From: Rabbi Moshe Sofer
To: Rabbi Eliezer (Lizer)
The Chasam Sofer rules that enameled iron pots of unknown composition should not be kashered through hagalah but rather by filling them with hot coals (a form of libun). If the coating cracks it is confirmed as earthenware, but the exposed iron is kashered by the heat; if it does not crack, the doubt about its composition combined with the thin layer renders it sufficiently kashered if heated to the standard of libun chamur. He also demonstrates that the Pri Megadim's lenient reliance on the Ba'al HaIttur and Rashba to permit three hagalahs for earthenware is based on a misreading of those authorities.
Source: Shu"t Chasam Sofer YD §113