The image above from Serious Eats might be making more than a few of you shudder. Conventional wisdom (myth) says that you should never use soap to clean cast iron because it will damage the seasoning.
Photo by J. Kenji López-Alt
Serious Eats' Kenji busts this myth and offers some advice:
The Theory: Seasoning is a thin layer of oil that coats the inside of your skillet. Soap is designed to remove oil, therefore soap will damage your seasoning.
The Reality: Seasoning is actually not a thin layer of oil, it's a thin layer of polymerised oil, a key distinction. In a properly seasoned cast iron pan, one that has been rubbed with oil and heated repeatedly, the oil has already broken down into a plastic-like substance that has bonded to the surface of the metal. This is what gives well-seasoned cast iron its non-stick properties, and as the material is no longer actually an oil, the surfactants in dish soap should not affect it. Go ahead and soap it up and scrub it out.
The one thing you shouldn't do? Let it soak in the sink. Try to minimise the time it takes from when you start cleaning to when you dry and re-season your pan. If that means letting it sit on the stovetop until dinner is done, so be it.
The key is you need to start with a properly seasoned cast iron pan. Also re-season promptly.
For more cast iron myth-busting, hit up the link below.
The Truth About Cast Iron Pans: 7 Myths That Need To Go Away [Serious Eats via Kottke.org]