Cooks who appreciate cast iron and carbon steel pans improve their easily corroded surface by building up an artificial protective layer. They “season” them by coating them with cooking oil and heating them for several hours. The oil penetrates into the pores and fissures of the metal, sealing it from the attack of air and water. And the combination of heat, metal, and air oxidizes the fatty acid chains and encourages them to bond to each other (“polymerize”) to form a dense, hard, dry layer (just as linseed and other “drying oils” do on wood and on paintings). Highly unsaturated oils—soy oil, corn oil—are especially prone to oxidation and polymerizing. To avoid removing the protective oil layer, cooks carefully clean seasoned cast iron pans with mild soaps and a dissolving abrasive like salt, rather than with detergents and scouring pads.