has to follow to stay profitable. The guy pushing the limits is trying to get the toughest, strongest, most reliable blade possible. And each heat-treating is geared to the specific blade. And they'll shatter them occasionally. The failures during heat treat will be obvious and spectacular. The factory will heat treat hundreds of blades within limits to ensure they'll almost all survive. All things being equal the blades made by the custom smith will be vastly stronger and more reliable. The factory blades can still be decent, usable, and good buys, but there simply isn't a comparison even if they're using the exact same steel. As a result I'd personally prefer to see thicker, beefier blades with stronger niku on production katana. It's simply a matter of erring on the side of caution.
Anyway, the bottom line is that niku is something that does not exist in isolation. We have to consider the cross section, the targets, the style of use, and the material and heat treatment used for the blade. It's not so simple to say that niku is a good thing. We can say that it's a good thing on certain blades for certain uses. So again we're left with the same observation. The details are important, but all the details are important. Everything has to fit together. No one feature makes a blade better than another. All the features together are what define a good sword. It's not whether the sword has niku, or a sharp edge, or the steel used, or the heat treatment, or the hamon type, or the folds in the steel. It's whether all the features of the sword work together as a harmonious unit to accomplish the task.
Antique swords balanced all those features marvelously and niku was a necessary component. Today's production swords probably could learn something from that lesson. The same is true of the design of the shinogi (high vs. low). There are many swords on the market that are bulky and unwieldy due to poor shaping. Bo-hi may not be the best solution. And today's custom swords add new ingredients to the recipe. With better steel and more controlled heat treatment and the control of microstructures that the top smiths bring to the game, well, new things are possible. And niku may be less necessary for blade preservation. But that also doesn't mean it's not a good thing for other needs. Complicated enough? Now I imagine there are more questions raised than answers given.