Cast iron has a porous surface. The seasoning process serves to fill and smooth the surface of the pan. It's true that the more you use and season a cast-iron, the more non-stick the surface becomes. Here is how you season a new or used cast-iron utensil:
1. If the pan is new, be sure any adhesive label is completely removed.
2. Wash, rinse and dry the utensil.
3. Grease the inside surface with Crisco or other solid shortening. A medium-light coating, as you would grease a cake pan, is sufficient.
4. Heat the oven to 350ºF, and position the oven rack in the top third of the oven.
5. Open your windows because there is going to be some smoke.
6. Rub a thin layer of shortening (like Crisco) or oil (bacon grease works great, too) all over the inner bottom and sides of the pan with a paper towel.
7. Place your pan upside down on the top oven rack with a rimmed baking sheet or a roasting pan underneath to catch the drippings.
8. Bake the pan this way for 1 hour. Then turn off the oven and allow it to cool with the pan inside.
When the pan is correctly seasoned, the cooking surface should be smooth and shiny. It helps if the first few things you cook with your newly seasoned pan involve oil, try frying or sautéing something.
To eliminate any concerns about using too much shortening, you can put the pan in the oven upside down. Put a cookie sheet or aluminum foil on the rack below to catch any drips.
A skillet or other utensil can be seasoned as often as necessary to maintain a good surface. Let's say you've just made tortillas and, after all that heat, the surface of your cast iron looks dry. Just season it again before you put it away.
Until the pan is very well seasoned, either by many uses or repeated seasonings, do not attempt to cook foods with a high acid content (tomatoes, for instance). The corrosive nature of high-acid foods will not react well to unseasoned cast-iron. Once a pan is well-seasoned, however, you can use it for just about anything. I do believe my mother could have fried plutonium in her big skillet.
Perhaps the most difficult thing for a particularly fastidious cook to do is refrain from using soap or detergent in the cleaning process. Seasoned cast-iron utensils may be cleaned very nicely with boiling water and a stiff-bristled brush. (I have a short-handled brush with stiff nylon bristles that I use only for cleaning my cast-iron skillets.) And often, say after making cornbread, I merely brush the skillet vigorously, wipe it with a damp sponge, and dry it with a paper towel. It is important to dry cast-iron utensils well after use; they will rust unless thoroughly dried. Cast-iron utensils will darken with use, turning from a steely gray, when new, to dark gray or black.
Over years of use, my cast-iron skillets have become old friends. Although I would never choose to replace them, if I had to (let's say an astute kitchen thief broke in and recognized my skillets for the treasures they are), I could buy new skillets, season them well, and have some new old friends.
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