The great mystery of black residue on cast iron skillets, solved. Here’s what it is, why it appears, and what to do about it.

Cast iron skillets have been used for generations upon generations of American cooks, and for good reason. Sturdy, easy to handle, and able to slide freely from stovetop to oven, you can prepare even the most demanding recipes in one go with a cast iron skillet at hand.

And yet, as trusty as cast iron skillets are, they are not without their share of quirks. For starters, cast iron is a reactive metal, so the skillets made out of it need to be seasoned and properly cared for. Otherwise, the iron will react to the elements, and the vessel will corrode and rust.

This requires you to be prudent with the food that you prepare in them. In particular, recipes that call for adding lemon juice, tomatoes, vinegar, and wine must be avoided—or the bare-metal cooking surface can react with the cooking liquid, eroding the seasoning and imparting a strong metallic aftertaste to your food.

The Main Cause of Black Residue in Cast Iron Skillets

This leads us to one of the most common complaints from first-time owners of cast iron cookware: black residue on their cast iron skillet. Which—no prizes for guessing—is probably what brought you here in the first place.

If you see black residue in your cast iron skillet, it is usually a sign that the seasoning has started to come off. This residue consists of dietary iron, and it isn’t harmful. However, it is a sign that the pan needs to be re-seasoned.

Black residue is most common on pre-seasoned cast iron skillets because the factory seasoning isn’t as reliable as manufacturers often claim. Another cause of flaky seasoning is a poorly seasoned skillet (for example, if you used too much oil, or you didn’t bake the pan face-side-down to prevent pooling).

That said, even a properly seasoned cast iron skillet can have this problem, especially if it is used to simmer liquids or cook highly acidic foods.

Cast iron skillets are great for searing steaks, sautéing mushrooms, frying bacon, and baking cornbread; less so for boiling water, simmering sauces, and cooking foods that contain lemon juice, tomatoes, vinegar, or wine.

Fatty foods and hearty sauces contribute to the seasoning of your cast iron skillet and build it up with every cook, whereas acidic and watery dishes erode it and can cause it to strip off. You know that’s the case when you notice black specks or, in some cases, black flakes coming off into your food.

When that happens, you need to re-season your skillet. Else, the seasoning will keep coming off until none of it is left, causing the cooking surface to corrode and rust. (A rusty pan isn’t safe to cook in, but it is salvageable. Still, restoring it will take significantly more elbow grease than re-seasoning it while you still have the opportunity.)

How to Strip a Cast Iron Skillet

To re-season your cast iron skillet, you first have to remove the old seasoning. It’s important that you don’t try to skip this step; the new seasoning will only adhere to the cooking vessel if you’ve stripped it down to the bare metal.

I have tried and tested quite a few methods for stripping cast iron cookware down to the bare metal, but nothing comes close to the effectiveness of Easy-Off Heavy-Duty Oven Cleaner. Below, I explain how this is best done.

Place the skillet face-side-down on a wooden block in your backyard. Put on rubber gloves and spray the underside and the handle of the skillet thoroughly with the oven cleaner. Then turn it over carefully and spray it on the inside.

Wrap the skillet in a garbage bag and let it rest outside, sheltered from the rain, for 24 hours. The next day, put on plastic gloves again, then give the pan a good soap down and scrub off any remnants of the old seasoning with the help of steel wool.

Give the skillet a soap down and a thorough rinse again, just to be sure. Before proceeding to the next step, pat it dry and heat it over medium heat for 4-5 minutes to get rid of any leftover moisture inside it.

If all went well, your skillet should be gray, free from corrosion and rust, and have a distinct metallic sheen to it. This is because the seasoning, which is what gives this type of cookware its black color, has been stripped off.

How to Re-Season a Cast Iron Skillet

Re-seasoning a cast iron skillet, as long as you’ve stripped it down to the bare metal and dried it properly beforehand, shouldn’t feel different from seasoning it from the first time in any way.

To season your cast iron skillet, preheat your oven to 450°F for 30 minutes. Using a clean cloth or paper towel, rub a very layer of cooking oil all over the cooking vessel—including the exterior, the interior, and the handles.

Put a sheet pan on your oven’s bottom rack (or, if you don’t happen to have one, line it with aluminum foil) to catch dripping oil. Place the pan face-side-down in the oven, so that the oil doesn’t pool on the cooking surface, and allow it to bake for 1 hour without interruption.

When the time is up, turn off the oven and wait for the skillet to cool down inside. Depending on the size of the cooking vessel, this step may take you anywhere from 1 to 3 hours. Once the skillet has cooled, it can be hung on a wall, stored in a cabinet, or cooked with.

If all went well, your skillet should be black once again. The bottom and sides should be shiny and slick, with no spotting or flaking. (Spotting or flaking are tell-tale signs that something with the seasoning process has gone wrong, and that it needs to be redone.)

Building Up Your Pan’s Seasoning

Contrary to what some people think, the process of seasoning your cast iron skillet doesn’t end in the oven. To maintain the integrity of cast iron cookware’s seasoning, it is important that you cook fatty foods in it, and that you add generous amounts of animal fat or cooking oil while doing so.

To keep your skillet in mint condition, especially in the first few days after seasoning, you need to prepare the right kind of foods in it.

Bacon strips, burger patties, French fries, latkes, hash browns, sautéed mushrooms, caramelized onions, and cornbread are all foods that build up the seasoning and help maintain your cast iron skillet’s slickness.