Cast iron care is something that cast iron-lovers take quite seriously. There are strict rules to follow depending perhaps on where you live and where you were raised. Some myths of cast iron persist even though they are tirelessly proven untrue. So consider this a public service reminder on the "reported dangers" of washing your cast iron skillet with soap.
Yes, you can actually use soap on your cast iron!
The myth that you shouldn't wash your cast iron skillet with soap is just that — a myth. It's driven by two theories. The first is that since oil is used to season the cast iron skillet and create a nonstick surface, soap would effectively wash away the cure that you worked so hard to build. The second reason that this myth persists is that soap isn't the most efficient cleaner of cast iron.
Soap doesn't wash away the oils.
Strictly speaking, oil is not responsible for the slick-hard surface of a well-seasoned cast iron pan — polymerization is. The oil combines with the porous surface of the pan when heated creating a surface harder and smoother than the pan itself. It would take a lot more than soap to remove the seasoning from a skillet. In fact, the best way to remove seasoning is to bring the pan to high heat in an oven or grill — not with sudsy water.
Get to Know Your Cast Iron
But you don't need soap.
Many cast iron purists simply wipe their pan clean after cooking instead of exposing the pan to water and harsh scrub brushes. But for those of us who'd prefer a cleaner cast iron skillet, oil and kosher salt are more effective than soap and a sponge will ever be. Pour a few tablespoons of each into the still-warm skillet and scrub the pan with a paper towel until the pan is shiny and the salt is near black. Rinse out the salt and thoroughly dry the pan. You can rub it with a touch more oil before storing, if desired.