Discoloration of carbon steel blades

Feb 18, 1999
I have a knife with carbon blade that started discoloring immediately (I used it to slice an apple and immediately rinsed and dried it off). I believe I read somewhere that it can be desirable for some carbon blades to have a bit of discoloration, as it may act as a slight protectant of the steel.

Is this true? By the way, the blade in question has a mirror-type finish, not a satin finish, and the discoloration definitely is not rust.
Certain acids in different types of food will react on Carbon Blades. Most of mine over the years have discolored some quicker then others. They have a nice patina to them that I think looks good. As long as you are not experiancing rust problems I would not be to concerned with the finish.
There are two different ways that oxygen can combine with iron. One cause red rust (ferrous oxide) and the other causes the gray patination (ferric oxide). Ferric oxide will act to a very limited degree to prevent the formation of ferrous oxide (the bad stuff). But is really is not all that protective. You will still need to protect your blade with oil, wax, or tuff cloth. You could polish off the grey, but you will have to do this frequently. You can even accelerate patination by using a mild acid like lemon juice to more uniformly patinate the blade. Patination of carbon blades is natural, normal, and over time, beautiful. Not to worry


Another great trick is i) pineapple or ii) mango. Both will create/erase patina when slicing. By erase I mean strip of the grey. Experiment with one of the two to "work" the desired patina.


I'm pretty sure the red stuff is ferric oxide so I guess the grey stuff is ferrous oxide. In my youth I bought red ferric oxide by the pound for making thermite. I've also checked some chemistry websites to check that Fe2-O3 is ferric oxide.
I stand corrected. Thanks Jeff. I guess I got the chemical nomenclature backwards, but my comments are still correct (IMO) for the most part. I guess I should check my references more carefully before shooting my mouth off.
I got what I think you guys are referring to as the "grey stuff" on my BM 940 (stainless steel) after cutting an apple. As you did, and I as almost always do after cutting fruit, the knife was rinsed and wiped. The grey seems to be "in" the metal, as opposed to "on" it.

Neither lighter fluid, nor oil, nor soap and water had changed the slight greying of the blade. I question if the coloration is even at the surface.
Try some Flitz metal polish. Good stuff without worry of abrasives.

I' ve also had quick discoloration experience with a Finnish puukko which too, had a mirror finish. It stained right after spreading raspberry preserves! Well, the Flitz did get rid of it. But it was a regular happening thing so I left it the way it was after a while and enjoyed the character of its discoloration.

Stabbing 'super hot' chicken legs off the grill while camping stained my ATS-34 BM Leopard.

I tried a few solvents, didn't work. It did look like the stain was IN the metal.

I got out the Revere Stainless Cleaner I use on pots and pans, it took the stain off immediately.
Metallic iron is Fe(0) or "iron zero". It's not oxidized or in a zero oxidation state.

Ferrous Oxide or FeO (O is for oxygen here not "zero") is Fe(2) or "iron two" and in the "two" oxidation state. This is the grayish patina as stated correctly above and is somewhat protective.

Ferric Oxide is Fe2O3, two iron atoms for every three oxygen atoms, and the iron here is Fe(3). This is red, sometimes black, rust. This is also part of the patina to different degrees.

These things are not soluble in common household solvents, they must be physically and/or chemically removed. The observations above are correct, these things are IN the metal surface to an extent. Acids, including some foods acids, will create and/or remove them. The best thing you can do is protect your blades and either remove the oxidized iron when it appears, and it will, or enjoy the patina. Metal polish works. As long as gross amounts of metal are not involved, for example pitting, they are quite harmless. Well...not really harmless...oxidation will trash a sharp edge.

Thanks Dr.V!,

I do enjoy a bit of patina on my carbon blades, but I do not like it on the 'stain-less' ones. Is there a reason for my likes and dislikes? Does a small amount of patina affect the 'stainless' more than the carbon?

I know it can affect the edge on the carbon blades...any good info on the other?


I had it happen to a Blackjack #1-7 that I used to cut some steak with. It actually looks better and well used.

"Don't interfere with anything in the Constitution. That must be maintained, for it is the only safeguard of our liberties."
---Abraham Lincoln

The chemistry is the same for stainless steels, as far as the iron is concerned, it just takes longer to occur and it doesn't occur to the same extent. The end result is the same, a small amount of patina will have no effect other than cosmetic on either steel. The edge of a stainless blade will also be slower to dull by oxidation than a carbon steel.

I think the reason for your like and dislike of a patina, depending on the steel, is the same as mine. Stainless is suppose to be "stain less" or at least it's suppose to stain LESS!

I have a Damascus Cuda which is getting some interesting color patterns to it.I wipe mine down with bacon grease.Tasts better than Tuff Glide.


have a"knife"day