GotGarlic
Chef Extraordinaire
I saw that quote. I cook on natural gas. Build-up of any sort has not been a problem for me. But I'll keep the coals suggestion in mind.
It seems reasonable that the gunk on the outside of the pan is carbon, and carbon is the non-stick surface, and hot coals are hot carbon. It would probably help to do the same thing but with the cooking surface down as another method of restoring your cast iron pan.
CI pans amaze me since they've been around so long, hundreds of years, and yet they perform in many cases as well as modern high-tech non-stick surfaces.
On mine, the gunk the outside is oil. Not sure if that's carbon - it's not burned - but I don't like touching it as I handle my pans.