The process of seasoning a pan involves coating it with oil, and then oxidizing that oil with heat and air. Every time you heat a pan, that seasoning continues to oxidize and burn off. The bottom of any cast iron used for cooking is always the most neglected spot, yet it receives most of the direct heat while cooking. Always check the bottom of any user piece of cast iron; because, the seasoning will both wear (from movement and sliding) and burn off (due to being directly heated). What Cheryl has posted is good advice. Just like any other piece of cookware, cast iron must be maintained. Just check the bottom of your user pieces every so often.
