# Making a knife with a hard to remove kurouchi finish.



## jwthaparc (Jul 25, 2022)

I've been wondering if there are things that can be done by someone making a knife, to make a kurouchi finish, that won't come off easily. I'm sure people have had knives that have the finish come off easier than others. So there must be variation from the different japanese makers on the process there blades go through when doing a blade with a kurouchi finish. 

One thing I was thinking, and this may be completely ridiculous. Is do something that would be sort of similar to how steel is case hardened, and when trying to do this, intentionally getting a layer of carbon built up on the surface. Maybe during the normalization process. Then going about heat treating, and grinding the knife like it normally would be for a ku knife. 

That above idea, might be stupid. But it's just a thought. I would love to hear from anyone that's got some experience with these types of finishes. 

(And I know about parkerizing, and bluing, I'm just talking about a "natural" ku finish).


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## Carl Kotte (Jul 25, 2022)

I don’t know anything about the details that go into making a lasting ku. But that there are tricks (and perhaps some magic involved) that go into making it, I do know since it is something discussed now and then in the Swedish knife maker podcast. @Isasmedjan has refined his recipe for quite some time.


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## M1k3 (Jul 25, 2022)

Carl Kotte said:


> I don’t know anything about the details that go into making a lasting ku. But that there are tricks (and perhaps some magic involved) that go into making it, I do know since it is something discussed now and then in the Swedish knife maker podcast. @Isasmedjan has refined his recipe for quite some time.


I think @RDalman has perfected his recipe also?


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## MSicardCutlery (Jul 25, 2022)

You can make the finish deeper, but that's about all I can think of in terms of making it last longer. Maybe a scraping after forging and a specific scale forming treatment would help make it more uniform and durable.


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## RDalman (Jul 25, 2022)

I believe, from seeing it in many japanese videos, lead bath austenitizing (with graphite on top of the lead) is common in jp, and I think your idea with building carbon on the surface might produce something similar. But you might need to do it when austenitizing.


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## inferno (Jul 25, 2022)

dual oil quench seems to create a quite durable kuro. at least with motor/hydraulic oil mix


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## DanielC (Jul 25, 2022)

Mine seem pretty durable.


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## inferno (Jul 25, 2022)

DanielC said:


> Mine seem pretty durable.
> 
> View attachment 190075


whats your "recepie"?


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## jwthaparc (Jul 25, 2022)

RDalman said:


> I believe, from seeing it in many japanese videos, lead bath austenitizing (with graphite on top of the lead) is common in jp, and I think your idea with building carbon on the surface might produce something similar. But you might need to do it when austenitizing.


Why do you think it would be necessary to do it during the austenizing soak?

Edit: also, lead bath... does that just sound worse than it is? Because that makes me think maybe it could be iffy if used on food afterwards.


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## superworrier (Jul 25, 2022)

Apparently they coat in graphite to release the lead afterwards. IDK how much would stay. But I've also read about salt baths. I wonder if lead bath is a misnomer or not.


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## deliyski (Jul 25, 2022)

Find Michio Ishikawa on YouTube. There's hours of videos of him explaining basically how to make knives which no Japanese bladesmith is ever telling. He has no apprentice and he will be the last of his name. He talks about molten graphite. I have his knife and can confirm the kurouchi finish is proper black and even and it's only missing where I've scraped it on the stones.


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## Kippington (Jul 25, 2022)

jwthaparc said:


> Why do you think it would be necessary to do it during the austenizing soak?
> 
> Edit: also, lead bath... does that just sound worse than it is? Because that makes me think maybe it could be iffy if used on food afterwards.





superworrier said:


> Apparently they coat in graphite to release the lead afterwards. IDK how much would stay. But I've also read about salt baths. I wonder if lead bath is a misnomer or not.



Lead baths were a common old-school method of getting to the quenching temperature. They switched to salt baths when they worked out how poisonous the lead was to the workers, who were hanging around the fumes all day.

I have no idea how either one effects the KU though.


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## inferno (Jul 25, 2022)

iirc "salt baths" can sometimes be cyanide salt baths. and thats probably not so healthy either.
or maybe those were just used for carburization?


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## inferno (Jul 25, 2022)

jwthaparc said:


> Why do you think it would be necessary to do it during the austenizing soak?
> 
> Edit: also, lead bath... does that just sound worse than it is? Because that makes me think maybe it could be iffy if used on food afterwards.


the steel is much more reactive at higher temps since the atoms are moving much faster/wider, and you can diffuse stuff into it deeper/faster then. the steel also stays at that temp longer than the actual quench phase. thats my guess.


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## DanielC (Jul 25, 2022)

Heat treating in molten lead "namara-nabe" is still seemingly somewhat used in many shops in Japan. As long as the lead is not overheated, it shouldn't give off any troublesome fumes.

Charcoal is often lain on top of the molten lead to consume oxygen, as O2 does enter the molten lead and can react with the steel.

I talked to Shosui Takeda a bit about his lead pots, because he still uses them and I asked him why he didn't use salts. He said there wasn't an immediate need to switch.

Lead transfers heat to the steel faster than anything I know of for this, including salts. I've actually looked into setting one up myself, but like Shosui, there isn't an immediate need to switch over from my current methods.


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## McMan (Jul 25, 2022)

Michio Ishikawa continues to use lead. He discussed it here: 
Apparently he has a good reputation for producing fine, homogenous KU.


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## jwthaparc (Jul 26, 2022)

inferno said:


> the steel is much more reactive at higher temps since the atoms are moving much faster/wider, and you can diffuse stuff into it deeper/faster then. the steel also stays at that temp longer than the actual quench phase. thats my guess.


The normalization would be when the steel will be the hottest (besides forging) and stay at that temp the longest.


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## Kippington (Jul 26, 2022)

DanielC said:


> I've actually looked into setting one up myself


I'm way more terrified of lead poisoning than you are, if you were thinking of setting one up. There's no way I'd want to be working anywhere near one of those.


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## jwthaparc (Jul 26, 2022)

Great video!

Good to know I wasn't entirely off base with my assumptions on what can be done to get a strong ku finish. 

I dont plan on getting a salt bath, or a lead pot anytime soon. However I have some ideas on how I could maybe get this to work in my kiln.


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## DanielC (Jul 26, 2022)

Kippington said:


> I'm way more terrified of lead poisoning than you are, if you were thinking of setting one up. There's no way I'd want to be working anywhere near one of those.


Possibly. Like anything in a hot shop like ours, proceed with caution.


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## jwthaparc (Jul 26, 2022)

I'll probably try some different methods with little bits of steel scraps I have laying around, and see what I can come up with. 

If all else fails I'm sure graphite wont be too terribly expensive.


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## jwthaparc (Jul 26, 2022)

What I'm wondering. Is what is that varnish they were talking about in the video?


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## DanielC (Jul 26, 2022)

Fwiw, molten lead poses less fire risk as well. Of something moist enters molten lead, it doesn't shoot a molten fountain to the shop ceiling, potentially burning the place down. It will grumble from moisture, but it won't pop in your face like salts.

I've used graphite in my clay mixtures and haven't noticed much of a difference.


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## jwthaparc (Jul 26, 2022)

DanielC said:


> Fwiw, molten lead poses less fire risk as well. Of something moist enters molten lead, it doesn't shoot a molten fountain to the shop ceiling, potentially burning the place down. It will grumble from moisture, but it won't pop in your face like salts.
> 
> I've used graphite in my clay mixtures and haven't noticed much of a difference.


For differential hardening? 

The way he was using it in the video. He just had graphite mixed with water, coated the blade, and let the water evaporate.


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## Kippington (Jul 26, 2022)

DanielC said:


> Fwiw, molten lead poses less fire risk as well. Of something moist enters molten lead, it doesn't shoot a molten fountain to the shop ceiling, potentially burning the place down. It will grumble from moisture, but it won't pop in your face like salts.
> 
> I've used graphite in my clay mixtures and haven't noticed much of a difference.


For a few years now, I've been curious about how dangerous, difficult and effective it would be to use molten aluminium. Looking into it at a casual level, I haven't found anything that says it wouldn't work out, but at the same time I think if it was any good, others would be using it instead of salts... Maybe one day I'll try it.

@Larrin - Would molten aluminium (if used to heat steel to austenitizing temperature) mess with the chemistry of the steel?


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## MSicardCutlery (Jul 26, 2022)

DanielC said:


> Fwiw, molten lead poses less fire risk as well. Of something moist enters molten lead, it doesn't shoot a molten fountain to the shop ceiling, potentially burning the place down. It will grumble from moisture, but it won't pop in your face like salts.


I have to respectfully disagree. A 20lb pot can be emptied in an instant from something a little too damp. Many amateur hand loaders casting bullets have found out the hard way not to immediately toss wonky castings back into the pot without letting them dry fully, I myself being one. Much of the time it will just fizzle, but having observed a 5' tall lead fountain up close once, and worn some of it, it's not something I would want to risk twice. 

As a side note, when I got into casting bullets I was very interested in the dangers posed by lead fumes. The general consensus on loading forums at the time was that bullet casting temperatures were too low to be a concern, and that so long as the lead were well below boiling point it would not emit vapors. However I did come across a few posts by a member who had access to the equipment needed to actually measure whether this was the case or not, and it wasn't. Even at the low temperatures used in casting, the lead was giving off a potentially concerning amount of vapor. My conclusion was ultimately that I should error on the side of caution and have always done my casting either outdoors, or with some sort of ventilation set up.


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## DanielC (Jul 26, 2022)

MSicardCutlery said:


> I have to respectfully disagree. A 20lb pot can be emptied in an instant from something a little too damp. Many amateur hand loaders casting bullets have found out the hard way not to immediately toss wonky castings back into the pot without letting them dry fully, I myself being one. Much of the time it will just fizzle, but having observed a 5' tall lead fountain up close once, and worn some of it, it's not something I would want to risk twice.
> 
> As a side note, when I got into casting bullets I was very interested in the dangers posed by lead fumes. The general consensus on loading forums at the time was that bullet casting temperatures were too low to be a concern, and that so long as the lead were well below boiling point it would not emit vapors. However I did come across a few posts by a member who had access to the equipment needed to actually measure whether this was the case or not, and it wasn't. Even at the low temperatures used in casting, the lead was giving off a potentially concerning amount of vapor. My conclusion was ultimately that I should error on the side of caution and have always done my casting either outdoors, or with some sort of ventilation set up.


This was the concensus explained to me by other makers who were very familiar with salt pots and the historic usage of lead pots apparently. Anecdotal admittedly.

I wouldn't personally dare put anything moist in one but I think it was explained as items not fully, completely dry or even a bead of sweat off the forehead won't make lead shoot I'm your face, but I don't cast bullets or have a lot of experience with it to be completely sure, maybe i misunderstood and it was only things like sweat rolling off your face, but it waa generally accepted that lead doesnt react as easily or violently as salts do, but I don't have direct experience so I will take your word on it, with experience playing with it.

As far as lead fumes go. Thank you for that. Luckily I haven't given it a go 

Edit: Maybe this is why Takeda is always so happy, or welds tangs to knives :0


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## DanielC (Jul 26, 2022)

Kippington said:


> For a few years now, I've been curious about how dangerous, difficult and effective it would be to use molten aluminium. Looking into it at a casual level, I haven't found anything that says it wouldn't work out, but at the same time I think if it was any good, others would be using it instead of salts... Maybe one day I'll try it.
> 
> @Larrin - Would molten aluminium (if used to heat steel to austenitizing temperature) mess with the chemistry of the steel?



Ever since I switched to a vertical HT barrel, I've lost most interest for molten HT options. It's just so easy to use and I don't mind applying clay coatings. But I want to see a video if you do it with aluminum


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## DanielC (Jul 26, 2022)

jwthaparc said:


> For differential hardening?
> 
> The way he was using it in the video. He just had graphite mixed with water, coated the blade, and let the water evaporate.


No, clay washes to prevent or slow decarb and scale.


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## MSicardCutlery (Jul 26, 2022)

DanielC said:


> Edit: Maybe this is why Takeda is always so happy, or welds tangs to knives :0


My father was a pipe fitter for a few years and told me about a day he spent working on some galvanized stuff, he had to come home early....something about not being able to work with so many pink rabbits running around the mill...


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## inferno (Jul 26, 2022)

if you guys dont want to breathe in some toxic molten lead fumes there is always the old classic:

cadmium! Melting Point = 321C

or why not any of these?

bismuth = 271C
zink = 419C
tin = 232C
selenium = 221C
antimony = 630C
barium = 727C
cerium = 795C
thallium = 304C
lithium = 180C
magnesium = 650C (this one could get exiting)
arsenic = 817C
or why not some good old plutonium  = 640C (i'd love to see this done)






Melting points of the elements (data page) - Wikipedia







en.wikipedia.org





I have a question though. how much energy does it take to even get a bath of liquid lead/alu going?
lets say you have a 10l bath, 10l alu = 27kg. and lead is like 110kg. there is a lot of metal to melt.


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## jwthaparc (Jul 26, 2022)

DanielC said:


> No, clay washes to prevent or slow decarb and scale.


Ah got it. 

What I'm thinking. Well first I'll test any results that can be had during normalization. If that doesn't have good results. I'll try the same plan, but while Austenizing. If that doesn't work, well. I can make more modifications. 


Basically what I'm thinking will be wrapping the blade with source of carbon, I've read tums works well, to help this along. It's going to be inside a controlled environment (stainless steel, tool wrap) so idk how that will effect quench speeds on lower hardenability steels. It will likely be fine, but I'm going to test it to be sure.


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## jwthaparc (Jul 26, 2022)

inferno said:


> if you guys dont want to breathe in some toxic molten lead fumes there is always the old classic:
> 
> cadmium! Melting Point = 321C
> 
> ...


I really dont think a lithium bath sounds like a good idea lol.


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