
Left: Unseasoned Right: Seasoned
From Wikipedia:
A seasoned pan has a stick-resistant coating of polymerized fat and oil on the surface. Seasoning is desirable on cast-iron cookware and carbon steel cookware, because otherwise they are very sticky to foods and rust-prone. For other pans e.g., stainless, aluminum, enameled), the same chemical phenomenon can occur, but seasoning may not be desired for cosmetic reasons (it makes a pan look splotchy), or the pan may already be stick-resistant (e.g., at medium heat, a clean stainless pan with oil is very stick resistant to many foods). ...
As with other cast iron vessels, a seasoned pan or dutch oven should not be used to cook foods containing tomatoes, vinegar or other acidic ingredients. These foods will damage the new seasoning. Instead, newly seasoned ovens should be used to cook food high in oil or fat, such as chicken, bacon, or sausage, or used for deep frying. Subsequent cleanings are usually accomplished without the use of soap. Because modern cleaning methods (detergent soaps, dishwashers) will destroy the seasoning on cast iron, manufacturers and cookbook authors recommend only wiping the pans clean after each use, or using other cleaning methods such as a salt scrub or boiling water.
From KitchenEmporium.com:
Cast Iron Cookware must be seasoned properly and it will last a life-time. ( I still use my Grandmother's cast iron skillets on a regular basis and they must be at least 60-70+ years old.)
New Pans
- Heat the oven to 250 - 300 degrees
- Coat the pan with lard or bacon grease. Don't use a liquid vegetable oil because it will leave a sticky surface and the pan will not be properly seasoned.
- Put the pan in the oven. In 15 minutes, remove the pan & pour out any excess grease. Place the pan back in the oven and bake for 2 hours.
Repeating this process several times is recommended as it will help create a stronger "seasoning" bond.
Also, when you put the pan into service, it is recommended to use it initially for foods high in fat, such as bacon or foods cooked with fat, because the grease from these foods will help strengthen the seasoning.