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View Full Version : kuro-uchi and reactivity?



Amon-Rukh
05-17-2012, 02:22 AM
I've never seen a kuro-uchi knife in person before, much less handled one, so I was curious how this sort of cladding affects reactivity and patina development in carbon knives?

Crothcipt
05-17-2012, 02:44 AM
It pretty much stops it until it is worn, or taken off. It is from what I read a cover left from forging, so there is no reactive with anything.

Taz575
05-17-2012, 04:49 AM
In short, patina or rusting is the steel on the surface reacting to the food and what is in the food. The kurouchi is basically forging scale that is left on the knife. It has already reacted when it formed the scale, so the scale doesn't have much if any carbon that can react since it already has reacted. I've also seen many Kurouchi knives come with a thin, clear lacquer, which further helps keep the surface from rusting. Like Croth said, if you grind off the kurouchi finish, the bare steel under it will rust/patina/react.

I may be off a bit (going on memory at 4am!), but I have read in various knifemaking books that when the steel is forged, heat treated, etc, the very surface of the steel has been decarburized to some extent and that the carbon is burned off. Wayne Goddard told about how the very surface of the steel should be ground away after forging because the surface is decarburized. I think they story he related was that someone had a problem with a knife not holding an edge until it got several sharpenings and got back far enough to get back into the carbon rich steel. It had something to do with the amount of forging needed to shape the knife and different steels and makers would need to remove different amounts of steel from the surface to get to the carbon rich steel.

TB_London
05-17-2012, 05:59 AM
Don't forget it's the iron that rusts not the carbon? Forge scale AFAIK is essentially FeO (iron II oxide) as opposed to rust Fe2O3 (iron III oxide). If they have oil Quenched the blade there'll be a coating of burnt oil as well.

You can think of the KU in the same way as the seasoning on a cast iron pan

Taz575
05-17-2012, 09:27 AM
Yeah, iron not carbon, sorry! 4am is too early for me to be posting usually :) Good analogy with the cast iron pan!

tk59
05-17-2012, 10:42 AM
The term kuro-uchi loosely describes any finish that's been left baked onto the finished blade to prevent corrosion and may not be actual forge scale or quench byproducts.

Taz575
05-17-2012, 10:57 AM
That's interesting about "baking" the finish on. I always saw references about keeping the forging or heat treating scale intact to preserve the soul of the blade. The Tojiro ITK gyuto I have had a different look than the Tanaka Kurouchi Nakiri and different than the Tojiro Shirogami Nakiri. The Tojiro almost looks like it was blued (ie old style gun bluing), the Tanaka was definetly forge scale as was the Ittosai Bunka Santoku I got from HidaTool amd you could see some of the hammer marks. The Tojior's were much smoother in finish and looked more artificial. I guess it depends on the manufacturer!

G-rat
05-17-2012, 10:59 AM
The term kuro-uchi loosely describes any finish that's been left baked onto the finished blade to prevent corrosion and may not be actual forge scale or quench byproducts.

Yep, that's what it looks like moritaka does for their supreme and blue 2 series. On my cleaver when it came it looked really nice but it looked almost brushed on

cwrightthruya
05-17-2012, 02:36 PM
I've also noticed that with my Moritaka KU knives. It's almost like a baked on spray paint and then an extra coating of something added afterwards. I'm assuming it is a type of baked on glaze with a lacquer added for extra protection because it could not be washed/scrubbed off. As a side note: this coating left a very foul taste in my food so I had to try and remove it (a very bad idea if anyone is thinking about it).The exposed steel underneath was so reactive that it would not take a patina, but would instead rust in under a minute, no matter what I was cutting.

I have not had this problem with any of my other KU knives, only the Moritakas.

Vertigo
05-17-2012, 09:09 PM
The Tojior's were much smoother in finish and looked more artificial. I guess it depends on the manufacturer!
About 1/3 of the KU on my Tojiro ITK 210 gyuto came off the first time I wiped it aggressively with a wet cloth. That's less like "baking on" and more like "coloring with crayon." Lol.

NO ChoP!
05-17-2012, 10:37 PM
+1 to ITK korouchi being horrid. One could completely remove it with a green scrubby.