Yaki ire, or differential quenching, is an ancient technique developed to harden the cutting edge, but leave a portion of the spine unhardened. This allows the blade to absorb a tremendous amount of shock without breaking, while still allowing an extremely high hardness level in the edge. Generally, this effect is produced by coating the spine of the blade in a thick layer of clay, and then performing the quench. The clay insulates the spine, stopping it from hardening, while the uncoated edge is allowed to fully harden. In rare cases, yaki ire is also accomplished without clay, by keeping the spine below critical temperature during the heat treat, but allowing the edge to fully austenitize before quenching.
A side affect of yaki ire is the hamon - the beautiful, wispy white line that can often be seen running the length of a japanese blade.
In this instructable, you will learn how to normalize a blade, rough shape a blade prior to quenching, make charcoal, apply clay to a blade, and perform yaki ire.
Step 1: Materials and Tools
I used the following materials and tools for this project:
*A knife made from hamon-able steel. Steels that are good for hamon are generally hyper-eutectic/shallow hardening, low manganese (.2%), and low alloy steels, like W2, 1095, and 1070.
*Clay/refractory cement. I use Rutlands fireplace cement
*Toothpicks and paintbrush for applying clay
*Wood scrap for making charcoal. more on this later
*Crucible for making charcoal
*Bucket for holding water
*Sandpaper and sanding block
*3 in 1 oil
*Carbide tipped masonry bit 3/16"-1/4"
*Accurate oven for tempering cycles
*Torch or heat gun for drying clay
*Angle Grinder (optional)
*Belt sander (optional)
*Safety gear like glasses, gloves, apron, ear protection, respirator, etc.
Step 2: Normalization Cycles
The first step in the process is normalization. After forging, the grain structure of the steel within a blade is often chunky and irregular. Normalization will help refine that grain structure, increasing the material's machinability and decreasing the chance of fracture or warp during yaki ire. In addition to this, it will also decrease the amount of time required to cool the blade and harden it (basically the blade will need to be quenched and cooled at a faster rate), which increases its ability to form a hamon.
To normalize, turn on your forge and heat the blade up past critical and allow it to cool to room temperature. You can test for critical temperature with the magnet, once the blade is past critical it will no longer be attracted to the magnet. Repeat this step 2 more times, heating it to exactly critical the second time, and then just under critical the third time. When cooling the blade, make sure not to just set it on a surface, as this can cause warping. it should be clamped somewhere with all of its surfaces open to the air.
Step 3: Rough Shaping
Before heat treating, you need to make sure that your blade is very close to its final shape. Any shaping done after the quench will be much harder to do. Start by cleaning off the scale. This is where you will find out how good your forging was, if you've left a lot of dents in the metal, this step can take a very long time. To grind off the scale, I mainly use a belt sander, and occasionally an angle grinder. If you don't have either of these, soak your blade in vinegar overnight to dissolve the scale, and then you can refine its shape with files. The blade should be very smooth and even, and the edge should end up a little under the thickness of a dime.
Now you will need to file the machi in. Clamp the blade in a vice, and file the notches in the bottom and top of the blade (called the hamachi and munemachi). Then, using drawfiling, smooth the spine of the blade. At this point, you can put in decorative doming on the spine if you want to, but, due to the thinness of this blade, I'll just keep the spine flat.
Sand the blade to 150 grit, PARALLEL to the edge. Perpendicular scratch marks can cause fractures in the quench.
Finally, drill the hole in the tang. To get through the steel, I use a 1/4" carbide tipped masonry bit. In my experience, they can drill through about 2.5" of semi hardened steel before dulling. Just oil the surface and drill through as best you can. It generally leaves a burr around the edge of the hole, so I just file that off after it is drilled.
Step 4: Hamon Basics and Design
Start by taking your knife and tracing its outline on a sheet of paper. Once you have the outline, you can start to draw what you would like your hamon to look like. There are way, way too many variations of clay patterns for me to get into it here, but the basic idea is that the edge and tip should not be coated by clay, and at least some portion of the spine or center of the blade will be coated. Try to keep the clay at least 3/4" away from the edge at all points, if the clay is too close to the edge it won't harden properly. The specific type of hamon I'm going for here is an unusual style called hitatsura which hardens the spine as well as the edge, but leaves an unhardened portion in the center of the blade. This is unusual because the spine of the blade almost always remains unhardened in normal clay patterns. Anyway, if you have any specific questions about other hamon designs, I'd be happy to answer them in the comments.
Once you have your design drawn, cut it out and tape it onto your blade. Trace it onto both sides of your blade with a sharpie. Inside the sharpie marks will represent where the thick layer of clay will be applied. Outside of the sharpie line is where a thin coat of charcoal and clay will be applied, followed by tiny lines of clay called ashi. I will explain this in further detail later.
Step 5: Charcoal Making
Charcoal is used in the claying process to speed up the quench in certain areas of the blade. Charcoal powder is mixed with clay and then spread in a thin layer over the blade where you want it to harden. When the blade is heated, the charcoal fragments burn out and leave behind a porous surface on top of the blade, which greatly increases its surface area, thus speeding up the rate of cooling. Charcoal is made when wood is burned in the absence of oxygen, so it's actually fairly easy to make it in a forge.
Grab some scrap wood and break it up into little pieces. You'll need a semi airtight container, I just used my crucible that I featured in my copper casting instructable. After the wood is inside, I drop an inverted 1" iron end cap to mostly seal the top. There should be space for air to escape, DO NOT MAKE IT COMPLETELY AIRTIGHT! It can blow up if you seal all the gases inside.
Stick the crucible into the forge and wait for it to get red hot. after a couple minutes, take the crucible out. There should be a large pillar of flame coming out of the top, this is the flammable wood gas burning off. Don't blow it out, just let it burn like that for a couple minutes until it extinguishes itself. Dump the contents out, and cover it with a pot so it doesn't burn away. Once it has cooled, you can remove the pot and you should now have charcoal!
Step 6: Clay Application
Clamp your blade to a benchtop and get out your clay. I take a couple tablespoons of Rutlands furnace cement and then mix a tiny amount of water with it until it is the consistency of glue. I use a paintbrush to apply the clay within the lines I drew to a thickness of approximately 1/8". Then I use a heat gun to partially dry the clay so it doesn't move around, flip the blade, and apply the clay on the other side. Try to keep both sides the same thickness, and make sure each side is the same thickness throughout the layer. Once you've done the second layer, hit it with the heat gun.
After this, I take a small amount of clay and mix it with a lot of water to make a watery solution of clay. I add a tablespoon or two of powdered charcoal and stir it in. Then, using the paintbrush, I apply a very thin coat of this over the entire blade. Like I said before, this will help cool the edge faster thus increasing its hardness and producing a better hamon.
Finally, using the batch of clay I used for the thick layer, I make a bunch of lines and dots on the blade called ashi. This will produce a lot of activity and waves in the hamon, and, if the blade is put under a large amount of stress, it will allow the edge to chip in small pieces instead of creating a large fracture. Once you have done all this, partially dry it with the heat gun, then put it in the oven at 250° F for 2 hours to fully dry the clay.
Step 7: Yaki Ire
Get a cup of salt (I use the kind that is put into water softeners) and find a bucket that is big enough to fit your blade comfortably in. Fill the bucket with HOT water, around 160° F, and then dump the salt in. Stir it and crush it until it is mostly dissolved, creating a hot brine.
Turn on your forge to low and turn off all the lights. You will need a dark environment to properly judge the blade's heat Get out a magnet on a wire and a temperature checking gun (optional). Slowly heat up the blade around the outside of the forge, to make sure there isn't any moisture left in the clay. Then, put the blade in the forge with the spine towards the source of heat. Run it back and forth within the forge, bringing it up to an even heat. If the tip starts to get too hot, touch it to a piece of brick or copper to draw the heat out.
When the blade is ready to quench, it should glow an even orange color. Continually check the blade when bringing it up to heat to find when it becomes non magnetic. When the entire blade is nonmagnetic and an even color, quickly move it over to your bucket of brine and thrust it in horizontally, edge first. The quench will take about 8-10 seconds to fully cool the metal, pull it out after that time period. Lightly tap the clay off with a hammer, and use the temperature sensor to check both the edge temperature and the temperature of the blade underneath the clay. The temperature of the blade underneath the clay should be significantly hotter than the edge temperature. If any part of the blade is over 400° F, especially the edge, it is likely that the blade did not get a good heat treat. To test the hardness of the blade, take a file and lightly rub it across the surfaces of the blade. It should bite into the area that was under the clay, but should skate off of the hardened parts. Finally, after you are reasonably certain that your blade has been hardened, place it in the oven to temper. The blade should be placed in the oven before it has a chance to fully cool to room temperature. For my differentially quenched blade's, they do not need as heavy a temper as other blades, so I usually put them in the oven at 350° F for 1 hour.
Directly after the temper, check your blade for warping. Sometimes slight warps can be fixed in a vice while the blade is still hot from the temper. If there is a large warp, the blade may needed to be heated up and bent back into shape, then re-heat treated. If there is a fracture or a very large warp, the blade will most likely have to be scrapped.
Step 8: Finished!
Here is the blade after the quench, I think the heat treat went very well. You can't see the hamon right now, but it will be brought out during polishing in a later step. Anyways, I hope you enjoyed this instructable and maybe learned a little something from it too. If you have any questions, I'd be happy to answer them in the comments section.
As for future instructables, I am currently writing one up on some mokume gane jewelry I made, and another on the making of a training karambit. I added some teaser pictures up above, and both of these will be released mid December, so make sure to look for them! I am also currently working on a full sized Naginata, I'm hoping to get this done for the knives and blades competition in December/January. It is going to draw from information that has been published in my other instructables, and expand on it, I am very excited for this project!
Please give me some other instructable topics in the comments section that interest you, I'm always interested to hear about what you would like to see me do! Thanks for reading! :)