In the class, we hardened the blades in the as filed condition. You can take them through the first stone, but any further will just create work for yourself. The secret of the Japanese katana is its ability to hold together even if the blade chipped or damaged. This is accomplished by a process of selectively hardening the blade through the application of clay along the area that is to be left unhardened. The Japanese smiths were very creative when it came to building strong, unbreakable blades. By carefully combining steels of varying carbon contents, sometimes using five or more different steels in the construction of their blades, they were able to adjust each portion of the blade for the maximum service. Since we were working with a piece of homogeneous steel, we attempted to achieve similar results though careful heat treating.
I won't pretend to know all the secrets of the clay formula, but I have found that any refractory that will stick to the blade and still be easily removed after hardening will yield satisfactory results. I prefer to a high temp mortar mix from Harbison-Walker Refractories called Satanite. It comes dry and may be mixed with water to provide a tenacious clay with excellent insulating characteristics. I also use #36 premixed mortar cement from Harbison-Walker #36 is an air setting mortar and has higher shrinkage than the Satanite, but is popular with many makers. I heard about it from Michael Bell and again from Bob Engnath. For the class, I brought a tub of the #36.
The clay works by providing a barrier to the quench, effectively slowing down the cooling rate so that the steel does not completely transform to martensite during the quench. Most modern eutectoid steels have a long TTT transformation time and the clay is not as effective as it is with the medium carbon steels. 1050 is particularly sensitive to the clay since it has to get below the nose of the curve in under a second to fully harden. By running a bunch of tests, we were able to determine that a thin layer of clay, around 1/8th inch, would stop complete hardening of the blade, create an interesting temperline and leave the back tough and relatively springy. All blades hardened in this manner will take a set if bent too far, but it is very important not to make the backs too soft or they will bend with nearly every cut. I have seen old blades that were easily bent in the hands and I do not find this acceptable. If it is too hard it will not stop the propagation of a crack so there is a balance that has to be struck. Obviously the Japanese smiths confronted the same problem hence the complex billet constructions.
A light, thin wash is applied over the entire surface of the blade. This is done with a brush which has had the excess water flung out and then is dipped into a thin clay mix. After this has dried enough not to be runny, a thicker layer of clay is applied to the back of the blade using a spatula. We used a broken bandsaw blade, but an old table knife will work as well. A layer about 1/8th is applied uniformly down to within about one third of the distance from the cutting edge. You can be as creative as you like with the application of the line remembering that if it is too busy, you may lose definition to the hamon. Since I wanted to stiffen the back of the 1050 all clay was scraped from the spine of the blade. This allows it to harden helping to control the curve and make it more flexible without being soft. On a tip from Bob Engnath, I wrap construction wire in a spiral down the blade and then go back and cover the wire with the clay that surrounds it. This wire is not necessary with Satanite, but helps the #36 stay in place when it is heated. The clay is then left to dry a bit before putting it into the heat treating forge.
You need a fast quench to harden this steel and water is the best choice. I was using brine, but Yoshindo says in his book that the clay has the same wetting effect as the salt and yields good results. It is best to use distilled or rain water for the quench, but lacking a rain barrel, we used water from the tap. My quench tank is made from plywood that was caulked and sealed with polyurethane. It works fine though it did leak because it had dried out during the trip. You should build the tank long enough to get the entire blade and tongs into it without a problem. My quench trough is 4'x1'x1' or maybe a little bigger. It is wise to dry run the quench before you apply the clay so you know what the movement will be and to see if everything will fit.
After the clay had set up enough so that it would not run or slide on the blade, it is put into the furnace. The furnace was set at 1600F and was up to temp before we put the blades in. The blades were put edge down in the grooves on the brick. When the forge is running it sounds like it is breathing. The flame cuts on and off as it maintains temp. Adding the spark plug ignition is really nice because all you have to do is push a button and it comes on. At heat treating temperatures, the flame would ignite by the interior heat of the forge, but as a safety factor, it is nice to know the spark plugs are firing.
To hold onto
the blades, we needed a specially designed pair of tongs, ones with an offset so that the
blade could be thrust into the quench tank and the tongs not get in the way. Chuck forged
out a pair in about half an hour that worked great. Besides the offset to the jaws there
is a lip that comes back at a right angle to pick up the bottom of the tang on the sword.
The blades were heated in a digitally controlled heat treating furnace. The blade slowly comes up to temp. When the back is at critical, we raised the blade out of the groove and let it come to the same color as the back. When I first went to the controller, I made the newbie mistake of relying on the controller to tell me when the blade was up to heat. What the controller is telling you is the temperature of the thermocouple and you have to calibrate your eye to that color and match it to the blade to be sure that you are at temp. This allows for heat treating in varying light conditions and is quite accurate when you get use to it. The eye is still the best judge of temperature because you can see if there are hot or cold spots and will notice subtle changes over the blade.
When the whole blade is at the proper hardening temperature, we were shooting for 1550F it is rapidly removed and quenched in the trough of water. It is held beneath the water and moved gently back and forth until all visible color has gone from the back of the blade, then it is taken out. If there is still color when taken out, it goes back into the quench again for a few seconds and then removed. Working quickly, you take all the clay off the blade and then check the edge with a file. If it has hardened it is then transferred to the low temp salt bath for tempering.
We do not really want to soften the steel at this point, but do need to relief the stresses created by the hardening, so we set up a salt bath at 325F and drew the blades from 1/2 to 1 hour. This seems to give the best hardness and toughness for this steel. Each steel will have to be treated differently. After the blades were done tempering we checked them again with the file and if they needed straightening we did that over the hardy hole on the anvil using a hammer. The back is soft on this style of blade and they adjust easily. I like to sharpen the swords at this point and go cut something. My standard tests involve cutting 2x4 pine or fir. I like ones with knots because it is hard on the edge and will tell you quickly whether you have the edge geometry and heat treatment right. If I happy with the performance of the blade it is real and I can proceed with the finishing. If I am not happy with it, it can be restraightened and rehardened. We had to do many of the blades over during the Folk school class and didn't lose any.