No, it isn’t black magic. Here’s what gives cast iron skillets their distinctive color, and why it even matters.
Recipes have step-by-step instructions and measurements, and celebrity chefs show you the ropes in their shows. And yet, in the heat of cooking, unanswered questions can, and often do, come up.
In “You Asked,” we answer these questions for you. Tell us your name, city, state, and question at email@example.com
Anne from Portland, OR, asked: “The other day I was cooking salmon in a Griswold skillet at my parents’ house and I wondered, why are cast iron skillets black?”
The answer lies in the seasoning, the thin coat of cooking oil baked onto the inside and outside of the pan, which protects it from corrosion and rust and gives the cooking surface a slick coating to which food doesn’t stick.
A well-seasoned cast-iron skillet is black because cooking oil has polymerized on the surface with the seasoning and the skillet’s daily use. If you remove the seasoning, the color of the cookware will usually change to metallic gray.
Cookware, you see, is made out of two types of metals.
There are the non-reactive metals, such as enameled iron, stainless steel, and aluminum coated with ceramic or non-stick. These metals don’t react with the elements or your food’s cooking liquid, and they’re not prone to rusting or leaching.
Then, there are the reactive metals, like copper, bare aluminum, carbon steel, and cast iron.
Copper and bare aluminum react with the acidity in your foods, and can leach significant amounts of the metal into your food. This is why all copper pans are lined (with tin, steel, or silver), and all aluminum pans are coated (with ceramic or non-stick).
Cast Iron Is a Reactive Metal
When it comes to cookware, carbon steel and, in particular, cast iron are the most reactive metals of all.
Not only will they leach metal if you cook highly acidic foods in them, but they will react with the oxygen and moisture in the air—and, in effect, corrode and rust—unless they are seasoned.
An unseasoned cast iron skillet is easy to recognize: It doesn’t have that distinctive pitch-black color that all well-seasoned cast-iron skillets tend to have. Instead, it’s dark gray, with a bluish hint and an unusual metallic sheen.
Left unseasoned, it won’t take long for the cast iron skillet to corrode in reaction with the oxygen in the air and rust in reaction to the humidity in your kitchen (in which case that corrosion and rust have to be stripped off and a seasoning has to be applied immediately after).
This brings us to the process of seasoning a skillet.
How Does Cast Iron Seasoning Work?
The seasoning is a thin coat of cooking oil baked onto the interior and exterior of cast iron cookware, giving it a black patina that keeps it from reacting with the elements and that makes its cooking surface slippery and slick.
Seasoning works because, when you rub a skillet with oil and put it in a hot oven, you trigger a chemical reaction called polymerization, in which the oil forms bonds with the bare iron on a molecular level.
These bonds that form between the oil and the iron are strong and durable.
However, you must be diligent with the cooking methods you use the pan for, as well as the recipes that you prepare in it: boiling liquids and simmering acidic foods can erode the seasoning and cause it to start flaking off.
Conversely, preparing fatty, hearty foods in your cast iron skillet will build up the seasoning and help to maintain its integrity over time.
Many cast iron skillet owners don’t re-season their skillet at all because they mostly use them for fried eggs, thick-cut steak, bacon strips, burger patties, and cornbread.
As a byproduct of polymerization, a controlled burn of the oil in which a significant amount of carbon is deposited on the surface of the metal, the cast iron skillet turns black.
This is not only normal, but it is desirable: a skillet that isn’t uniformly black (for example, with gray spots or a flaky cooking surface) is a skillet that hasn’t not been seasoned well.