Last week, I offered a quick review recommending my favorite, inexpensive pocket knife: the Opinel #8. In examining one of my carbon steel blades, I was disappointed to find that it had not yet formed a protective patina. I decided to change that.
A big complaint about carbon steel is that it rusts quickly, unlike stainless steel. Carbon blades can be sharpened finer and will retain their sharpness longer than stainless steel. Adding a patina, or acquiring one through use, on carbon steel will slow down bad rust. I say bad rust because the patina on carbon steel is rust, it’s just good stable black rust (Fe3O4,) as opposed to the bad pungent red stuff (Fe2O3.)
How to get a beautiful patina on your carbon steel? Treating him with mild acid seems to be the answer. The forums all suggest dip your blade in heated apple cider vinegar to coat 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 hours, I wiped them clean. The lemon had turned black where it touched the steel. The steel has started to develop a nice grey/black film. Wiping the blade coats the patina and helps to coat more evenly. I then put the blades back in said lemon and left them for a few more hours. I repeated until I was happy with how everyone looked. One knife took fewer applications than the other.
For a more even and less random application, you can try the above method or immerse the blade in a solution of 50% water and 50% vinegar for 4-24 hours. The forums have hundreds of recommendations for the brave or the bored.
If you want to try forcing a patina on carbon steel and need a knife, I suggest Opinel N°8 Carbon.