Is not using soap when washing cast iron pans safe? [duplicate]

That residual fat is leftover from what you cooked in there before, so it would only be an issue after cooking something with a very low smoke point, washing the pan, then cooking something with a high smoke point.

Wiping the pan with a paper towel will get rid of the vast majority of this, and if you dry it over heat you can carry on to cook off the last little bit, like a sort of half-baked or top-up seasoning. Doing this before storage gives the smoke smell time to clear before you next use it, but this should be minimal if you cook it off gently. I recommend turning on the cooker hood extractor if you have one.

I like the heat-retention of cast iron but I've gone over to enamelled (le Creuset). I still use a carbon steel wok that needs reseasoning after using the traditional metal spatula, so I actually rinse, dry with heat, wipe over with oil and heat.

Once your cast iron is seasoned. It is perfectly fine to use soap to clean it. Soap will not remove the polymerized oil. I often soak my cast iron in soapy water, particularly after roasting a chicken, for example. If you are not going to use soap, you can pour in a layer of kosher salt, heat the pan, and scrape up the residue with a wooden spatula. Then wipe out with paper towel. In either case, there is no need to leave any residue.

Never put soap in or on a well seasoned cast iron pot or pan. Scrape them out rinse them with water. Then wipe out with a course towel or paper towel. Heat on burner till warm to remove water. It removes the oil to use soap in them. Every now & then wipe or scrub them out with a old rag with salt in it & oil on it. A old sock works well for this. course salt.