This myth seems to make sense—after all, you use fat or oil to season cast iron pans and soap removes fat, case closed, right? It's not that simple.
The seasoning layer on the surface of a cast iron pan is formed when the pan is heated while in contact with some fat or oil. This may happen during normal cooking or during a special seasoning process that many people use with a new pan. The result is a chemical reaction in which that fat polymerizes, meaning that multiple individual fat molecules join together to form larger molecules. It is these larger polymer molecules that bind to the metal of the pan and form the seasoning. And, guess what, the polymer is not dissolved by soap. So, it's perfectly OK to use a mild soap on your cast iron if you are so inclined.
Never use harsh detergent, put the pan in the dishwasher, or scrub with an abrasive, however, as these will take off the coating. This myth originated back in the days when soaps were harsh, often containing lye, and would indeed damage the seasoning on cast iron. Modern chemistry has brought us milder but still effective dishwashing soaps that can be used on cast iron without issues. Just don't tell grandma!
Of course, many people get their cast iron perfectly clean without soap, so there's no real reason to use it. The idea that soap will ruin the pan is, however, a myth.
Much new cast iron comes already seasoned. If not, it is likely coated with wax or shellac to prevent rusting, and you must remove this coating before seasoning. Check the pan's instruction to be sure, but generally a good scouring with detergent will do the trick. Dry thoroughly, coat the entire surface, inside and out, with oil, and put in a 325 degree oven for about an hour. Let cool and wipe off excess oil and you are ready to cook. The best oil to use for seasoning is flaxseed oil. Lacking that, use lard or hard vegetable shortening like Crisco. Regular vegetable oils such as corn and peanut will work but are not as effective.
