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.