Sword Shop: Basic Japanese Sword Forging
Part 2: Heat Treating and Clay Coat Application

By Randal Graham

This article is a continuation of Sword Shop from the last issue of SFMO in which we covered the forging of a basic Japanese-style sword. In this issue and article of Sword Shop, we're going to cover the heat treating process of a katana blade.

Preparing to Heat-Treat Katana Blades

So, now we have blades that are forged to shape, with the scale and pitting from the forging ground off to show clean steel.

Before anything else happens, I like to normalize the blades at this point. Normalizing accomplishes a couple of things: First it will equalize/remove all of the stresses caused by the forging process, which can be considerable, due to a large number of cycles to the forge and the heat, and back to the anvil where the blade is hammered and constantly cooling.

Second, it helps put the steel into a softer condition that is much easier to file and carve.

Third - and the most important feature - is that it sets the condition of the steel up for best results in the hardening cycle. I'm looking for the finest grain I can get, and going in with fine grain means coming out with fine grain.

Left: "Scud" - my high-temperature salt rig. Right: Blades rough ground and ready to normalize.

I normalize my sword blades with "Scud", the name Don Fogg has given to my high-temperature molten salt forge. It's constructed from an old hot-water tank, and inside there is a ceramic lining to insulate the salt tube, which is made of stainless steel.

Two venturi burners come in from the side and the flames from these swirl around the inner tube, heating the salts.

The salts I use melt at around 1100F and remain stable to about 1650f, and meets the needs of the simple and low-alloy steels I use.

There is a thermocouple, which is a heat probe, connected to a digital controller, that keeps me appraised of the exact salt temps and can be used to controll the temperature automatically, though I don't regularly use this feature.

I use the molten salts for a couple of reasons, the first being that they can supply an almost perfectly even heat to a blade placed in them. Since the salts are liquid at temp it flows around the blade heating it evenly from all sides. The second reason is the speed in which it can bring a blade up to temperature, so in the context of making swords regularly this feature adds considerably to the convienience.

Since it takes scud around 45 minutes to melt the salt charge and stabalize, I'll use that time most often to do a quick clean-up grind on the blades I'm working on. I do very little shaping at this point, I'm most concerned with getting off the scale and the pitting associated with the forging process.

Left & Right: Feeding the blade into the salts.

For W-1 steel, I like a normalizing temperature of 1400F degrees. I simply feed the blades into the salts untill they are immersed up to the tang, and watch for the temperature of the salts to drop from the introduction of the cold steel blade; once the temperature recovers to 1400F I'll then hold the blade at that temp for about 2.5 to 3 minutes. Then I bring the blade out of the salts and simply set them aside where they can air cool, at least to a full even black, which is about 850F, cooler is better though. Care is taken to make sure they are going to cool evenly in nice still air, a few gusts or breezes under the door or through a window can play havoc with the whole process.

This process of heating and cooling is repeated two more times, for a total of three normalization cycles. On the last cycle blades are sometimes placed in an insulative material to provide for an even slower cool than in air, but with W-1 and the mass of a sword blade this is not nessecary, so I can consider the blade both normalized and annealed.

Left: Blade coming out of the salts at 1400F. Right: Blade at an even 1400F.

Some higher-alloy steels, like L-6 for example, will need to be annealed (softened) in a separate fourth thermal cycle to make it an acceptable condition to work with. Alternatively, the third cycle can be modified to become an annealing cycle as well.

Top: Blades air-cooling. Right: Wire-brushing each blade.

The blades are cool to the touch, straight, and soft. The salts leave a film on the blades, that I simply remove with a rotary steel brush.

Top: Drawfiled and ready for clay

Now, I will draw-file the blade to get it flat, smooth, and all the lines where I want them to be. All of the major shaping is done at this stage, all of the surfaces must be even and consistant before the hardening is done. A blade that is rippled or uneven will not stay straight in the water quench, it will warp or twist, and when this starts to happen cracks are invariably the result. So I can't cheat here, you simply have to resign yourself to doing what needs to be done, and taking the time to get it as close to "perfect" as you can. Anything less and the water will bite ya.

Left: Draw-filing the shinogi-ji Right: Draw-filing the ji

I've found that there are some techniques with drawfiling that can help alot at this stage. I always file large sections to an even smooth state, instead of little sections that can end up being a dip or wave in the blade. I try to do all the surfaces to an even texture before I go back and address problem areas. Always work on high spots to bring them down to the rest of the blade, working on low spots only makes them deeper. And it is very helpfull to do ajoining surfaces in opposing directions, the difference in cut direction will make it much easier to see the separate surfaces in the light, and helps you to see the ridgelines between these surfaces clearly and allows you to make them sharp and straight. The edge is rounded with the drawfiling as well.

Top: Draw-filing directions

After the blade is drawfiled to completion, it's in a perfect state to apply clay. With practice the surfaces can be made suprisingly smooth and even with a file, but it's an aggresive texture that holds onto the clay very well.

I mix up some clay and use a wet paint-brush to "scrub on" a thin wash coat of clay, which is the base coat and also helps the clay adhere better by forcing it down into all the little scratches and grooves. This thin coat does not insulate, and can actually enhance the quench speed somewhat, insulating effects don't seem to begin untill the clay gets to a thickness of around 1/16th of an inch.

Top & Bottom: Applying a preliminary thin layer of clay

Then I go back and start to apply the main body clay, with the W-1 high carbon steel and others like it there is a little different mind-set at work here. The clay does insulate the steel, but very little. Rather, it's the effect the hot clay coating has on the water as it's being quenched that I think provides the controlled cooling necessary to form a hardened edge and a springy body.

Left: Applying body clay, smoothing clay to consistant thickness Right: Forming clay around tip

So to this end the clay is kept consistant, and rather thin... about an 1/8th of an inch. This differs greatly from other methods where the clay is the sole source of insulation and is applied very thick, sometimes over a 1/4 inch thick, which will supply a hard edge but soft body.

In this case, I'm wanting "small choji" hamon. I apply the clay on the top two thirds or three quarters of the blade. Once this is done, I go back and add "ashi" to the clay coat, small thin strips of clay over the whole surface of the blade at right-angles to it's length. These provide some insulating action in the quench as well, and will form little sections of softer material in the hardened edge, giving it more ability to prevent and arrest any cracking that may form in a blade under hard use. They also contribute to the formation of crystalline features within and around the hamon itself.

Inset: Applying ashi Above: Ashi laid out for creating a "small choji" pattern

After the ashi are in place, the final step is to go back and adjust the clay between the ashi, I do this with a little dental tool, to pull it up in little ridges and to also even out the average line that is formed between insulated and un-insulated blade, to set the hamon width so-to-speak.

Inset: Correcting clay application with a dental tool Above: Forming ridges between ashi lines

With the high carbon steel and a water quench, this is fairly important. The little ridges I form between the ashi will help control the width of the hamon, without it it's fairly common for the hamon to "jump" up under the clay and it can ruin the looks and the structure of the blade, which can be fixed only by going back and starting with the normalization cycles all over again, re-filing, and re-claying the blade.

Now the blades are set aside for a while, to start setting up. When the clay is getting fairly "stiff" I'll use a torch to fire it to a nice deep brown, so it's dry and solid, or sometimes I'll hang it in my low-temperature salt rig, where it cooks at around 500F for a while, to achieve the same result.

So, now the blade is ready to harden! Coming up next issue!

About the Author

Randal Graham is an industry-leading smith of Japanese-style swords of high performance and aesthetic beauty. His blades are sought after in Japanese sword art practitionary circles. Randal's website is at http://swordforum.com/randalgraham/

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