# Kurouchi finish



## palyujl (Aug 21, 2017)

Any one here knows how to put kurouchi finish back on knife? Gyuto blue steel 1 thanks


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## Pensacola Tiger (Aug 21, 2017)

As far as I know, once it's gone, it's gone. Some people have used gunsmith's cold blue to darken the cladding, if I recall correctly.


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## Kippington (Aug 21, 2017)

It would be easy to add a kurouchi finish if we could heat the steel up above its tempering temperature... but we're not allowed to do it. :nunchucks:

Lately I've been experimenting with the same techniques used to get cast iron pans to look black, but it's not looking so crash hot without going above 200°C.


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## HRC_64 (Aug 21, 2017)

Pensacola Tiger said:


> As far as I know, once it's gone, it's gone. Some people have used gunsmith's cold blue to darken the cladding, if I recall correctly.



If the KU is a black oxide finish, this is would be correct.
BO finish by itself is not something that can be repaired at home.

here's some notes on black industrial finishes;

https://www.epi.com/black-oxide/

I'm not familiar with the technical specs on KU finishes, 
if these are standard oxide finishes or some unique trade secrets

the cold/DIY grade finishes are generally significantly lower quality, but a home-remedy 
of sorts for lost black-oxide finishes, recommended commonly on the internet
industrial finishes, however, need heat and specialized chemistry (expensive/bulk etc)
to set a much thicker, more durable, and more uniform protective oxide layer.

also, the heat (200+degrees) for setting industrial BO finish is enough to be get nervous about HRC on blades
Also, make sure to research any food-safety issues, so this is for a specialist
if its something you absolutely need to have refinished.



good luck either way.


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## aaamax (Aug 22, 2017)

If you take a piece of carbon steel and heat it, even well below a red heat and dip it in a food oil, you will get a black finish. Not quite the same thing as the scale on it originally, but more in line with hot gun bluing and very durable. Albeit a natural way as opposed to nasty chemicals. You are monkeying with the heat treat though.
If you want real scale as the maker had originally, you need to take her up to a red heat and you can kiss any fine edge you have on there good bye.

the unnatural chemical stuff is scary from a food safety standpoint.


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## Pensacola Tiger (Aug 22, 2017)

aaamax said:


> the unnatural chemical stuff is scary from a food safety standpoint.



Not if you understand what the process is doing. Cold bluing reacts with the metal and is removed afterward. There is no residue left to contaminate food. 

Rick


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## Nemo (Aug 22, 2017)

Pensacola Tiger said:


> Not if you understand what the process is doing. Cold bluing reacts with the metal and is removed afterward. There is no residue left to contaminate food.
> 
> Rick


What's the chemical reaction that it causes?


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## Pensacola Tiger (Aug 22, 2017)

Nemo said:


> What's the chemical reaction that it causes?



I'm not a chemist, but the cold bluing solution I'm familiar with (Brownell's Oxpho Blue) contains phosphoric acid which reacts with the metal to form bluish/black phosphates on the surface.


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## natto (Aug 22, 2017)

Cold rust bluing leaves a thin film of iron oxides. The color changes with the thickness of the film, like colors from tempering. 
Some cold bluing solutions are pretty poisonous. I don't like the idea to use it on knives. The result should be iron oxide coating again.


KU from quench should be burnt oil. But some look like paint to me. I like fading Ku on a forged surface. No idea how poisonous it is. Funny how I don't mind burnt oil and mind cold bluing solutions.


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## Kippington (Aug 22, 2017)

natto said:


> KU from quench should be burnt oil



I don't think this is right. Steel brought up to forging temp has a black coating over it. Fe3O4, black iron oxide.

You can get it after quenching in water - no oil involved...
Well actually, its already there before the quench.


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## natto (Aug 22, 2017)

Sorry, what is there before the quench?
I'd be glad if being wrong in this case.


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## Kippington (Aug 22, 2017)

I'm also happy to be wrong.



natto said:


> Sorry, what is there before the quench?



Fe3O4, black iron oxide, also known as forge scale. Happens to simple steels in an oxidizing environment at high temp (above 800°C).


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## natto (Aug 22, 2017)

I try to remember, most of my steel turned grey. That was scrap spring steel for exercise, but contained silicon alters the forge scale. Fe3O4 is nicer on a kitchen knife than burnt oil.


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## milkbaby (Aug 25, 2017)

You could do a dark patina with vinegar etch. Paint the edge and blade road with a resist like nail polish. Saturate a paper towel with vinegar and wrap the blade for a while. You could also dip it in hot vinegar instead.


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## tommybig (Aug 25, 2017)

milkbaby said:


> You could do a dark patina with vinegar etch. Paint the edge and blade road with a resist like nail polish. Saturate a paper towel with vinegar and wrap the blade for a while. You could also dip it in hot vinegar instead.



I wanted to do this on some spots where the kurouchi went off.

Could vinegar hurt/weaken the kurouchi that is still on?


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## dwalker (Aug 25, 2017)

Find someone who does black parkerizing in your area. I've done it to one of my Moritakas. It is more durable than KU and takes a very trained eye to tell the difference. Parkerizing is a common firearm finish and will work on iron cladding but not stainless cladding. It is safe for the steel as it only reaches 100°c maximum. Mask off the blade road with a waterproof tape like used in powdercoating. You can then media blast the remaining finish down to bare metal. It takes about 1-2 minutes in the solution and you will have an even, durable KU finish. I'll do it for you if you want to send me the knife. It would cost a bit of postage, but I would do it for free to help out a member here.


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## HRC_64 (Aug 25, 2017)

tommybig said:


> I wanted to do this on some spots where the kurouchi went off.
> 
> Could vinegar hurt/weaken the kurouchi that is still on?



Acids in general eat black oxide(s), 
so do some research or test a sample
before messing around on your knife

http://www.practicalmachinist.com/vb/general-archive/stripping-black-oxide-95208/#post270746


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## playero (Aug 30, 2017)

Some of the options mentioned seem like electro coating. Most of them don't work. I have tried vinegar and it leaves like a shadow. Lemons more or less the same. Tried plantains skins last night didn't do a thing. Gun blue when applied it turned the blade a light black color. Left for 30 minutes and when I cleaned 2 knives it disappeared. Have muriatic acid and blood at my house but haven't the time to use them.


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## Silky (Aug 30, 2017)

Not trying to push any services, but I believe that Jon Broida has posted some pictures of restored kurouchi finishes on carbon knives that he has done. Might be worth it to send him a call. If the knife is stainless-clad though, you're out of luck (I'd love to hear a solution if you found one though)


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## TurboScooter (Aug 31, 2017)

How about rust bluing?

Maybe you can make a tacticool kitchen knife - like Duracoat/Cerakote or DLC/PVD.


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## Nomsdotcom (Sep 1, 2017)

dwalker said:


> Find someone who does black parkerizing in your area. I've done it to one of my Moritakas. It is more durable than KU and takes a very trained eye to tell the difference. Parkerizing is a common firearm finish and will work on iron cladding but not stainless cladding. It is safe for the steel as it only reaches 100°c maximum. Mask off the blade road with a waterproof tape like used in powdercoating. You can then media blast the remaining finish down to bare metal. It takes about 1-2 minutes in the solution and you will have an even, durable KU finish.  I'll do it for you if you want to send me the knife. It would cost a bit of postage, but I would do it for free to help out a member here.


I've been wondering about this. Saw a blade Straub posted on I.G. a while ago with a "parkerized" finish. Is there a significant difference/advantage between this and an acid etch? My understanding is that it is similar to a heated acid etch with additives that create the corrosion resistance.


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## StonedEdge (Sep 1, 2017)

Might be easier to sell it and buy a new one?


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## shipbuilder (Sep 10, 2017)

Given all the discussion above, plus 40+ years in the shipbuilding business (including with Bethlehem Steel), the simple answer is, you can't. The series of steps that an experienced smith goes through to create his version of KU cannot be undone and redone without creating a completely different knife than the one you started with. [I've never understood the value or purpose of KU finishes anyway - if they were so damn good, why do we not see them deployed in other "using" metal products, whether swords, machined parts, industrial products, whatever ? They are decorative and not particularly and not particularly attractive to my eye. I have gradually been removing the KU from my two Takedas and like them better and better.]
Tom


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## HRC_64 (Sep 10, 2017)

KU is a forced patina....
inherently dubious longevity IMHO

black oxide is basically rust
in terms of chemistry


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## aaamax (Sep 10, 2017)

shipbuilder said:


> Given all the discussion above, plus 40+ years in the shipbuilding business (including with Bethlehem Steel), the simple answer is, you can't. The series of steps that an experienced smith goes through to create his version of KU cannot be undone and redone without creating a completely different knife than the one you started with. [I've never understood the value or purpose of KU finishes anyway - if they were so damn good, why do we not see them deployed in other "using" metal products, whether swords, machined parts, industrial products, whatever ? They are decorative and not particularly and not particularly attractive to my eye. I have gradually been removing the KU from my two Takedas and like them better and better.]
> Tom



There is the answer in spades.
Plus, when a long time member, but a reluctant poster writes something, it is probably well thought out advice.
Cheers.


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