Kuro Uchi quenching ???

[jlwamp]

This is similar to a post I've made here before.

One time I've gotten lucky, but pretty much every time I'm going for a kuro uchi or forge finish and have a deep black, smooth forge scale built up.
I grind it, love the look, then quench it and my precious black scale blasts off into the oil.

Is there something I'm missing?

(this last time was 1084 into canola oil)

 

[Tenebr0s]

You want the blade clean of all scale before quenching. Thick scale buildup is not kurouchi (kurouchi is relatively thin compared to heavy forge scale). If you heat and oil quench a clean blade, you should get a nicer black finish on the knife.

 

[JTknives]

There are other ways to get a nice black finish then wanting to keep the nasty forge scale on the blade. It's good that your quench "blasts" the forge scale off. I don't consider forge scale to be a stable substance. Last thing I want is to have forge scale come off the blade into food. Forge scale always seams rough and snaggy. But on black smith items it's a little different. I will forge and wire brush/wheel off the loose stuff and kinda smooth out the roughness. Then I will oil/wax coat it and then re heat it. Then I will wipe it down again with a oil or bees wax. Makes a nice vintage blacksmith finish that is durable.

 

[jdm61]

One possibility is that most if not all Japanese kuro ichi blades are clad in "iron" which does not harden, ergo, no scale blowoff.

 

[Stacy E. Apelt - Bladesmith]

jdm61 has the main answer. Also, the final finish is a patina. The blade is etched after quench and temper to make it black. The soft iron will take on a much deeper tone, ad it will stay in the recesses. Even with an all steel blade, this is how I get the dark lines and dents.

 

[Tenebr0s]

Stacy, I'm curious what etch you have in mind that leaves a darker finish on iron cladding? I find ferric chloride doesn't do a very good job of this for me (its tends to leave the mild-steel a light gray).

There are lots of ways to do kurouchi in Japan (some that involve coatings on cheaper knives, etc), but the most traditional - and most common - is a very thin layer of black iron oxide left over from the quench itself. The thin wash of clay, which all blades are coated in, helps prevent heavy buildup. It's not something that will flake off in use.

 

[Tenebr0s]

This is a little fish knife I did with a traditional kurouchi finish.

 

[lanternnate]

This was just a happy accident that I kept.

It's 5160 and it was a clean 600 grit satin finish prior to heat treat. The black went on from the canola oil during the heat treat. One way I have read about but not tried is a mix of beeswax, linseed oil, and turpentine. That's a traditional blacksmith finish to put that classic black look on pieces. My understanding is that is traditionally done at full red heat, so I would think it would need to be before heat treat. I don't know if it would then blow off during the quench.

 

[Stacy E. Apelt - Bladesmith]

Many patina solutions will blacken iron. I have used 15:1 FC, slate black ( a commercial iron black), brass black, stove black, and telluric acid. Some of the commercial Japanese knives have a baked on lacquer in the recesses. There are several blacksmith recipes for black finishes, too.

 

[Tenebr0s]

Other than FC, do you know if any of these leave a food safe finish?

Although I don't mind leaving black from the quench if I use water, on steels I oil quench, I prefer to clean it completely if the knife is going to be used in the kitchen.

Many patina solutions will blacken iron. I have used 15:1 FC, slate black ( a commercial iron black), brass black, stove black, and telluric acid. Some of the commercial Japanese knives have a baked on lacquer in the recesses. There are several blacksmith recipes for black finishes, too.