Some people believe that cleaning cast iron kitchenware is easy – just throw it into a campfire! The iron will get red hot, all gunk will turn to ash, and voila! Clean pan!
Please, please do not do this. If you put your cast iron cookware into a fire to clean it, it will irreparably damage the pan. The pan may crack; it will surely warp.
When you come across a vintage pan and it has an odd red color to it (not rust), I bet you that it will rock and / or spin when you press along the upper edge. That is because someone tried cleaning it in fire, or placed it into a fire, and the high heat changed the composition of the pan and it turned that odd color and also warped.
Here are a few pics of a vintage Griswold number 8 slant logo EPU skillet that has been heat damaged. This pan has been cleaned to bare iron, but has not been seasoned. The red you see is not rust; it is fire damage.
And here’s that same pan, when pressing along the upper rim:
So please, don’t even think about cleaning these precious old heirloom pieces by throwing them into a fire!