On the folding of sword steel

Discussion in 'Weapons' started by Count Duckula, Apr 15, 2014.

  1. Count Duckula

    Count Duckula Valued Member

    After writing an article about steel and one about magic swords, I feel that perhaps this would be a good opportunity to dispel a rather persistent sword myth. There are plenty of them to go around, and the most well known one is the myth that a sword smith making a truly exceptional blade folds the billet a thousand times. In that billet he may or may not (depending on which version of the myth) use hard steel and soft steel to make the blade sharp yet impact resistant.

    Picture a scene of a lonely smithy at a remote location. There is a forest or mountain in the background. The smith is working hard on a sword, hammering the billet flat, then folds it in on itself and hammers it flat again. He does this by hand and does it thousands of times. This is fantasy, yet there is just enough truth at the basis to make it appear plausible and recognizable. And it sounds cool, hence the fact that this is one of the most widely spread sword myths in e.g. youtube comments.

    Actually, this was going to be an article on sword myths (plural) but it became obvious early on that even an article on folding alone would quickly run past a 1000 words. As with my previous articles, this has been proofread by Mike Blue. Any remaining errors are of course wholly my own.

    Folding a sword billet

    The origin of this myth (or a big part thereof) is the fact that a sword might have a layer count of 1000. But that can be achieved by folding a single bar of steel 10 times. Each folding doubles the layer count. When Japanese sword billets are folded, this is usually done 7 to 12 times. Folding a billet 1000 times would result in 1.07e+301 layers, each one theoretically trillions of times thinner than the width of an individual iron atom. Ignoring this fact for a moment, there are a couple of big problems with the folding process itself. As with anything in smithing, there are tradeoffs to be made.

    Let’s start with the reasons why a smith might fold a billet. There are essentially 3 reasons.

    1. Pattern welding.
    2. To drive out slag and impurities.
    3. To fold one layer of steel around another.

    Pattern welding

    Pattern welding is the process whereby 2 or more different types of steel are fused together and folded repeatedly. The resulting piece is then etched, at which point the differences in oxidation will cause visual patterns to appear.

    Obviously, a flat bar of steel won’t have any visual appeal, since the surface would just be 1 type of steel. However, if –during the processing – the bar is twisted along its long axis, or ground in a way that crosses different layers, or has small divots of various shapes removed, the end result will have wildly varying patterns. With some creativity, almost any type of repeating pattern can be achieved. This is all done via folding. The number of folds depends on the number of different bars at the start and what you’re trying to do with them. Since this is about visuals, the idea is you can still see the surface patterns with the naked eye.

    As such, the usual number of layers is counted in the tens or hundreds. Depending on the number of bars you start with, a handful of foldings are enough. After all, if all you want is a billet with 16 layers as a starting point for further twisting and hammering, it is much easier to make a stack of 16 layers of steel and forgeweld that single stack together than it is to start off with 2 layers which you’d need to fold 3 times. And if you want 5 layers, you need to work with a stack anyway because you can’t get to 5 layers by folding.

    Regardless of the makeup of the starting billet, for pattern welding you don’t fold more than a couple of times because otherwise the result will just be visual mush with the texture of mashed potato.


    Purification of steel is needed when you don’t start out with clean, solid bars of steel, but with raw nuggets of steel that contain inclusions and dirt. The most obvious example of such is the Japanese steel known as tamahagane. In my first article about steel, I briefly mentioned Tamahagane and how it is made. After the smelting process, the near mythical material known as tamahagane is nothing more than ugly, dirty nuggets of steel, filled with slag and inclusions.

    The smith will process those nuggets, and eventually hammer them together to a single bar of steel. Or rather, his apprentices will do so under his watchful eye. These days, a lot of this is done using automatic power hammers, but traditionally made swords from Japan are still hammered out by apprentices using sledgehammers. A solitary smith doing this with a hand hammer is an unrealistic proposition. At this point, the bar of steel is still nothing more than a collection of nuggets welded together. There is still a lot of crud inside. To get rid of the crud and also to homogenize the carbon, the steel is flattened, folded in on itself and then flattened again. The idea is that this process will drive out the inclusions and homogenize the steel into a single uniform bar. The fact that the smith can do this using primitive tools, that is the real magic.

    To fuse 2 layers of steel together, the steel needs to be at welding temperature. Forget about ‘red hot’. I am talking about bright yellow colors, a few degrees below the point where it would actually start to burn like one of those sparkling stick fireworks for kids. At this temperature, the carbon inside the steel is very energetic and the carbon at the surface of the billet will oxidize and form scales. These scales are the grey flakes breaking away from the billet as it is hammered. The longer the billet is at those high temperatures, the more carbon is lost. In the olden days, this folding and flattening was done using sledge hammers, which is time consuming. It takes quite a while to flatten and then fold a 2 inch thick bar of steel. To fold it 10 times requires 10 times as long. All the while the steel is shedding carbon.

    In its raw form, tamahagane starts out at 1.4 to 1.5% carbon content. After 10 foldings (give or take a couple) this will already have dropped to 0.6 and 0.7%. This is ideal for impact weapons such as swords. You want the edge hard enough to take a wicked edge, yet forgiving enough that it doesn’t shatter upon impact.

    In any case, at that point you’ve folded as much as you can. More folding means more time at welding heat which means more carbon lost. 12 is about the maximum number of times a typical billet can be folded. If you’d keep going until 20, the billet would no longer have any meaningful carbon content and essentially be a bar of lump iron. Folding a thousand times… nope. Not going to happen. Long before that you wouldn’t even have a billet left to fold, given that it would have burned off completely.

    As an interesting bit of trivia: a smith starts out with 10 pounds of raw tamahagane to end up with a 3 pound bar of purified sword steel. The rest is burned off or otherwise lost during the process. Folding and forge welding come at a hefty price, considering Tamahagane is priced per gram. These days you can just buy a bar of steel that is chemically identical to such a bar of purified and folded tamahagane. It costs a mere fraction of the cost of such a tamahagane bar.

    For a bar of a single type of steel, folding does not make an excellent bar even better. It just makes crappy steel less crappy. Another way of looking at it is that if the smith has perfect control of every single variable and circumstance and he does a perfect job, he will do a perfect job of driving out impurities and controlling the carbon content, and ends up with a bar of homogenous steel that is the same as the bar from the foundry. If you already start out with a bar from the foundry, folding can only make it worse.

    It is a testament to the depth of knowledge and experience of good sword smiths that they make a bar that is nearly as good as what a modern foundry can make in a scientifically controlled environment.

    Folding a piece of steel around another one

    This is the part where the hard steel / soft steel angle of the myth comes into view. This idea is that you take a bar with high carbon content (which would be the hard steel) and low carbon content (which would be the soft steel) and fold them over and over to end up with a sword that is both very sharp, yet flexible enough to withstand impact. Like pattern welding, but for mechanical properties instead of visual properties.

    Aside from the fact that the idea is incorrect, it wouldn’t work anyway. Carbon is very mobile in a steel billet at welding temperature. If you weld a 0.8 and a 0.3% carbon bar together, then after only a couple of foldings will you end up with a more or less homogenized bar of steel with a 0.4 to 0.5% carbon content. As with the other parts, there is some truth to give birth to the idea. In some sword designs like the Japanese swords, and also some types of knives and other tools with a cutting edge, the piece is made up from different parts of steel with different carbon content.

    For a weapon like the Japanese sword, it needs to have a hard outer surface that takes a sharp edge and which does not scratch that easily. And yet it needs to be able to take an impact without shattering. To achieve this, the smith shapes a bar of high carbon steel to that its cross section looks like a U. In the hollow he then inserts a bar of low carbon steel that fits as snugly as a jigsaw puzzle. This construction is then heated and welded solid and flattened and drawn out to a single bar of steel that will be shaped like a sword.

    This is not the only way to make a sword with the hard / soft concept. There are various ways of making such a design, and indeed various methods were used throughout history. It is worth noting that this was done not only for swords, and not only to be able to cope with impact. Before the industrial revolution, good quality steel was an expensive resource. A smith would use it as sparingly as possible. It is common for old tools to have an edge region made from high quality steel and a body of plain iron or low quality steel. They wouldn’t want to waste good steel on the back or tang of a kitchen knife. Today this is still being done, but more for aesthetics than to save money.

    Then again, it all depends

    Everything I’ve written so far is completely true in a historical context of making blades. Today, the answer is a bit more complex.

    I know a smith who has replaced his power hammer with an automatic 50 ton hydraulic press. He can bring a large 2 inch billet of steel blanks to critical temperature and then squeeze it to a blade sized piece of steel before it even has the time to cool below welding temperatures. As you can imagine, flattening and folding a blade can go very quickly. Combine this process with a vacuum oven to keep oxygen away from the steel, and it should become obvious that a smith could fold a piece of steel many, many more times than a dozen and still have enough carbon left.

    It would still be a pointless exercise of course. We’ve seen the 3 major reasons why steel is folded. For aesthetic reasons, folding more than a handful of times is counter-productive, because the layers would be so thin that the end result is just a more or less uniform mush.

    For purifying tamahagane it would be pointless as well. After a certain number of foldings, the steel is as pure as it is going to be. And with Tamahagane, the whole point of starting with a high carbon content is to end up with 0.6 to 0.7 % carbon at the same moment that the steel is considered sword quality pure. Sure, you could use modern methods, borax flux, etc to start with tamahagane and fold it a hundred times under controlled circumstances in order to make the carbon drop. And you’d still have the same end product. Besides, if you want to go the techno route, just buy a bar of Swedish carbon steel, because that is what you are going to end up with after a lot of time, effort and money.

    And for structural reason, folding more is actually worse because you lose and sort of structural differences between pieces of steel. Although, my friend the master smith told me he could make a layered sandwich of 2 very different types of steel, with very different heat treatment requirements. And with a lot of effort, he could end up with a piece of steel that had actual hard and soft layers. But there again we have to acknowledge that there is no point. From a microscopic pov, such a steel would have lousy qualities because the edge would cross overlapping hard / soft layers. From a macroscopi pov, the steel would also have lousy qualities because the overall homogeneity means we lose the benefits of having a sharp edge and a softer body.

    So all in all, the conclusion still holds. Folding more than a dozen times in normal circumstances is logically pointless, counterproductive in reality, and practically impossible. Using modern technology and, a lot of skill, effort and resource, it is possible but still equally pointless and still counter productive.
  2. Count Duckula

    Count Duckula Valued Member

    Steel is fun :)
  3. 47MartialMan

    47MartialMan Valued Member

    Does this apply to Damascus steel?
  4. Count Duckula

    Count Duckula Valued Member

    What is these days called damascus is pattern welded steel: 2 (or more) different types of steel forge welded together and folded a couple of times. What was called 'damascus' steel in the olden times is nowadays called 'wootz'. I mention both types in my first article about steel.
  5. PointyShinyBurn

    PointyShinyBurn Valued Member

    If you wanted to make the best possible sword using modern materials, would you just machine it out of a high-carbon steel?
  6. boards

    boards Its all in the reflexes!

    If someone was remaking a broken sword would you use patten welding or would they melt it down and start again? And would it be as strong as the original sword if you patten welded it?
    And can you do this with a sword that has the two different types of steel wrapped around each other?
    If you had a rusted sword, can you purify the sword through the folding process? Clearly you would lose material but could you make something like a gladius length sword out of a rusted longsword?
    And fourthly, is there a calculation to work out how much raw material you will need to make a sword given that you can lose so much? Or is it just down to experience? If you lose to much can you just add some more purified material into the sword or do you need to start over again?
  7. Count Duckula

    Count Duckula Valued Member

    Assuming for a moment you mean a Japanese katana, no. I'd forge it out of a bar of 1060 steel. That is carbon steel with 0.6 % carbon, and no (or as close to none as possible) alloying elements. That kind of steel is chemically identical to purified tamahagane.

    For that type of sword, the combination hard edge + softer spine is still the ideal sword construction. Using something like 52100 (ball bearing steel) you could make a sword that is much tougher than a normal katana.

    But in order to make it durable you have to compromise a bit on how sharp you can make it. And katana were intended for certain type of use that did not involve hacking through armor.

    Other types of steel would require / benefit from other types of steel. After all, what you want to do with it has an effect on structural requirements. So a viking sword would benefit greatly from e.g wootz steel (the fabled Ulfberth swords).
  8. Count Duckula

    Count Duckula Valued Member

    Ok in order:

    1) you cannot remake a broken sword. You can use the shards to forgeweld a new billet and forge that into the shape of a new (smaller) sword, but you cannot put the 2 pieces back together like in Lord of the Rings. Melting and recasting a billet will work if the only differences are carbon content. Then the mix will just be proportional. And obviously, you'd have a monosteel blade afterwards. If there are a lot of alloys in the mix, that is a more difficult question to answer obviously.

    2) yes, you could purify the steel of a rusted sword like that, with a couple of caveats. First, there are different ways of removing rust. And second, folding removes carbon from the steel. If the steel is already at the point where it has the ideal carbon content, the new billet is going to have less than the ideal carbon content. If the material is really rusted through and through, it'd be far better to melt it again and cast a new bar of steel. Folding and forge welding a lot of sword steel together will cost too much carbon.

    3) It should be noted that I am not an experienced sword smith, working with tamahagane. The tamahagane purification process is where so much material is lost. Tamahagane is extremely expensive. By the time you are working with it, you've had a lengthy apprenticeship and you've learned how to know what you need at the start to end up with a given amount at the end. Once you start the folding, you're commited. Adding stuff halfway through is not really a good idea because it would not get flattened and distributed across as many layers as the rest of the billet.

    If you start off with a purified bar of steel from a steel mill, there is no need to go through the length folding process and you would not lose as much. If you start out with 1060, you can just forge the blade to the shape you want, and only need to account for material loss caused by polishing.

    Btw, I mention melting and re-casting, but that is also a process in which material is lost, and which needs a lot of experience in terms of figuring out carbon loss etc. It is also very, very, very dangerous in terms of personal health and safety for anyone who is not skilled in the working and processing of molten metal.
    Last edited: Apr 17, 2014
  9. David Harrison

    David Harrison MAPper without portfolio

    Great stuff, once again :)

    Keep 'em coming!
  10. Count Duckula

    Count Duckula Valued Member

    Thanks David.

    Btw, on the subject of melting steel, it is very dangerous and someone I know online still has burn scars from his neck all the way to his big toe from a single drop of molten aluminium that went behind his collar and left traces and open burns along the entire route it took from there to his shoes. thankfully, it missed Mr. Willy.

    Molten steel also has so much heat trapped inside it that the most innocuous things can become death traps. One beginner mistake is to pour molten metal in a shape that is sitting on a concrete floor. Concrete can retain trapped moisture because it is porous. But the structure is also closed enough that pressure can build up if the moisture evaporates because sudden extreme heat is nearby. This can then cause the concrete surface to explode, violently throwing the casting mould and the molten metal into the air and onto any bystanders.
  11. yorukage

    yorukage Valued Member

    Great article, thank you. I have a Last Legends Tsunami 5k Folded Katana, does that mean it was folded something like 12 or 13 times then?
  12. Ero-Sennin

    Ero-Sennin Highly Skilled Peeper Supporter

    I learned a lot from reading the article, but I learned the most important lessons in this post. :eek:
  13. Count Duckula

    Count Duckula Valued Member

    Something like that yes. They specify the number of layers. Obviously, if you start with 1, you need to fold once more than if you start with 2 :).

    They don't explicitly tell what type of steels they use, only that they use 'dual steel'. If you take 2 types of steel and fold them that many times, you lose the structural advantages of having 2 types of steel. So while they may or may not have folded that many times, the main 2 reasons are a) because it sounds cool and b) because 'everyone knows' that folding makes steel better :)

    The only technical reasons for folding that many times for quality purposes is to purify tamahagane. But they don't use that, they use tool steels. So the folding is done mainly for aesthetical / marketing purposes. I am not saying anything about the quality of the swords itself. I don't know them. I can only say that the folding was not done for practical reasons.
    Last edited: Apr 17, 2014
  14. pseudo

    pseudo Padawan

    So just out of curiosity, I know that a lot of knife makers really like D2 tool steel, would this also be a good steel for swords?
  15. Count Duckula

    Count Duckula Valued Member

    No. D2 is wrong steel for a sword or anything that is long and slender and subject to impact.

    It has a very high carbon content, which makes it brittle. With a sword you are going to have hard impact that is applied to a long slender body that is very hard and brittle. It will shatter. Maybe not the first time or the second time, but sooner rather than later, will a piece of sharp steel fly away.

    If I was going to use a tool steel, it would be something like O1.
  16. boards

    boards Its all in the reflexes!

    Thanks for the answers Count Duckula :)
    The Lord of the Rings thing was what started my questions :p
  17. erickjhonson

    erickjhonson Valued Member

    There are different kinds of steel now in the world
  18. Hannibal

    Hannibal Cry HAVOC and let slip the Dogs of War!!! Supporter

    There always were
  19. Mushroom

    Mushroom De-powered to come back better than before.

    Watching copius amounts of Man At Arms:Reforge. Makes me wish I learned the manly art of blade making.

    I seriously think that I could end up with a big old Brian Blessed like face mane after making 1 decent sword.
  20. Ben Gash CLF

    Ben Gash CLF Valued Member

    I think what he means is that modern mono-steels have effectively eradicated the need for folding except for reproduction and cosmetic purposes.

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