Seasoning of cast iron cookware is a way to protect the metal and keep it from rusting. A layer of fat is oxidized on the surface to create a hard, dry barrier against water and air.
Nearly all cast iron cookware is sold without a coating of seasoning on it. Giving those grey metal pans and ovens a signature black sheen is up to the new owners of those pieces. The process involved is called "seasoning the cast iron."
Since cast iron is extremely susceptible to rusting when exposed to air and water, a layer of protection is required to keep it in good condition. When it comes to cookware, that protection comes in the form of an oil or animal fat. A thin layer of the fact is rubbed into the clean, dry surface of the pan. Next, the pan is placed in an oven at a moderate temperature between 350 to 400 degrees Fahrenheit for about an hour. When done properly, the final result is a black shiny pan with a hard, non-stick surface.
During the heating process, the fat or oil melts and settles into the porous surface of the pan. As the fat molecules oxidize it bonds together creating the dark, hard and protective layer on the outside of a cast iron pan.