Now comes the critical step in developing the beauty and functionality of the blade. Most will refer to this process as hardening and it started already when I thermal cycled the blade in what has become known as normalizing. At this point I will clay-coat the blade in preparation for quenching and hardening the blade. That is called differential hardening (hard edge - soft spine). And to finish the process the blade will be tempered or drawn back a little to reduce the brittleness of the edge.

In order to gain as much knowledge as possible from this steel I am planning on using three different clayformulas applied to the blades in different sections and with different thicknesses. I will also vary the soak time at austenizing temperature in the heat treating forge to see how that affects the steel.

I will start by mixing my own clay formula: 50% fireclay + 50% charcoal.

I have develop a set of simple tools for laying down the clay: a brush and a spatula with some shaping of the tips to form the clay as I want. A little plastic box to mix stuff to the consistence that I need and a hard plastic board as an easel.

Here I am mixing my clay to a thick paste first.

The second formula that I will be using is well-known by everyone: satanite.

And the third formula is based on a recommendation from Don Fogg: an anti-scale compound that is quite plastic and easy to work with.

I will start by applying the satanite to the back of the blades. One blade will have a thick coat and the other a thinner coat.

Detail view.

Then Don's anti-scale compound. Thick coat and thin coat again.

Detail view.

Then my own formula for the kissaki. A tricolor blade.

After letting the clay dry over night I am ready to quench on the next evening. There will be no pictures of the actual quench. Sorry about that but I was by myself and this step requires all my concentration and could not take pictures at the same time. But here is a picture of the setup. I have a long heat treating oven made out of a water tank, a pyrometer to guide my temperature although the final decision as to when the blade is ready to be quenched is a visual one based on the colors of the blade and not the reading on the pyrometer. And I have a fish tank for tanto-size blade. The fish tank contains water saturated with salt and some soap at about 110 degree F.

Another useful tool is a blade holder. A piece of rebar to which I have welded two attachments on the ends to hold the blade. One small size for tantos and the other for larger blades.

Here is a detail view of the holding attachment.

And at this point yaki ire (quenching) has been completed and the blades are in the oven cooking at 300 degrees F for one hour and a half to temper the edge and reduce the brittleness of the martensitic edge. Prior to putting them in the oven I have checked the edge with a file to see that they hardened properly.

Next morning this is what they look like.

After a quick clean-up I noticed that the satanite and the anti-scale compound did not leave any residue but my clay mix interacted with the steel a little deeper and left some residue (carburization?).

I now have two twin blades ready for polish.