Black patina on japanese kitchen knives

[FatFrumos]

Hello. I'm trying out different ways to darken my knives. So far I used ferric chloride and cold bluing. Next up is coffee. But I also have a question - what method is used to darken Japanese kitchen blades? It doesn't look like ferric chloride and it's thick, feels almost like some kind of paint. I tried to Google it, but Google is useless these days.

Any info would be very appreciated.
Thank you!
Luka

 

[Drop bear]

I have done coffee on a hultafors. And it came up really dark.

 

[Hubert S.]

Try googling kurouchi, maybe that's what you are looking for. You mention paint. Some Japanese knives have lacquer on them for protection during shipping, but that should be removed before use.

 

[hhmoore]

Hello. I'm trying out different ways to darken my knives. So far I used ferric chloride and cold bluing. Next up is coffee. But I also have a question - what method is used to darken Japanese kitchen blades?

You aren't using cold bluing on kitchen knives, are you? Everything I've read indicated that it's toxic & not to be considered food safe.

 

[Stacy E. Apelt - Bladesmith]

Kurouchi is the black surface left by forging to shape. It is a layer of decarb and black iron oxide. The lower ha-bevel is ground away exposing bright hard steel. In many cases the kurouchi area is a soft steel san-mai or ni-mai.

Coffee and other coatings are not durable on things like kitchen blades.

 

[DevinT]

Cut brined meat and a patina will form.

Hoss

 

[Pinoy Knife]

you could use "Hot salt bluing" but you might lower your hardness is you use too hot of bluing set up..
i knew a "Damascus Maker" years ago that did the salt bluing and it was fairly "Low temp" as to not mess with the heat treat too much..

 

[FatFrumos]

Here are a couple of examples I was referring to:

Blenheim Forge 180mm Bunka Iron Clad Aogami Super Walnut & Copper

Blenheim Forge Classic Range 180mm bunka knife. Aogami Super carbon steel core with iron damascus acid etched cladding with kurouchi finish. Walnut & Copper oct

bernalcutlery.com

Blenheim Forge 170mm Funayuki Stainless Clad Aogami Super Fumed Oak & – Bernal Cutlery

Blenheim Forge 170mm Funayuki / short chef knife, fumed oak and brass ferrule handle. Aogami Super carbon steel core with stainless steel cladding. Forged, ground and handles made by hand at the Blenheim Forge works in Peckham South London. Blenheim Forge knives are made with Japanese aogami...

bernalcutlery.com

Is this decarb left after forging? I'm asking because the surface underneath the dark area looks extremely smooth. When I forge a blade, even if I use a flattener or a power hammer it still feels rougher then these knives here. That's why I was wondering if it's some kind of agent covering the blade.

 

[FatFrumos]

Erm, as I posted it I noticed that it says kurouchi right there I guess that answers this question.

 

[Nick-D]

Here are a couple of examples I was referring to:

Blenheim Forge 180mm Bunka Iron Clad Aogami Super Walnut & Copper

Blenheim Forge Classic Range 180mm bunka knife. Aogami Super carbon steel core with iron damascus acid etched cladding with kurouchi finish. Walnut & Copper oct

bernalcutlery.com

Blenheim Forge 170mm Funayuki Stainless Clad Aogami Super Fumed Oak & – Bernal Cutlery

Blenheim Forge 170mm Funayuki / short chef knife, fumed oak and brass ferrule handle. Aogami Super carbon steel core with stainless steel cladding. Forged, ground and handles made by hand at the Blenheim Forge works in Peckham South London. Blenheim Forge knives are made with Japanese aogami...

bernalcutlery.com

Is this decarb left after forging? I'm asking because the surface underneath the dark area looks extremely smooth. When I forge a blade, even if I use a flattener or a power hammer it still feels rougher then these knives here. That's why I was wondering if it's some kind of agent covering the blade.

Bernal use a rolling mill for their San mai afaik. Would explain the smooth kuroichi