Japanese heat treat clay mix

[Mende]

As you all probably know. .to achieve a good cutting edge and a nice hamon, Japanese bladesmiths applied a mixture of clay, coal dust,ashes and some other ingredients on the blade except for the edge to keep the spine of the blade from cooling too fast and thus keep it flexible while allowing the edge to harden to its full potential.

Recipes varied from smith to smith and were very closely kept secrets.
What I am interested in knowing is a good recipe for this mixture.

Has anyone tried this selective hardening successfully ?
I have read all there is online and seen a lot on youtube. . but the exact recipe is never shown.

Any info will me much appreciated. Thank you.

[Ice Czar]

a clay is like a salad, there can be any number of ingredients in it
what they have in common is their particle size

the Heat Capacity of minerals varies considerably, but silicates which are often the particles in many clays are fairly uniform in heat capacity.


in other words, even with a recipe you have an uncontrolled variable in the clay you have available, added to the other variables of mix proportions, the heat vs time regimen, steel structure, its going to be trial and error, and goes towards explaining why it was individual recipes even in Japan.

Hope you get some better answers

[ApprenticeMan]

It is a STOOPID myth that you have to use some magic clay mix, just get Satanite refractory clay, or AP green. It is not so much about the clay as about the insulating properties. The fact that it is clay doesn't mean DIDDLY SQUAT, it's the fact that the clay on the spine insulates the spine from forming martensite, and the exposed edge forms martensite, which gives the hamon effect after polishing.

[Robert Mayo]

I agree with Sam i use plain old furnace cement with exelent results.

[Mende]

furnace cement you say . .. Thx guys! makes sense to me

[Jmercier]

Satanite castable refractory cement works great for getting a nice active hamon, mix it a bit liquidy and 'paint' it on with a popsicle stick or something, let it dry out a little before you cook the rest of the water out of it so it doesnt just fall off the blade, and you're set to go. You can really easily help controll (to an extent) just where your hammon will be by painting lines up and down the sides of the blade with the cement.

[Mende]

ok . .but I do remember reading somewhere that coal dust has a special role in the mixture. ..because it makes it porous after burning and that helps the insulation process by trapping air or something. .

I think I'll experiment with that a bit. . .satanite+ coal dust mixtures versus pure satanite.