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Carbon Steel versus Cast Iron

By Cook's Illustrated Published September 2015

Carbon steel is composed of roughly 99 percent iron to 1 percent carbon, while cast iron normally contains 2 to 3 percent carbon to 97 to 98 percent iron. Why does such a seemingly small difference matter?

Carbon-steel cookware is often compared with cast iron. While the two materials are very similar, carbon steel actually contains less carbon and more iron. Carbon steel is composed of roughly 99 percent iron to 1 percent carbon, while cast iron normally contains 2 to 3 percent carbon to 97 to 98 percent iron. Why does such a seemingly small difference matter? As it does in chef’s knives, the metal in cookware has a grain structure that determines how it performs and how it can be shaped. The extra carbon in cast iron makes it more brittle, because carbon wants to clump up into lumpy carbides (an iron-carbon mix) or form flat sheets of graphite (pure carbon), and both disrupt the grain of the iron, making the grain irregular, more brittle, and less strong. By contrast, with less carbon in its makeup, the metal in carbon-steel pans has a more uniform grain structure, helped along by mechanical processes such as rolling the metal between heavy rollers while it’s hot, and/or dousing it with cool water or oil, all of which helps “freeze” the grain structure in its best configuration, which makes the resulting metal stronger. As a result, carbon-steel pans can be made lighter and are more pliable (referred to as more “ductile”); they’re made by cutting and pressing the shape of the pan from sheets of carbon steel, like cookies cut out of rolled-out cookie dough. That’s why carbon-steel pans are less weighty than cast-iron pans but perform very similarly.

When you’re shopping for carbon-steel pans, you’ll see references to “blue steel” and “black steel” pans. Usually, the terms refer to surface-hardening treatments that help prevent rust, which leaves the surface of the new pan looking either blue or black rather than shiny silver. (Our winner is an exception. The manufacturer etches the surface to remove the black color.) Like the blackish surface appearance of anodized aluminum cookware, these finishing treatments are relatively stable but can wear off over time or after cooking or cleaning with acidic ingredients; the pan will eventually look brownish-black like regular carbon-steel cookware as it acquires layers of seasoning. Our winning pan is black steel and our runner-up is blue steel, but in the end it didn’t seem to make a big difference in their performances.