Kurouchi questions

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spoiledbroth

spoiledbroth

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I have always had a few questions about kurouchi finishes:

My understanding is that the black stuff on the knife is actually the product of forging (forgive my ignorance). Usually it is ground off.

Without removing the scale how can the smith fine tune geometry? Does this mean the geometry is usually inferior?

It's pretty common to see knives where the black stuff has worn off leaving a kind of nashiji look. Why does this happen? Are all KU knives equal in the sense that the finish will wear off eventually? Is this finish food safe???

Not very often do you see single bevel knives with a KU finish. Any ideas why?
 
There are plenty of ways to produce a Kurouchi finish, I'm not the most knowledgable about this, it can be done just by forging, like on Takeda san's knives and it can also be done chemically like on Mutsumi Hinoura's knives. When it is done chemically usually it is after the surface is ground, so the maker has extra chances to tweak the shaping process. Generally knives that have not been ground on the kurouchi surface tend to be thicker, but Takeda is an exception to this.
 
spoiledbroth said:
I have always had a few questions about kurouchi finishes:

My understanding is that the black stuff on the knife is actually the product of forging (forgive my ignorance). Usually it is ground off.

Without removing the scale how can the smith fine tune geometry? Does this mean the geometry is usually inferior?
Click to expand...
I think you're presuming a bit about how smiths "fine tune" geometry. Further, if the knife is a conventional kurouchi knife, the blackened flat (hira) will be at a different angle than that of the kireba (blade bevel). Cutting geometry will be optimized by adjusting the height of the kireba (raising or lowering the shinogi, which is the ridge separating the kireba and hira) to suit the knife's thickness and desired convexity. Makers do not always fret over nuances in geometry...that is something that will be defined more by the user over time as they thin and adjust.

It's pretty common to see knives where the black stuff has worn off leaving a kind of nashiji look. Why does this happen? Are all KU knives equal in the sense that the finish will wear off eventually? Is this finish food safe???
Click to expand...
Scale comes off. Soak a kurouchi knife in hot vinegar and watch the edge turn black and the kurouchi start to dissolve away. Pitting will obviously be retained, just as you would get if you try to remove old rust. Some kurouchi finishes are more stable than others and are not always achieved with the same techniques. They should be food safe and should not have dangerous impurities in meaningful amounts.

Not very often do you see single bevel knives with a KU finish. Any ideas why?
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There are a few, but there are a few possible reasons there aren't many. First I can think of, apart from the visual asymmetry, is just that professional environments that use kataba knives tend to clean aggressively and a kurouchi finish won't survive long. Mind, I'm no expert on kataba knives and there may be better answers than I've got for that.
 
Hmm interesting. Thanks for your answers XooMG. I didnt mean to be presumptuous to be honest I still don't understand how stamped knives are made, much less forged (need to buy Springhammer documentary I guess) and I understand there are several methods of forging a knife in japan depending on the steel. Perhaps more. I really don't know much about how knives are produced.

One assumes that KU knives from reputable smiths/firms (ie sakai takayuki, tsukiji masamoto, kiya, shigefusa etc) would be free from dangerous impurities in meaningful amounts.
 
spoiledbroth said:
. . . to be honest I still don't understand how stamped knives are made, much less forged . . .
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Not that I'm any expert either but I think the point is that they are not? Two basic ways of making knives . . . Forging them by heating and pounding steel or stamping them from cold rolled steel. In both cases you grind after to shape, then polish, sharpen, etc. A gross oversimplification I'm sure but that's my (limited) understanding.
 
Oh, I get it, I'm overthinking it (the knifemaking process) maybe :)
 
The KU finish is baked on after the blade has been ground, for the most part it isn't material which is left behind from forging.

Stamped knives are stamped from sheet metal to create the blanks.
Mass produced forged knives are usually drop forged through a series of dies to shape the blanks.
Handmade forged knives (which includes some higher end big-brand knives such as ZKramers) are forged with hammers.
You can also use stock removal.


The composition of the steel (in terms of layering) is unrelated to the fabrication process. There's no reason for example why you couldn't buy pre-made sheets of san-mai damascus and stamp knives from it.
Another point to remember is that all of the steel has been forged at some point. It's just that sheet steel has been forged to that thickness at the mill while forged knives will often begin with bar-stock (or similar) and then forged flat by the knife maker.
 
See that's a bit of a revelation to me. That would imply that there's an extra step (ergo higher production cost) in making a kurouchi knife. This doesn't necessarily mesh with my impression that the least expensive knife you can find from most smiths will be a KU (if they make it). (shig and muteki come to mind as examples)
 
MAS4T0 said:
The KU finish is baked on after the blade has been ground, for the most part it isn't material which is left behind from forging.
Click to expand...

I beg to differ. The kurouchi "finish" is produced by not polishing the surface after heat treating. It is not applied and baked on after grinding.
 
spoiledbroth said:
See that's a bit of a revelation to me. That would imply that there's an extra step (ergo higher production cost) in making a kurouchi knife. This doesn't necessarily mesh with my impression that the least expensive knife you can find from most smiths will be a KU (if they make it). (shig and muteki come to mind as examples)
Click to expand...

Exactly. Leaving the blade "rustic" saves a lot of time (and money) in finishing. Nashiji finished knives are also not as labor intensive, so they are also less expensive.
 
which brings me back to the question I suppose of... the hira, that blackened flat part of the blade. I wonder if that (lack of) geometry is more acceptable on a KU single bevel knife than on, say, a wide bevel KU gyuto where you might want to have a little convexing on that "hira" part for food release.... anybody have thoughts about that? I would assume the geometry of the hira on a deba or yanagiba is less important.

I could I suppose understand a "baked on" KU finish for custom made knives where a request is made, or perhaps it's the smith's philosophy that the KU finish should last a long time, and so they can contrive that rustic finish by somehow causing the scale (or some other chemical reaction to produce a more stable finish) to reform once they've massaged the geometry a bit by grinding (because I figure custom makers - we're not talking Japanese here - do worry more about geometry than the Japanese big knife firms)
 
In my opinion neither are very important, on a wide beveled knife there is rarely much contact between the ingredient and the hira, same with single bevels
 
Certainly for Heiji, but on thinner wide beveled knives such as Takeda's maybe it could have some effect on softer ingredients. I really don't know
 
Pensacola Tiger said:
I beg to differ. The kurouchi "finish" is produced by not polishing the surface after heat treating. It is not applied and baked on after grinding.
Click to expand...

My apologies, I didn't phase that properly. Thank you for clarifying.

What I was referring to is smiths applying extra scale to the blade (during heat treating) in order to create a thicker more uniform kuroucho finish.

Sorry Spoiledbroth, I didn't mean to say that it was an extra step at the end.
 
i was once told by a supplier that traditionally people just regard kurouchi blades as being "cheap", so they dont sell well among professional chefs as the restaurant may be regarded as "cheap" (we all know in a traditional Japanese kitchen customer sits right in front of the chef).
 
MAS4T0 said:
My apologies, I didn't phase that properly. Thank you for clarifying.

What I was referring to is smiths applying extra scale to the blade (during heat treating) in order to create a thicker more uniform kuroucho finish.

Sorry Spoiledbroth, I didn't mean to say that it was an extra step at the end.
Click to expand...

And thank you for clarifying what you meant. It's nice to add that bit of information to my understanding.
 
chiffonodd said:
I think it might be time for a class field trip to a forge . . .I live close to Murray Carter's shop. I think they do open forges once a month. :bladesmith::knife::knife:
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I've got a family member who lives an hour or so from him. :)

Hmm. It seems that this thread has left me with even more questions than I started with! :laugh: I guess that's learning.
 
I'm not sure there's much good reason for confusion.

If you're doing a rough and simple knife...maybe a Tosa style thing...
  • weld a piece of core steel into an iron taco
  • forge out to rough blade + tang shape.
  • grind or cut rough knife shape to a template
  • heat treat (details omitted)
At this point, the knife "geometry" is basically flat and mostly uniform thickness (except for longitudinal distal taper). There may be texture or concavity to the flats near the spine, but it really varies.

  • grind the edge in
This leaves you with your bevel, a hard (abrupt) shinogi, and a flat (hira). The hira geometry will not massively affect sticking characteristics if the bevel is less than ~20mm high because the bevel should sufficiently separate most foods, though tsuchime-type texturing and a light concave cross-section might reduce some of the "climbing" above the shinogi that you sometimes get with flexible slices and thin (at the shinogi) blades.

Most kurouchi knives I've seen are ground with rather flat bevels, and have not been optimized for product separation, including some that get a lot of praise. There are exceptions to this, but the average bevel will be pretty dead flat, and some will have some vestigial concavity (hollow). Sticking and suction-wedging can be noticed within 1cm of the edge even in thinner knives, if cutting challenging targets.

If you're going to tune the geometry of the knife, you'll be tuning the bevel, rather than the entire knife. Since the shinogi can act as a sort of spine (most food does not adhere above it unless the knife is very thin at the shinogi), this is not a major limitation most of the time.
 
Hmm. Very pragmatic comments. Thank you again.

You are right, there doesn't seem to be much good reason for my confusion. :p

Obviously the taco is a double bevel but presumably your comments apply to single bevel knives too (grinding the ura being an extra step)
 
Here I have found information from Mizuno Tanrenjo about the step by step knife making process (forging) for anyone interested

Process of making Japanese knives(Kata-ba-haganetuki-houcho)
1) Heating process
1.Hagane-kiri The blade steel is shaped and cut it into the correct amount for the knife.
2.Hagane-tuke The blade steel is joined with base steel using Kinsetsuzai or special
bond
3.Wakashi-tuke It is heated to between 800C - 900C for temporary adhesion
4.Sakinobashi The part which is going to be the top of knife is shaped
5.Nakago-nobashi The part of the end of the blade is shaped
6.sou-hidukuri It is heated to make the final shape of the knife
7.yaki-namashi It is then put in ash to cool down slowly avoiding any irregularities

2) Finishing process
8.Beto-tori The oxidized film or oxidized iron produced through heating process is
removed
9.Aratataki The knife is on normal temperature to make it tighten
10.uramigaki The face of the blade is polished
11.ji-tachi Extra steel after hit is cut off
12.Hon-narashi The back is pounded to make the quality perfect
13.Kokuin-uchi The maker's name is stamped on the blade
14.Kei-biki The knife is put on the template to mark the correct size
15.Kata-tachi The excess is cut off to make it into the required shape
16.Migari-mawashi The rough edges are polished off with grinder or file
17.Mae-shori All oil or dirt is removed from the blade

3) Annealing
18.Doro-nuri Mud is put on the front to avoid oxidization and irregularities
19.Yakiire It is heated up to 800C and cool down in water
20.Yakimodoshi The same process is repeated to make the blade stronger
21.Hizumi-uchi Irrgularities that may have happened through this process are adjusted

4) Polishing
22.Aragiri-hatogi The blade on its front is polished with round polish stone
23.Ara-hira-togi The whole front is polished
24.Ara-ura-togi The back is polished
25.Ara-mune-togi All through from the top to the end of blade is polished
26.Hizumi-togi Irregularities happened polishing process are adjusted
27.Nakame-togi It is polished with smoother stones
28.Hafu-migaki It is then polished with a cloth glued with diamond powder
29.Kito-shiage Diamond powder is put on the woden board to make line
30.Kesyou-shiage It is polished with whetstone powder to get the color
31.Kobatuki-shiage Then next it is polished the top delicately

5)Making the handle
32.Genboku-kezuri A dried piece of wood is curved into a correct size
33.Kiwa-tuke Then it is curved the part for a ring
34.Ana-ake A hole is made for the shaft (gross)
35.Wa-hame Buffalo's horn is cut into the same size of the ring and pushed into the
handle
36.Migaki Both the horn and the wood are polished till it gets smooth

6)Finishing
37.E-tuke The shaft is heated and pounded into the handle (ouch)
38.Yugamitori-Kenpin The whole balance is adjusted
39.Bousei-shori It is lubricated with anti rust paint
40.Housou It is packed into the box
Click to expand...
 
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