Jaki-ire
Hardening of the Japanese swords blades

Hardening of the blade is definitely right after forging, one of the most important but also most dramatic steps in making the sword. It gives the blade its characteristic incurvation, some of its extraordinary features and some visible (after polishing) effects are created – especially hamon (hardened line on a blade). Successful hardening can create very beautiful and perfect blade, if it be to the contrary it can ruin all the previous hard work or even destroy the blade. The purpose of this process is to create tenacious blade with hard edge. Following steps are necessary to be made to achieve that.

Preparation of the blade

Forged and roughly adjusted blade is necessary to shape but most importantly to clean with harder file. Such adjusted surface of the blade should not be touched any more. Sweat and impurities that stick on the surface avoid perfect connection with the pasta. The blade in this stage has no or only very small curvature. Some types of the blades can even have monouchi - last quarter near the tip curved down, so the opposite way than final product. It is necessary to count with the flexure that originates from the hardening. Now comes thin layer of pasta.

Preparing the pasta is the matter of every individual sword-cutter. The basic content is usually the same – loam. There is a lot of mystery about what type of loam to use. Generally it doesn´t really matter, it must hold on the blade after desiccation. That is important. I am using usual argil. Some sword-cutters add pulverized wood-coal or its ash or other essences in the pasta. Pasta must protect the steel (except the edge) from water. There is no or just really thin layer of pasta on the edge. Thin layer protects the blade from oxidation, while warming up makes bigger plane that contact with the water while sousing.

The layer around the edge is thicker - 1-2 mm in my case. As I mentioned before, this loam protects the blade from water during hardening. Other option would be covering the blade above shinogi (ribs) only, or not using pasta at all. Hamon is made by very careful setting the temperatures during warming up before the own hardening. I consider this as one of the most difficult ways, but it generates the best results when successful. But let´s get back to hardening with pasta. The line of hamon clearly visible after polishing can create many different patterns. It can be straight line – suguha, even wavy lines – gunome, uneven waves – midare and many other simpler or more complex patterns. The type of hamon can be influenced by the pattern we “draw” in the pasta on the edge area. After placing the pasta and possible drawing the pattern we let the pasta desiccate.

Hardening

The most important is even warming the blade at full length on the right temperature. Every material has different features which differ the temperature necessary to create hamon. Generally the better steel you use, the lower temperature during hardening. It is necessary to overreach the temperature level where hamon starts to be created (it is formed by grains of martensite). But if the temperature is to high, the blade will crack in hamon line (hagire) while sousing. If it is too low, hamon will not be created at all. In this case we can repeat the process, but from the very beginning. Even warming creates balanced and periodic hamon. If the temperature is higher or lower on some parts of the blade it will be visible after polishing even in the hardened line. The next very important aspect is the time of warming. Now I am going to be talking about my own experiences so somebody using different material can get different results. The longer the blade is being warmed up, the more the martensite grains are growing. That is maybe why there is then the thin line nioi that separates the line of hamon from the rest of the blade. Nioi is very thin after short warming and hardening in the lowest temperature limit. If the warming is longer it gets thicker but not so sharp. Good results in hardening are possible to be reached in a certain temperature limit. Color of the steel is the only aspect the smith is led by. According to my experiences with the material I use, the lowest temperature is indicated by bright red color while the highest by deep orange. However bright red indicates the highest temperature within the good quality folded blades. For the final look of hamon line it is also very important where in that temperature spectrum while hardening the blade is. When the blade is equally warmed at full length it is then soused. Some schools prefare to souse the whole blade including nakago. I souse the blade diagonally with the edge facing ground and only the edge itself, without nakago. This way the blade shows another effect – mizukage (shadow of water). Cooling lasts approximately 15 – 20 seconds.

In this time the blade at first bends down (for about 6 seconds) and then starts bending up to its final shape. That makes tension in the blade. After taking out from water and removing the pasta I usually warm up the blade again on about 200°C (to get rid of that tension). If the blade is crooked it is then straighten up (by local warming or hammer); small corrections can be fixed by file. Now it is only about the first polishing on the hard stone that will show the result. If everything is all right we proceed with polishing and we can start to make the blade a set.