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Differential hardening with clay

by Bob Engnath

Clay and other refractory glops used in tempering (Hardening) the Japanese style edge, are probably the most confusing part of the entire process. Drop a bit on the floor while you're working and you'll understand why I use the term "glop."

First, the clay has a twofold purpose. One, it keeps the quenching medium, oil or water, from contacting the main body of the blade, so that portion of the blade will not harden. Second, the clay helps hold some of the heat in the main body of the blade so that it cools slowly enough so that it will not be shocked into hardening.

Steel has a very discouraging tendency built into the process of selective hardening. It bends. I don't mean warps, it bends, curving upwards from the cutting edge. You have to heat a carbon steel blade to around 1500 degrees F to make it harden The entire blade expands. This is one of those facts of life and elemental physics that one cannot escape. When steel is converted into martinsite, the hard stuff, it is frozen in the expanded size. The back of the blade does not harden, and slowly shrinks back to the original dimension.

There, in one paragraph, you have at least half a dozen ulcers and enough heartburn to use up a case of anti-acid tablets.

Water quenching produces the most desirable looking temper line, close to duplicating some of the ancient types. Water quenching also produces the most extreme differential between the size of the hardened edge and the soft back. One will find a perfectly straight sword hardening into a cutlass shape, with over an inch and a half of curvature. 1045 and 1050 move about an inch and a quarter, W-2 might approach two inches. Oil quenching a 5160 bowie with a hardened edge, will produce about one eighth of an inch of curve over a foot of length.

In ten seconds of quenching, the blade goes through an incredible combination of cooling and stress. You'll hear it creak while seeming to sing in your hand, and when conditions are a bit too much, it goes ping. You do not want to hear the ping. When it pings, it is the sound of the spine shrinking to such a degree that it literally pulls the cutting edge right apart.

When this happens, one hurries over to the steel distributors manual, flipping pages and searching. Yup. It says this stuff can be water quenched, right there in black and white, plain as day. To make a long story short, the distributors and manufactures are not thinking about a knife sort of cross section when they talk about this type of hardening. They are talking about axles, ball bearings and other items with a nice, even cross section and no thin edges. This is a problem that can really discourage the knifemaker. On one occasion, five swords left my shop with undetected cracks and put me in such a state that no more were made for about a year.

I wouldn't tell you about this if there weren't a way to fix it. You have to scrape the clay off the spine of the blade so that part of the back will harden just a bit, reducing the tendency to curve so much. Now they'll only curve about three fourths of an inch. That, coincidentally, happens to be just about the correct curve for a Japanese sword.

You do have to adjust just a bit, because a proper sword is a little thinner at the tip than at the guard. Bend the tip down about three eighths of an inch, and the butt end upward about the same amount. Pray a little. Try Buddha too. It pays to cover all the bases.

The bending gadget in my shop is just a couple of eight foot planks with bolts holding them together. There are spacers on the bolts, between the planks, so they're snug on the blade when you slide it in between. Put a six foot pipe on the other end of the blade and you can easily put a kink in five sixteenths steel that is an inch and a half wide.

The last fly in the ointment is, the clay will not bend along with the steel when the sword starts to curve. That makes it fall off, often while there are still sections beneath it which are hot enough to harden. You do not necessarily want odd islands of hardened steel messing up the neat look of your cutting edge. Kuzan Oda helped me solve that problem by suggesting that wrapping mild iron wire right into the soft clay would help it stay on tight enough to keep water from getting under it. It only takes about a dozen turns for a sword, and you are able to form the clay right back over the wire. If it appears to crack in drying, just patch it the next day.

The clay, as I've been calling it all this time, really isn't clay. The best sword hardening glop that I've been able to find, through six or seven years of trying, has been A.P. Greens No. 36, High Alumina Refractory Cement, suggested by both Mike Bell and Francis Boyd. They brought this up after I'd spent about three years in rather futile experiments. It comes in fifty or one hundred pound pails, mixed and ready to use. Keep it sealed with a plastic cover, right on the clay, inside the closed lid, and it will last for months. Fifty pounds will be enough for at least twenty swords and costs around thirty five dollars. A.P. Green has distributors all around the country, and they can even ship the buckets by U P S.

Getting clay onto the blade in a useful pattern isn't too hard. First, you need a couple of those Dreem Whip containers that your wife saves when they're empty. Run down to the art shop for an artists pallet knife. Get two, one straight small one, and one longer big one. At the hardware, pick up some number 22 iron wire and a putty knife. Three or four shop rags are handy.

The work area need not be anything more involved than the workbench.

Work while sitting, it's easier since putting the clay on a single sword might take as long as forty five minutes. If you're right handed, sit with the bench at that side. One of those plastic containers should be full of #36 that you've scooped out of the bulk bucket with the putty knife, and the other full of water. Throw the putty knife and pallet knives in it when they're not being used.

The clay should be heaped in its bucket, so you are able to scrape a roll of it about the thickness of a wooden match onto the side of the small pallet knife. If the clay is too stiff, dunk the pallet knife into the water bucket, picking up a few drops and stir a little extra water into the surface of the small patch that you're working on. Don't use too much, because it makes the clay shrink a lot more than the medium that they use to mix it at the factory.

Hold the sword blade horizontal in front of you. I wedge the tip into one of the many holes that have been worn into the side of the bench. Then I usually go over to the grinder and knock the sharp edges off the tang because they really tear up the hands.

OK, try again. You have to rap the clay laden edge of the pallet knife onto the side of the sword with a fairly brisk motion, hopefully transferring a line of clay to the blade so that it runs from the cutting edge to the grind line. Well, try again. It takes a little practice. They go on both sides of the blade and your spacing should be approximately equal, but doesn't have to be symmetrical.

These are called ASHI lines, and produce softer sections of steel projecting down into the temper line. The Japanese used them to help stop edge chipping. ASHI means "rat leg". Yes, that has puzzled me too.

Those ASHI lines might only be one fourth of an inch apart, or as much as one half inch. They will have quite an effect on upper shape of the temper line. They also have a bit to do with curvature, with less lines producing more pronounced curvature.


Switch to the large pallet knife. It should be about the size of a small kitchen spatula. If you can't find one that size, get a bigger one and whack some off with the grinder.

Now you have to work from the back of the blade, so reverse the blade in your hand, so you're holding the tip and the tang is on the bench. Yes, that little point hurts my fingers too.

The clay that you put on now will determine how high the temper line will go on the blade. The proper height is one third of the total width, but suit yourself. Begin plastering clay onto the side of the blade, stopping about halfway down. Once you have about three sixteenths of clay on the steel for some length, come back and work it down a bit lower. Do not cover the spine.

As you get to the last few inches towards the tang, make the temper line a little narrower by pushing the clay down farther. It's tradition. The temper line should extend into the tang about an inch past the end of the cutting edge. Cover the tang, leaving enough clean to grab with the tongs when you're heat treating. If you want to make the tang stiffer at the junction of blade and handle, scrape about half an inch of clay away along the top sides of the spine, tapering the cut in and out smoothly over about six or eight inches.


Turn the blade around, holding the tang and repeat this on the second side. When that's done, go back and carefully make a neat turnback at the tip.


Stand the blade on its tip and wrap about a dozen turns of number 22 iron wire right into the clay, working from tang to tip. Don't let any clay get pushed around at the tip, because it'll mess up that nice turnback later. Spatula the clay back over the cuts that the wire made

You may heat treat immediately, or wait until the clay has hardened. If you go for "right now", be careful not to scrape the clay off in the forge. I would not try this in a coal forge until the clay was hard.


Copyright Bob Engnath 1997
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