Why the dark spots on steel are seasoning

The dark, mottled patches that build up on a carbon steel pan are seasoning, not something gone wrong. They are a sign the pan has been used properly and often. Rust is a different thing entirely, and once you know what each looks like, you will never mistake one for the other again.

Carbon steel is not stainless

Stainless steel has chromium mixed into it, which forms an invisible protective layer and keeps the surface looking much the same for its whole life. Carbon steel has almost none. It is closer to cast iron in character, which is exactly why cooks prize it: it answers a change in heat quickly, and it darkens with use rather than staying blank. That darkening is the pan building a relationship with whatever fat has touched it. Expect it. Do not try to scrub it back to grey.

Patina versus rust

Patina is smooth, dark, and stays put under your thumbnail. It ranges from a light straw colour through brown to near black, and it will not flake, bubble, or come away when you wipe it with a dry cloth. Rust is different to look at and different to touch. It is orange to reddish-brown, it sits on top of the surface rather than in it, and it flakes or rubs off as fine powder or grit when you run a fingernail across it. If a patch comes away in your hand, it was rust. If it does not, leave it alone.

Building seasoning

1. Wash a new or bare pan with warm soapy water and dry it completely, including the handle rivets.
2. Set it over medium heat until fully dry, then add a thin film of a high smoke-point oil, such as grapeseed or refined vegetable oil.
3. Wipe out the excess with a paper towel until the pan looks almost dry rather than glossy.
4. Heat until it just starts to smoke, then let it cool.
5. Repeat this two or three times before first use, and let ordinary cooking do the rest. Each meal with fat in the pan adds a little more.

When it is genuinely rust

If you find true rust, usually from being left wet or stored somewhere damp, it needs to come off before you cook with the pan again. Scrub the affected area with a stainless scourer or fine steel wool and a little soap, working until the surface underneath is bright bare metal with no orange remaining. Rinse, dry immediately and thoroughly, then re-season using the steps above. There is no shortcut that leaves rust in place and calls it patina. If it flaked off in your hand a minute ago, it needs to be gone completely, not just knocked back.

Drying is the whole game

Nearly every rust problem on carbon steel starts the same way: the pan sat wet, even for an hour on the draining board. Wash it, then put it straight onto a burner on low heat for a minute to drive off every trace of moisture before you put it away. A pan left to air dry on its own, especially near a sink or a window, is a pan that will show orange spots by the following week.

Treat the dark patches as evidence the pan is doing its job. A carbon steel pan that still looks like the day you bought it after two years of cooking is either unused or stainless, and it was never going to give you the same cooking in the first place.