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Carbon steel, rusting, patinas, time period

Sun Jan 05, 2014 8:30 pm

So I've been wondering about patinas and rusting on carbon steel knives. I've done some research but I can't seem to find the answers to my questions.

How long does it take to build a patina, I know the steel matters so lets say White #1?

Is this something I need to be truly cautious of, or something I can keep in the back of my head?

Patinas protect the knife, but rusting is a huge issue, how much maintenance and care needs to go into a carbon steel knife to build a natural patina but inhibit oxidation?

Re: Carbon steel, rusting, patinas, time period

Sun Jan 05, 2014 8:43 pm

Time to build a patina: it depends.

Many knives made of something like white #1 are going to be clad in mild steel, so the patina properties of white #1 (or any particular steel) might be less relevant than you might think.

I think if you are careful with a new knife, you can protect it from rusting while it develops a patina. After it is developed, it's not quite as sensitive (but the care regimen is simple enough that it's good to get in the habit).

From what I understand, a patina is a type of oxidation, but it is different from rust. And there are a variety of ways to force a patina to give it a head start.

If you're someone who is careful and takes care of their tools, it's probably not that big a deal.

Re: Carbon steel, rusting, patinas, time period

Sun Jan 05, 2014 9:24 pm

Some steels will develop a patina over the course of a single cutting session. I recently polished the patina off a white steel knife (can't remember 1 or 2), within an onion I was seeing discoloration. Many people will force a patina to get to the "safe" place with the knife more quickly and uniformly. The patina will darken over time.

Rust, the orangy stuff, is a cause for concern because of pitting. You do not want rust to get so established you loose steel. If the rust is surface, it can be polished off with a scotch-brite type pad, bar keepers friend, or some other similar abrasive cleaner/polisher. I polished up the knife above because it had developed some surface rust.

The timeline for rust is quick, but not lightning quick. Depending on a number of factors, rust can form over an hour or so, to overnight.

I typically rinse and wipe dry my blades between ingredients, if I am doing big batches, or whenever I move away from the cutting board, ie to check a pan. Then I wash and dry the knife immediately after finishing prep, before sitting down to eat. I also store my knives on a magnetic knife rack where they can get air circulation so any residual moisture can dry up. If storage in open air is not an option, consider leaving a clean fry knife on the counter overnight and storing it the next morning.

Ultimately, don't worry. Patinas happen naturally with no help from you. You can force a patina to have some control over the process if you like. Rust is not good, but can usually be repaired. Carbon upkeep is not different than stainless, you just have to do the clean up immediately and keep a your knife clean during use. Develop good habits, apply them to all your knives stainless or carbon, and you will never have to worry about it.

Re: Carbon steel, rusting, patinas, time period

Mon Jan 06, 2014 12:46 am

Simplified, patina = black oxide = iron + oxygen. Rust = red oxide = iron + oxygen + water (vapor). A patina can be protective, but the protection is limited. Professional knife users in Japan consider a patina "dirty," and will clean their blades after every session with a daikon/cucumber/cork and baking soda. As others have said above, water is the enemy. Keep your blades dry, whether or not they have a patina.
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