I've cleaned over two hundred cast iron pans in an open fire and never
had one crack or warp. When scrubbed and oiled they look like new.
Everybody that I've given them to don't have any problems with them.
If you just throw them in a fire and let them cook for hours you might
have a problem. I fire them in the evening so I can see the color turn red
it's time to remove them the fire or if a small fire just let it go out.
As for collectors around here most of the antique stores bring their
old pans to me to clean them up.