A little carbon build-up on a cast iron pan is a good thing. Like any good thing, though, there is a point where the build-up becomes a problem rather than a benefit. It can be difficult to find the balance between retaining the non-stick, rust protective qualities of cast iron cookware and preventing unsightly or unhealthy situations. Uneven carbon build-up can cause uneven heating, which will lead to scorched food. When build-up becomes too thick, the pan can take longer to heat to the correct temperature, leading to undercooked food and wasted fuel.
Place the cast iron pan in a basin and pour boiling water over it. Drain the water. Make a paste of 1/4 cup of baking soda and 1 tsp of water. Wipe a thick layer all over the outside of the pan; allow it to stand for 30 minutes while the pan cools.
Rinse away all the baking soda with warm water. Place pan upside down on grill or inside wood stove. Heat pan to black, which is between 1,-17.7 and 648 degrees C. If you're heating the pan in a standard kitchen oven, heat to 260 degrees C for at least one hour. Allow the pan to cool slowly to room temperature, over the course of an hour or two.
Use a polyester pad to scrub away any visible oxidation. This will appear as rust streaks, reddish dust, black flakes or any other discolouration. The scrubbing process will create brown/black dust. Turn pan upside down and tap it gently to get the residue out of the pan.
Repeat the previous step, using 0 and 00 steel wool in that order until the entire pan is an even, dark colour.
Wipe entire pan with a damp paper towel. Pour 1 tsp of vegetable oil or olive oil into the pan and wipe it around the entire inside and outside surface of the pan. You may also rub a slice of bacon fat or 1 tbsp of vegetable shortening all over the inside of the pan instead.
Wipe away any excess oil, fat or shortening. Place the pan right side up on the hob, grill or wood hob. Heat it for 10 to 15 minutes on high or until the metal appears glossy black.
Linda stradley of What's Cooking America states, "...the cooking surface (of cast iron pans) develops a non-stick quality because the formerly jagged and pitted surface becomes smooth. Also, because the pores are permeated with oil, water cannot seep in and create rust that would give food an off-flavour. Your ironware will be slightly discoloured at this stage, but a couple of frying jobs will help complete the cure, and turn the iron into the rich, black colour that is the sign of a well-seasoned, well-used skillet or pot."