Cleaning a cast iron skillet can usually be done with a stiff brush or sponge without soap.  Using soap can remove the cast iron’s seasoning, which is important to protect because it imparts flavor, improves browning, and gives the pan its semi-non-stick properties.

However, every once in a while hot water and a brush isn’t enough.  To more thoroughly clean your skillet without damaging its seasoning:

1. Pour in a bunch of kosher salt.

 
2. Put the salted skillet on the stove over medium heat.

 
3. Use a paper towel to scrub the salt around the inside of the pan until it looks much cleaner.  If you’re having difficulty scrubbing because of the heat, use a pair of tongs to hold the towel.

 
4. Carefully discard the dirty salt (it’s still hot), let your skillet cool, and put it away for later.