What clay/material/coating to use to make Kurouchi?

[andur]

Hi all!
I hope someone can help me out who has more experience with knife making. I've seen many types of Kurouchi finishes: it seems as though every smith has their own secret recipe and method for making a Kurouchi finish. Has any of you tried their own? Is there a clay I could try? I'm guessing the clay/coating goes on before hardening. Maybe there are even commercial clay products available? Or should I try and mix my own?
Any tips would be welcome!
Thanks,
Andur

 

[HRC_64]

pretty sure...its just industrial black oxide

And...the romantic talk about it is just marketing...
(often if not always)

FWIW "proper" black oxide is not a DIY finish...cannot be done at home...
(requires heat + non trivial chemistry)

 

[andur]

For example my 10-year old Tanaka which has seen everyday use for about 6 years still has most of the kurouchi finish on it. It's very strong, hard and looks like a special material or finish baked onto the blade. So that got me thinking they're dipping the blade into something or coating it before heat treat.

 

[milkbaby]

It's just forge scale from forging and heat treat. When you heat up the steel to temperatures required for forging or for heat treat in an environment with oxygen, you get dark iron oxides formed at the surface. The kurouchi layer is also known as brut de forge in western knifemaking.

There is talk of some knives that have some dark color applied, but whatever that stuff is, it's probably less stable than true forge finish. You can generate a stable rust bluing, but that is more work than simply leaving the forge scale. Edited to add: Some people have used gun bluing chemicals to refresh a kurouchi finish, but I've never tried it myself.

Clay is used for hamon. During heat treatment, the clay insulates the steel from cooling fast enough to form the hardened phase. The soft steel and hard steel will polish and etch differently, so when you do either of those (or both) to the blade, you'll see the hamon develop.

Edited to add: You can also get autohamon when heat treating a thick blade. The very thick spine retains more heat and might not cool fast enough to fully harden in some cases. So when that happens, you can generate a hamon without an insulation layer like clay.

 

[user 29468]

the clay insulates the steel from cooling fast enough to form the hardened phase.

yes, if it's a thick clay layer, but if it's a thin layer, you have the exact opposite effect than what you describe.
the Ku finish can also be a burnt oil, it's simple to differentiate from a real ku because it's shiny.

 

[HRC_64]

...you get dark iron oxides formed at the surface

AFAIK black oxide and iron oxide are the same surface...

the question is do you believe they use samurai hocus pocus
or proven industrial finishing methods?

 

[andur]

I know what forge scale and burnt oil finishes look like and I know how to achieve them. Except none of those look like any of the kurouchi finishes I'm seeing on Japanese blades. That's why I'm wondering if they're doing it differently.

 

[RDalman]

I know what forge scale and burnt oil finishes look like and I know how to achieve them. Except none of those look like any of the kurouchi finishes I'm seeing on Japanese blades. That's why I'm wondering if they're doing it differently.

Austenitize in a lead bath topped with graphite. Have fun

 

[milkbaby]

Here's some comparison, from top to bottom are: Japanese knife (Shinji Fujishita from Echizen), my knife, Japanese knife (Takeda), my knife.

My knives were heat treated in propane forge and quenched in canola oil. I don't know the full process of the Japanese knives, only seen some on YouTube. The bottom one is monosteel 15N20 and the upper carbon Damascus. I believe the Fujishita is Hitachi prelaminated suminigashi clad Shirogami, and the Takeda is the carbon (non-stainless) clad Aogami Super that they made in-house (I believe they did the cladding in-house?). To my eye, whether it's cladding or monosteel, the color and character of the scale is not too much different between all these.

What do you feel is the difference between what you are achieving versus what you see on your Japanese knives?

 

[andur]

This is the best one I've seen, the Tanaka. After years of use, almost looks like new:

 

[madelinez]

The difference between the Takeda classic and the Takeda NAS (stainless clad) is pretty substantial, I could believe the classic is a result of forging but the NAS kurouchi almost looks painted on. It's very consistent and shiny. Would love to know the difference.

 

[RDalman]

The difference between the Takeda classic and the Takeda NAS (stainless clad) is pretty substantial, I could believe the classic is a result of forging but the NAS kurouchi almost looks painted on. It's very consistent and shiny. Would love to know the difference.

If it looks painted on it's probably because it is, chemically gun blue.

 

[Matus]

What about the Kurouchi on Carter knives? I know he cold forges the knife before HT and the surface is pretty smooth. I do not think he uses gun blue or similar stuff.

 

[user 29468]

stainless do not react the same way to the forging process, very uniform black smooth finish, iron is more "grainy" and the black is less durable

 

[HSC /// Knives]

What about the Kurouchi on Carter knives? I know he cold forges the knife before HT and the surface is pretty smooth. I do not think he uses gun blue or similar stuff.

good day
The Kurouchi on Carter knives is the same as any other Kurouchi finish, meaning its simply a raw forged finish.
Cold forging is independent of the final finish and done on all of their laminated steel blades,
There is no gun blue used at Carter or anything similar. I can attest to this having spent 9 days at his shop.

regards
Harbeer

 

[Pensacola Tiger]

The difference between the Takeda classic and the Takeda NAS (stainless clad) is pretty substantial, I could believe the classic is a result of forging but the NAS kurouchi almost looks painted on. It's very consistent and shiny. Would love to know the difference.

It's just the difference in how the stainless cladding forms scale. The kurouchi on Kochi blades is another example of how stainless reacts.

 

[Matus]

I have hear from many sites that using stuff like gun blue is widely used in Japan on kurouchi kitchen knives. It would be great to know a few examples where this is the case so that it would be easier to identify. There are wast differences in the kurouchi finishes on kitchen knives though. Just compare something like a Zakuri, Kochi and Takeda (carbon clad) - just to name a few examples.

 

[keithboyle13]

Hi,

This style of finish can be achieved with a clean surface prior to heat treatment and a neutral to carburising fire. The key is minimizing excess oxygen when heating. Without excessive oxygen you can get this finish without heavy scaling, and associated pitting.

A wash coating of clay can promote faster quenching and provide an oxide barrier to stop scaling and decarburisation. The viscosity of the clay wash used can have significant effects on the oxide surface from light to no oxide for thicker coats to a full black oxide with very thin coats.

Like I mentioned control over the heating will have the best response to obtaining this finish and if using clay to make sure it actually sticks to the blade. Satanite or similar products are easily obtained, and pretty user friendly without any magic or voodoo home brew clay mixtures.

If you wish to look at putting a stable oxide coating onto an existing Carbon steel blade. Research rust bluing with Hydrogen peroxide, Salt, and Vinegar. This is a little intimidating to start with, but works very well.

 

[Cuna]

this yellow copmound at 9:10 is what we used when making our own knives in at Takefu Knife village in japan.
i Never asked what it is tho.
does anyone know what it consists of?
apparently it is responsible for blackening the steel.

 

[Blank Blades.]

this yellow copmound at 9:10 is what we used when making our own knives in at Takefu Knife village in japan.
i Never asked what it is tho.
does anyone know what it consists of?
apparently it is responsible for blackening the steel.

I would be interested to know if you found out what that stuff was.

 

[user 29468]

it's a mix of clay , sand ( or stone dust) and other stuff, each maker have is own recipes .