The process of folding steel, known as shita-kitae, could be done anywhere from 10-20 times, in some cases creating up to 1 million layers of steel. Performed when forging samurai swords such as katana and wakizashi, the purpose of this ancient swordsmithing tradition is very misunderstood. Most people don’t understand why this is done, or what exactly is happening to the steel when it is folded.
Folding the steel is not a magical process that is needed to make the best blade in the world. The need for steel to be hammered down and folded several times comes directly from the fact that the blade material you’re working with is impure. Pure, homogenized, high quality steel does not need to be folded, and can in fact be weakened from the very act. Folding is a process to remove impurities, and to homogenize the steel. In feudal Japan, swordsmiths used a material called tamahagane which consisted of iron sands and charcoal smelted in a clay tatara furnace. This blade material was very impure, with gaps in the steel, less desirable elements, and an uneven distribution of carbon throughout. You want the strength in a sword to be uniform through the whole blade rather than have one or several areas with concentrated levels of carbon, creating brittle weak points in the blade that could lead to the wielder’s demise if their sword is broken.
Now why would steel have an uneven distribution of carbon? The simple answer is that the steel wasn’t fully liquified during the smelting process. To fully liquify iron, you need to reach a temperature of 2,800 degrees Fahrenheit, and medieval levels of technology couldn’t always get there. The traditional Japanese tatara furnace, for example, could only get up to 2,500 degrees at it’s apex. However there are many other elements in the steel that do reach a melting point lower than iron. So what else remains in the steel besides iron? This stony waste matter known as slag is counted among the blade’s impurities, however there are a couple elements that can be beneficial in small amounts, such as silicon and titanium.
The real question is, how does folding remove impurities? It seems intuitive that folding the steel several times over would even out the carbon content and homogenize the steel, but how are the impure elements of the steel removed? One answer can be found in the differing melting levels of the steel you’re working with. Iron has a very high melting temperature, but when you get iron red hot, the other non-metallic inclusions in the steel will have reached their melting point and turn into pockets of liquid. When you fold the steel over and compress it, hammering the steel down, those little pockets of non-metallic liquids are squeezed out to the surface. This is personified by the flying sparks you see when a steel bar is hammered.
Another answer to this question is that the oxygen present in the air is able to burn out surface level impurities during the folding process. If you’re able to burn out impurities on the surface of the steel and then you fold it, you’re moving what was once on the surface further into the middle, and you’re moving the middle material of the metal onto the edges as you go, exposing more impurities to be burned away by oxygen. There is another side to this fact however. Oxygen will bond to anything that it can, and will also burn away some of the carbon on the surface of the steel.
If you ever see someone working on a bar of steel, you’ll see black flakes forming on the surface of the steel and flaking off. This is actually the surface of the steel oxidizing, and it happens at a much faster rate when steel is hot. Swordsmiths would quench the steel in oil or water between each fold, to clean these rusted flakes off as to not let them get caught in the next folding, promoting a pure steel. This brings us to one of the drawbacks of folding steel, which is decarbonisation, burning of carbon on the surface of the steel due to it coming into contact with the air.
This is why when making blades with impure steel that will require a lot of folding, it’s important to choose a steel with a high level of carbon content. During the folding process the steel will lose a good amount of it’s carbon. The end result is steel with the right carbon content and enough impurities removed to make a very strong blade. Though this entire lengthy process could be avoided if you start out with high quality steel in the first place.
Nowadays steel is folded only for tradition’s sake, and to produce the beautiful flowing grain pattern it creates on the blade. The idea that folding pure steel somehow enhances the blade to be stronger and sharper is completely incorrect. With only performance in mind, it’s better to leave the blade unfolded, as this process opens the steel up to more detrimental results. It enables the possibility for rust to be caught in between the folds, creates a higher level of decarbonisation through the forging process, as well as introducing the possibility of gaps left in the steel if not performed correctly by a master swordsmith.
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