Finish on Japanese kitchen knives

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Hello folks - I was hoping to get some insight into the blackened surface appearance on the knives in the attached photos. Is this an applied treatment or merely the raw finish after hardening?

Much appreciated - Ed

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I suspect it's both....... mill scale, with some type of darkening treatment for a consistent color.
 
Thanks Ed. I wonder if it’s some residue from differential hardening. I guess it’s going to be more research and testing.
 
I'm fairly certain the black is either mill scale or forge scale. With how think those types of blades are, they were likely hardened/tempered while in profile only form, and then ground. There's a number of different ways to get that looks.... with the simplest being what I mentioned.
 
some Japanese knives of this style are wrought iron with a small piece of blade steel forge welded in(san-mai). the dark finish is on purpose, from Zknives (http://zknives.com/index.shtml) " kuro-uchi finish. In general the terms kuro-uchi refers to blackened, rustic finish on the knife, and so far no two makers I have knives from, made the same kuro-uchi finish. However, Watanabe made this one different from his other kuro-uchi knife I have, 150mm Pro Nakiri. The Sakekiri finish is more rougher and somewhat grayish color. " old style Japanese blades were made this way to reduce the amount of blade steel needed to make a knife. also, the blade was enclosed with softer, tougher wrought iron. there seem to be 3 groups made now: blade steel with wrought iron, blade steel with low carbon steel and blade steel with low carbon stainless.
 
Ahhh. Right. I should have thought about San-mai. It would explain the pitted look as well as the darker color. The mill scale on my O1 is never that dark nor consistent in color.
I had researched how to possibly recreate the look artificially using cold or hot metal treatments (such as EPI.com) but in the end I think I’ll pass.
Thanks for everyone’s time.
 
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