Unusual patina

[dickb]

I have made a bunch of kichen knives from 1095 and 1084 and I use them every day. 

A couple of them have begun to take on a very light blue or straw color. It's not completely even, but it's definitely there. They look similar to colors you get when tempering steel, but not as strong. 

The blades havnot been exposed to anything hotter than what comes out of a hot water faucet or any chemicals other than dish detergent. 

I would like to create this finish on a few blades, but don't know why or how it came about and would appreciate any suggestions. 

[Frazer]

It's just an oxidation layer. It's the same process as tempering colors only instead of them being caused by heat they're caused by moisture, the food you're cutting etc. Those same colors appear on all my knives over time too.

I'm not sure why the colors tend to be blue most often and I'm not sure if you can force them to appear more uniformly somehow. Perhaps someone else will be able to provide those details.

[ThomasPowers]

What he said; I see it on my stainless steel spoons as they tend to get the most cycles through washing.  What alloy are the blades you are seeing it on?

[Frazer]

It appears 1095 and 1084.

I see it with those, spring steels and on the 15N20 in damascus as well. Always the same color (more or less) and mostly when they get used for food prep. Whenever I give a knife to someone I give them a disclaimer that these aren't SS kitchen knives and some discoloration over time is natural. Especially if you're cutting acidic foods like tomatoes or citrus fruits.

My guess would be that if it has tempering colors (from heat) it will have the same oxidation colors from other sources regardless of the alloy.

*Note this doesn't mean the alloy doesn't play a role at all, I'm just saying that for most of the iron-based ones we tend to work with it may not play as major of a role. Except maybe for some stainless. I don't have much experience with forging stainless so I don't know.

[Frosty]

I hadn't noticed a blue patina on my kitchen knives but just went and looked. Sure enough a number of them have a bluish gray patina, sort of like smoke haze. If we're seeing the same or similar patinas on different steels it must have a common cause.

I don't see it on my knives non-kitchen knives, that I think eliminates blade alloy.

Dish soap? That's hard to believe, all my blades experience a little Dawn dish soap to cut the oil and crud, especially my pocket knife.

I believe it has to be something about cutting food, probably vegetables, all my kitchen knives cut veggies only a few cut meat.

What chemical or compound do vegetables share that only vegetables share?

Frosty The Lucky.

[JHCC]

I've noticed the same thing on some of the knives I've made from 5160. 

1 hour ago, Frazer said:

It's just an oxidation layer. It's the same process as tempering colors only instead of them being caused by heat they're caused by moisture, the food you're cutting etc. Those same colors appear on all my knives over time too.

I'm not sure why the colors tend to be blue most often and I'm not sure if you can force them to appear more uniformly somehow. Perhaps someone else will be able to provide those details.

I suspect this might have something to do with the thickness of the oxide layer, but that's largely supposition.

[Frazer]

Huh... I'm reading on t'internet that you can force a patina with warm blood, mustard (basically vinegar), vinegar and various other acids... Doesn't seem like it's a very durable finish, but no real surprise there. 

The first one interests me. Next time I cut my fingie (shouldn't take long) I'll have to test that internet mumbjo jumbo.

Anecdotally, I will say that I have had knives patina after cutting exclusively meat too. Especially if I don't do a good job cleaning it off afterwards. Now whether that's due to the salt, the spices or the meat itself I just don't know.

[ThomasPowers]

Frazer; do you need a second?  John is a lot closer to you than I am...

The thickness of the oxide layer is what causes the color we see.  Heat increases the thickness for temper coloring; I wonder what chemicals would do it in the kitchen?  (I bet the big cutlery companies have this all documented in their vaults somewhere!)

[George N. M.]

We occasionally get a colored, sometimes blue, on the interior of our stainless steel cooking pots.  It doesn't scrub off well.  I think it is some chemical reaction with the metal from cooking and/or washing.  We may be seeing interference colors (which, like and oil on water colors, interfere with the wavelengths of the reflected light) rather than an actual colored pigment.  You may be getting something similar.

"By hammer and hand all arts do stand."

[Frosty]

Mustard is a whole lot more reactive than vinegar. Vinegar is added to prepared mustard to stop and stabilize it at a specific spicy heat level. Coleman's dry mustard and water make frighteningly HOT mustard for your Chinese dishes and you won't believe what leaving it in the fridge over night will do. 

I've gotten some neat colors on copper with Frenche's plain yellow mustard, nothing consistent though. <sigh>

Frosty The Lucky.

[Latticino]

This is a pretty common thing to see on good quality carbon steel kitchen knives.  Most of my older ones I've inherited have a patina that develops over an extended time.  I find it a great indication of a well loved tool and appreciate it developing on the knives I have made myself (mostly 1084, 1095, 5160, and W-2).  I agree with the consensus that it most likely comes from interaction with raw vegetables or meat.

I've had some success forcing a similar patina on a knife I forged myself by fuming it in the vapors rising off of gently boiling apple cider vinegar.  Blade needs to be very clean and oil free.

[TWISTEDWILLOW]

I don’t know how true it is, and I’ve never tested it,

but I had an old guy tell me once that blue grey color you see on old pocket knife blades could be made by slicing up apples with it and not washing the blade afterwards just leave the apple residue on it and you would get that color, 

 

[Daswulf]

This one old carving knife I have has the blue and yellowish patina going on. I even cleaned it up a bit and sharpened it. I think this is a pretty old knife but I couldn't find much on it. 

Used it to carve up the thanksgiving turkey. It has a little flex to it and worked surprisingly well. 

Hope the colors show in the pictures.

[1forgeur]

Onions?

[Irondragon Forge ClayWorks]

I have a very large 12 1/2 inch bladed carbon steel chefs knife that is used mostly for cutting up pizza's. The tomato pizza sauce etches it in an interesting pattern even though I wash it within an hour of using it.

[SLAG]

Mr. Dragon,

Could you quickly rinse the tomato juice off and quickly run a little baking soda/water solution over the blade whilst you are in the frantic heat of culinary battle?

And then come back to said knife when you are not so hectic,  to properly clean the blade?

But wait a minute Mr. SLAG, you may like the etching of said blade.

Just a stray thought.

SLAG.

p.s. Have a Merry Christmas and a wonderful productive, interesting, and healthy new year.

[Irondragon Forge ClayWorks]

I kinda like the etching, like a false Hamon.