What causes a pan to become concave or convex?

[PeteLangevin]

My daily use pan , a wagner 1088 chicken fryer spins like a top on the counter but still cooks great. Just curious what causes pans to change shape.

 

[Bonnie Scott]

It's cause by overheating. You can read about it here.
http://www.castironcollector.com/damage.php

 

[Sharon Shuman]

Thanks for the reference link, Bonnie. Certain markings on pieces I have are explained well in the essay. Also many thanks to the creators and all the contributors to this site; the forum daily produces laughter and info., and I appreciate all of it!