Kuroushi finish discoloring food

Hi,

I recently got my first kurouchi-finished knife, and discovered that, despite having washed it thoroughly, the black finish continues to discolor foods cut with this knife. Is this normal? Is there anything I can do to stop it?

I'm not mentioning the maker because I don't want to blame them if this is my fault.

Thanks,
Dave

I imagine is less of the kurouchi and more of just oxidization that occurs with carbon steel knives and foods... in that case, it can be managed easily

What are you cutting?

What Jon said. Cutting someting like an onion with a 'fresh' carbon steel blade will cause oxidation of the steel which then gets 'washed down' during the next slice and darkens (black or brown) the cuts. The easiest way around is to let patina to build up (a few cutting sessions) and this behaviour disappears quickly.

This has been my experience as well. OOTB my Tanaka kurouchi B2 wasn't very reactive and didn't cause any discolouration but after sharpening and exposing fresh steel I was getting some slight oxidation, particularly on onions as Matus said. This has now stopped after a few sessions and a new patina has built up, again as said previously.

So I wouldn't worry about it Dave. Get a few sacrificial onions and keep chopping.

I would only add. thet when kurouchi does get off the blade (eg. when sharpening a wide bevel kurouchi knife), then it is in a form of small black hard pieces (that can scratch the blade as they are very hard!), it does not dissolve in food acids.

I've had new KU that must have been touched up after forging because some of the black just wiped off with acetone

I believe that some KU finish is not from forging but painted on with chemical that turns the steel black.. rgdsz

zitangy said:

I believe that some KU finish is not from forging but painted on with chemical that turns the steel black.. rgdsz

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there is some of that too... also, some that is applied before heat treatment, and some that go on after

Thanks, everyone. I suppose it could be oxidation. I don't know how long the knife was sitting around before I got it. Living in AZ, I'd have had to wash it and put it away wet several/many times to form that much oxidation in the time I've had it, which I did not do.

It's a small petty-ish knife (technically, it's a deba, but it's only 90mm, so more useful for odd jobs than for filleting fish), and my first use of the knife was to cut some slices off a block of Havarti cheese. Not much of a test, but I wasn't testing, I was preparing a snack. Washed it, cut the first slice, saw the black whatever smudged onto the cheese, washed and dried the knife again, threw it in the drawer (not literally), got another knife and cut a second slice, threw those slices away, and moved on.

I like the knife, but if it's going to transfer small bits of itself onto anything I cut with it, I'm going to have to relegate it to a gardening tool. An overpriced one.

If you got some discolouration on the food, then there must have been some loose stuff on the blade. Cheese is not acidic so it would not oxidise the blade like a tomato or an onion would. I would give the knife a proper rub and test it again.

Hmmm, very strange. Sounds like you got the knife second hand. Do you know the history of it? Has it been coated with anything? Any chance there was a thin film of oil on the blade which transferred to the cheese?

If all else fails maybe just use it for slicing ash coated chèvre?

Reactivity in carbon steels is nothing to worry about. One way to try and defuse it is by way of passivation. This is the process of treating your steel in order to reduce the chemical reactivity of its surface. To be very practical, get your hands on some mustard. And I dont mean any fancy organic Dijon or anything like that either - I mean the yellow stuff that comes in little packages in fast food places. Now, clean that blade impecably. Use acetone and make sure it is completely clean. Next, cover the entire surface of the blade with a thin layer of "mustard" and wrap it up in a paper towel. No need to make it into a bulky knife mummy - just enough to have one or two layers of paper flat on the mustard and the blade. Leave it there over night, and in the morning spray it with some plain water - keeping it wrapped of course. Let the paper dry again and repeat this process 5 or six times. Your blade will come out patinated to the gills. Scrub it clean with a soft towel and some soap water so you dont damage the layer of patina. You will need to sharpen your micro bevel again, but once you do - I think you will see a difference in how this knife reacts to food.

Note that as soon as you thin out the blade again it will expose a new reactive bevel so you may need to patinate over that again before you put it to work.

I hope this helps.

That's an interesting process Tsuriru and a very thorough explanation, thanks. I've heard of forced mustard patinas before but never tried one. Can I ask what is the purpose of the repeated water spraying step? Thanks.

OliverNuther said:

I've heard of forced mustard patinas before but never tried one. Can I ask what is the purpose of the repeated water spraying step? Thanks.

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Patina takes time. when you first apply the mustard, it will start to react with the surface of the blade, but experience shows that dry mustard is not as effective in reacting with the blade as wet mustard, otherwise perhaps we would be able to use mustard powder rather than paste. Repeating the wetting and drying step is conducive to building several layers of patina that will shield the blade (somewhat) from reacting "violently" with certain food types.

You can also try boiling vinegar - but this will produce a far more potent etch on your blade and Im not at all certain that you will like the outcome aesthetically speaking. To do this, clean your blade immaculate with acetone. Then, boil some 5% vinegar in a pyrex pot. Use the ladle to drench the blade over and over and keep this up for about 15 - 20 minutes. Then, wash the blade under running water and submerge in a bucket containing a 15% baking soda - in - water solution to neutralize the reaction. Make sure you stir the bucket right before you submerge and if possible re-stir is carefully. wash blade with soap and water. Dry, and you are set to go. Note: boiling vinegar requires a well ventilated area, and you should wear a respirator , safety goggles, gloves, and an apron (at least) when doing this. Safety first.

This is often done to re-etch Damascus, accentuate san-mai etc. but will grant the blade some protection against reactivity as well. To what extent? - Each blade is a bit different.

Ok that makes sense. I've got a mono steel W2 that I was thinking of experimenting with a mustard patina so I'll give your wrapping technique a go. Thanks for the advice.

Sorry OP, hijack over. Back to you.

tsuriru said:

Reactivity in carbon steels is nothing to worry about. One way to try and defuse it is by way of passivation. This is the process of treating your steel in order to reduce the chemical reactivity of its surface. To be very practical, get your hands on some mustard. And I dont mean any fancy organic Dijon or anything like that either - I mean the yellow stuff that comes in little packages in fast food places. Now, clean that blade impecably. Use acetone and make sure it is completely clean. Next, cover the entire surface of the blade with a thin layer of "mustard" and wrap it up in a paper towel. No need to make it into a bulky knife mummy - just enough to have one or two layers of paper flat on the mustard and the blade. Leave it there over night, and in the morning spray it with some plain water - keeping it wrapped of course. Let the paper dry again and repeat this process 5 or six times. Your blade will come out patinated to the gills. Scrub it clean with a soft towel and some soap water so you dont damage the layer of patina. You will need to sharpen your micro bevel again, but once you do - I think you will see a difference in how this knife reacts to food.

Note that as soon as you thin out the blade again it will expose a new reactive bevel so you may need to patinate over that again before you put it to work.

I hope this helps.

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Do you have any photos of what kid of patina would develop from this approach?

strumke said:

Do you have any photos of what kid of patina would develop from this approach?

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You can see what was once (long ago) the original fire scale right where the handle connects to the blade, and the micro bevel wich is the lighter line running along the cutting edge, where the fresh metal has been exposed.

OliverNuther said:

Hmmm, very strange. Sounds like you got the knife second hand. Do you know the history of it? Has it been coated with anything? Any chance there was a thin film of oil on the blade which transferred to the cheese?

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It's supposedly brand new, fresh from the maker. It came with a clear plastic sleeve on the blade. I see no signs of oil on either. And I have washed the blade twice before it's most recent use. (and again, afterwards)

If all else fails maybe just use it for slicing ash coated chèvre?

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:laugh:

Well, you guys may be right, but it doesn't make sense to me that it would be rusted before it was ever used. And things don't really rust, here. Right now, the humidity outdoors is 10%. But, I guess I can try cleaning it with acetone. Seems like that shouldn't be necessary, either, but if it is, it is.

Iron builds different oxides. Red or orange ones discolored your cheese, grey ones are chemical and mechanical stable and protect your knives and food. I rub reactive knives with veggie remnants and clean with boiling water. Boiling water speeds up the process and makes patina more stable. IIRC it increases the number of oxygen atoms bound to iron.