Activities in the Steel
by Jim Kurrasch
By activities in the steel we are talking about things such as sunagashi, kinsuji, inazuma, and chikei. But here I am also going to include things such as utsuri, and chirimen hada. These are various things or extras to look for. Their mere presents tends to tell you that this is a above average sword. And if they are there in great numbers, with great variations it will point out the really very super swords. Basically I will freely admit that in the following article I assume quite a bit. This is because the actual methods for making the better swords, were trade secrets and died out along with those smiths. If anyone alive knew the exact methods used to make Ichimonji, Rai, or Aoe blades, then their blades would be far better than any made in the last 600 years. So I am stating what could have happened to make those great blades of the Kamakura.
Basically we had better start with the definitions for nioi, and nie. Because it is from these that the activities are built from. If we start off with low carbon steel we can not harden it to any extent. So we must add carbon. Carbon will migrate into or out of red hot steel depending on the exterior conditions. If red hot steel is immersed in carbon, it will be absorbed into the skin of the steel. This allows surface hardening in modern metalworking. But red hot steel in a oxidizing condition with no source of exterior carbon will give off carbon, reducing it's content. It is through this that the Japanese swordsmith is able to raise or lower the carbon content in their steel. If they want more carbon they coat the iron with a clay containing rice straw, and then heat it red hot. To reduce the carbon they take the steel and just heat it red hot with lots of air.
Now that high or low carbon content can be made, what can be done with it. If we take high carbon steel heat it orange hot. Then immerse it in water it will harden. With increased temperature, decreased water temperature, or increase carbon content we will end up with a harder steel. The opposites will may a softer steel. With the softer steel we will noticed a change in appearance in the polished metal. It will take on a soft hazy appearance. With the hard steel there will be a hard looking shine. The Japanese swordsmiths are masters at making it exactly right for what they want. And good swordsmiths can pick the effect they want.
Now there is another thing that comes into the game. The method of refining the steel. This is done by repeated folding. A stack of small pieces of the steel (about 3 X 5 X 2 inches), is taken, coated with the a clay flux. And a piece of rice paper is wrapped around it to hold it together until it starts to bond itself. This all is heated red hot and hammered together. It is then partly split, again coated with the clay / rice straw slurry, heated red hot and folded over onto itself. Then the process is gone through again. As more and more folds are done the number of laminations in the steel increases as does the carbon content. The lamination numbers go up by a factor of 2. So we get 2 layers, 4 layers, 8, 16, 32, 64, 128, 256, 512, 1,024, 2,048, 4,096, 8,192, 16,384, and 32,768 layers. If we did 16 folds we would have 65,536 layers. When making a sword approximately 5 folds would be used for the center /core steel, and this would be 32 layers. Approximately 10 folds - 1,024 layers would be used for the side steel - jitetsű. And approximately 15 folds - 32,768 layers would be used for the edge steel. Different schools would vary on the basic formula depending on their raw materials (what type of wood, and sand iron is used). And another reason for variations was the finished sword desired (nioi deki, nie deki , or loaded with ara nie).
Now to say that your edge steel was folded 15 times so therefore it has exactly 32,768 layers in it is wrong. When making the sword this iron is cut into smaller pieces. These pieces are then again stacked giving the desired pattern. That pattern will be hammered by a extremely skill swordsmith into something that will resemble the finished sword. The back edge of the tip will be cut away at a 45║ angle and the front edge will be hammered up into a b˘shi shape. This allows the grain of the steel to flow around the b˘shi.
And if we use more folds we also tend to get a muji tetsű . This often appears to be a over refined steel. I will also assume that the refined near muji appearance of Rai tetsű, or Bitchű Aoe is due to using more layers for their jitetsű, than other schools that had ˘-hada. And the ha of Bitchű Aoe had a grain pattern, so they probably used less folds for their ha tetsű .
Another thing to know about the folding process. The steel is never melted. It is heated just hot enough to tightly bond. If it starts to melt, the impurities are driven back into the steel. With proper temperature control each folding will drive the impurities out of the steel. By the way, there is a tremendous loss of steel with all of this folding. The smith starts with several times as much steel as the final sword will weigh.
Now comes the tempering. A fine clay slurry is put onto the edge area of the sword. The pattern of this initial clay will dictate the tempered pattern. This clay is allowed to dry somewhat. Then a second layer of clay is put onto the sword. This layer will make the surface basically smooth, and it also protects the metal from oxidation. Some of the swordsmiths in Japan are now experimenting with not using clay in the tempering. With only a oil coating they have been able to form wild ch˘ji temper patterns similar to Ichimonji. The sword is heated up to a orange red color, and plunged into water. The sword temperature, water temperature, and the plunge rates are very controlled trade secrets.
It is very important to understand the method behind the folding and layering methods. Without that one can not understand just why the activity form. Remember that the increasing of the carbon content as the steel is folded, is a surface effect. It does not penetrate the steel to any great extent. And as the steel is folded each of the prior layers narrows by Ż. So the last layer forms Ż of the thickness of the block of steel. 2 folds back is ╝ of the thickness, and 3 folds back is 1/8 of the thickness. Added importance is due to the fact that the skin thickness of the added carbon content is also decreased. Another thing that would vary the thickness of these conditions is the relationship of that line to the surface of the blade. If the line runs at a 90║ angle the line appears thin. At other angles it appears wider. Actually the line is the same in either condition, but at a larger angle, more is exposed, and it appears wider. I also assume that the better swordsmiths are able to vary the amount of carbon added in each layer by adding more or less straw, or varying the type of straw in the clay slurry. They might even use the charcoal of certain woods to increase the carbon content of a specific layer.
Why is it so important to understand the layering effect and the carbon effect? Well they allow the activities to form. As the tempering takes place, finely tempered lines will form sunagashi, kinsuji, and chikei . What forms where depends of where the carbon content is patched correctly, with the metal temperature, and rate of cooling. The ha is formed where the carbon content, the temperature and the rate of cooling are all fairly high. Kinsuji is formed where the grain is basically masame, and the localized carbon content is higher than the general hamon carbon content. This may have been caused by that extra carbon content added. Masame is necessary because kinsuji is fairly straight, and masame would be required to placed that localized high carbon content in that straight line. Mokume or itame would be used to make inazuma. This brings that layer of high carbon skin steel into and out of the hamon , giving us the lighting - inazuma effect. A similar effect happens with ayasugi-hada where the wavy masame goes into and out of the hamon. And thus the shiny line goes into and out of the hamon .
Chikei is a similar condition as kinsuji or inazuma but it is in the ji. Since the ji is a thicker body than the ha, with a increased heat sink effect. And since the ji is better protected by the protective clay. The chikei does not harden nor shine to the same extent as if it had been in the ha. A very good smith could take this into account and increase the carbon content in some layers of the ji, thus causing increased brightness of the chikei .
Another thing to mention. The shininess does not have to be strictly due to carbon. After the arrival of the U.S. Navy and it's fleet of Black Ships to Toky˘ Bay, in 1854, the Japanese swordsmiths picked up some tricks from the foreigners. New metals, possibly nickel, or chromium were sometimes added to form activities. I own a blade with this, the blade is tempered in ara nie, and the kinsuji formed are wide, giving a somewhat un-natural appearance. But this appearance would fool most collectors. What one looks for is a bright line that stays shiny no matter what the surrounding material does. It shines equally in the ji as in the ha. Actually there is nothing that says similar methods highly refined, were not used in Kamakura. And without very localized minute analysis with something like SEM-EDX (Electron Scanning Microscopy - Electron Dispersive X-ray) we may never know.
Sunagashi is a condition where the carbon is more spread out. The high tempered steel are like a line of sand spread across an area. So in some small areas it shines, but right next to it the conditions were not satisfied for making hard steel. The line follows the skin of one layer, or it could follow several layers close together.
The ji nie or ha nie are formed when the carbon is spread out much thinner. This is not really a situation where the skin is involved, except it may be due to a condition similar to sunagashi, involving layers across all of the ji. Scattered ha nie are similar, but obviously in the ha.
Utsuri is really a stumper. Just what causes it I have no real clue. But it could simply be a contaminant in the metal, something like silicon, or molydeum. Anyway the some swordsmiths knew how to make it, and making it has only been recently re-discovered. Those that can do it state that it the blade must be heated to a very exact temperature range prior to tempering. But there is another factor that what we see now in the old swords may be the same thing that we see in the new swords, after aging 500+ years. Maybe cooling every evening, and warming everyday does something to the metal. Or maybe the metal is acting like a super-cooled liquid. After 500+ years that liquid is a bit more solid, causing utsuri.
A interesting thing about utsuri is that it can exist in the same metal that has ji nie. True utsuri tends to follow the metal pattern. By this I mean that one group of layers of the metal has utsuri, and the next group of layers does not. But both of these groups can have similar ji nie formations.
Now just sit back and think what the supreme swordsmiths during the Kamakura could do. They were able to make a sword that had masame to make the kinsuji, itame, mokume or wavy masame to make the inazuma . Metal that formed utsuri, and metal that did not form utsuri . They applied clay in a way to give very complex patters in the ha . And then they brought this metal uniformly to the right temperature, and immersed it in the water of the right temperature, at the right rate so that it all came together to make a Marvelous Sword. And this sword had to be strong enough to live through many battles, little the worst for it. Being able to cut through armor as well as flesh, to hit up against other swords, and various weapons, and to still function as weapons until they could be resharpened. All of the layers of the metal had to be brought together as one. Each layer had to have it's own function that enhanced the function of the layer next to it.