How to force a patina on carbon steel

Last week I offered a quick review recommending my favorite, dirt cheap pocket knife: the Opinel No. 8. Examining one of my carbon steel blades I was disappointed to find it had not yet formed a protective patina. I decided to change that.

A big complaint about carbon steel is that it rusts quickly, while stainless steel does not. Carbon blades can be sharpened finer and will hold their edge longer than stainless. Adding a patina, or acquiring one via use, on carbon steel will slow bad rust. I say bad rust because patina on carbon steel is rust, it is just a good, stable black rust (Fe3O4,) as opposed to the evil, pitting red stuff (Fe2O3.)

How do you get a nice patina on your carbon steel? Treating it with a light acid seems to be the answer. Forums suggest everything from dunking your blade in heated apple cider vinegar to coating it with mustard. I stuck mine in a lemon. The photo above is the result.

After letting the knives sit in the lemon for 24 hrs, I wiped them off. The lemon had turned black where it was in contact with the steel. The steel began to develop a nice grey/black film. Wiping the blade smears the patina and helped coat more evenly. I then returned the blades back to said lemon and left them for a few more hours. I repeated until I was happy with how each looked. One knife took fewer applications than the other.

To get a more even, less random application you may try the apple cider method above, or submerging the blade in a solution of 50% water and 50% vinegar for 4 to 24 hours. Forums have hundreds of recommendations for the brave or bored.

If you'd like to try forcing a patina on carbon steel and need a knife, I suggest the Opinel No. 8 Carbon.

Notable Replies

  1. As a knifemaker, I feel the need to add: you should also neutralize the acid anywhere it isn't easily rinsed off -- just dissolve a bit of baking soda in water and soak like you did the knife. Lightly coat the blade with a good food-safe oil or wax to increase it's rust resistance. Never machine-wash your blade, hand wash with mild detergent, then re-oil the blade. Hone the blade whenever it stops cutting easily. These basics will keep your knife usable for decades.

  2. I think you're missing the point completely, Medievalist. This process actually protects the blade; it's not just for appearance, unlike "prestressed" clothing. It's more akin to prestressed steel or other structural materials, which actually strengthens the final structure.

  3. Alternate title: "How to keep unruly citrus in line."

  4. Naw, I understood, just making a wisecrack. Poorly, though, I guess! Sorry about that. Acid patinas are corrosion, or rust, as Jason pointed out himself (he also points out that a few of the iron oxides are mildly passivating, which is particularly important to cooks).

    Black or grey rust will hold some oil and can keep your steel from developing red rust when exposed to moisture. But it doesn't strengthen the blade in any meaningful way, and isn't a substitute for keeping the blade clean between uses. It mostly just makes it prettier, bringing out any grain in the metal, and prevents mildly acidic foods from reacting with the raw iron and generating "off" tastes.

    My blades and other metal accoutrements get patinated with use pretty quickly; for one thing, my sweat seems to be more corrosive than that of other people, for another, blood does the job up a treat (I'm a meat-eater as well as a tool user and occasional smith). Most of my armor has a heavy brown patina, most of my blades have a dark grey to black patina, with mirror edges.

  5. have you read/heard of Alexander Weygers "Modern Blacksmith"?

    I feel, as a fellow motorcyclist and tool junkie, you might enjoy it very much.

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