Cast Iron Seasoning Flaking or Peeling

Black flakes lifting off your cast iron are bits of seasoning — hardened, polymerized oil — shedding because a layer was built too thick or did not bond well underneath. It is not rust, it is not dangerous if a fleck ends up in your food, and the pan is not ruined. The fix is to scrub the loose material back to a sound surface and re-season in thin coats.

First, the reassurance, because this one worries people: flaking seasoning is not a health hazard. Seasoning is oil that has been polymerized into an inert, carbon-like solid, chemically similar to the coating on any well-used pan; an occasional black fleck in your food is harmless. It is also not rust — rust is orange-brown and powdery and is a different problem with its own rescue guide. Black and flaky is seasoning; orange and powdery is rust. Once you know which you are looking at, the fix is easy.

Why seasoning flakes off

Seasoning flakes when the layer that is lifting never properly bonded to what is beneath it. A few things cause that:

• Too thick, built too fast. This is the usual culprit. Thick seasoning coats — or many coats piled on quickly without each fully curing — form a brittle shell that cannot flex with the iron and eventually cracks and sheds, the same way thick paint peels. Thin, fully cured layers bond; thick gummy ones do not.
• Poorly bonded factory or old seasoning. Sometimes an original layer was not well adhered, and once one patch lifts, the edge keeps peeling.
• Seasoning over a dirty or greasy surface. A coat baked over food residue or soft, un-polymerized oil sits on top rather than bonding, so it flakes later.
• Sudden temperature shocks and acids. Repeated harsh thermal shock or long simmering of acidic foods in a thinly seasoned pan can loosen a layer at the edges.

How to fix flaking seasoning

1. Scrub off everything loose. Get the pan warm and, with a chainmail scrubber or steel wool and a little soap, work over the flaking areas until nothing more lifts. You are not trying to save the failing layer — you are removing it back to seasoning that is sound, or to bare metal if you have to. A little soap and elbow grease is exactly right here.
2. Feather the edges. Where sound seasoning meets a bare patch, scrub the transition so there is no sharp lip for the next coat to catch and peel from. You want a smooth surface, even if it is a patchwork of dark and lighter areas.
3. Dry it completely. Towel it and then heat it on a burner until bone dry, so no moisture is trapped under the new seasoning.
4. Re-season in thin coats.Rub on a few drops of neutral oil, buff it back until the pan looks almost dry, and bake it upside down past the oil's smoke point for about an hour. Repeat two or three times. Follow the full method in how to season cast iron. Thin and patient rebuilds a layer that will not flake again.
5. Or strip and start over. If the seasoning is flaking all over, it is often faster to take the pan back to bare metal (a self-clean oven cycle, a lye bath, or plenty of steel wool) and season from scratch. A completely reset pan is as good as new.

How to keep it from coming back

Flaking is a thickness problem, so the prevention is the same as for a sticky pan: build seasoning in thin, fully cured coats, never thick ones, and let cooking do most of the work over time. Season a clean, dry, grease-free surface so each layer actually bonds; avoid slamming a cold pan into violent heat; and go easy on long acidic simmers in a bare pan until the seasoning is robust. Do that and the layer grows even and tough instead of brittle. If you are just getting started, cast iron for beginners covers the fundamentals.