If you learned anything from your granny’s granny’s granny, it’s that a cast iron skillet is meant to be seasoned, used, and never, ever, washed with soap. You’ll ruin the seasoning, you’ll rust the pan, you’ll throw away all the years and years and embedded flavor in that seasoning.
So you can’t wash your cast iron skillet with soap.
Except you can.
Here’s the thing: Your granny’s granny’s granny isn’t wrong, necessarily. Using soap on a cast iron skillet will ruin the seasoning. The misconception is that this is a big problem. It isn’t. You can just re-season the pan and resume cooking food just as delicious as before. When you season a pan, you are merely creating a bond between the oil and the metal, resulting in an oily seal on the surface. Soap will eat into it, but so will a lot of other things — acid from tomatoes, lemons or other fruits, alcohol, starches. It’s a good idea to clean and re-season your pan every once in a while, and it doesn’t particularly matter how you do it.
Many people prefer to clean their cast iron skillets with rock salt and a sponge, and that’s a perfectly acceptable way to do it. But if you’d rather just hit it with some Dawn, you don’t have to feel bad, as long as you season it again.