814. Kashering for Pesach

115:6 On Friday when Shabbos is erev Pesach, every man should be especially careful to make sure that challah has been separated from the loaves that were baked for Shabbos. If one did not separate the challah and remembered on Shabbos, there is a great problem as to how to proceed. This is because we may not separate challah on Shabbos and it is also prohibited to retain them. The Magen Avraham says that in such a case one must give all the loaves to a non-Jew as a gift before the time when it becomes forbidden to have benefit from chometz. Other authorities differ and offer other courses of action but none of them solve the problem as satisfactorily as avoiding it altogether. Because of this, one must be extremely cautious in this matter.

116:1 An earthenware vessel that had been used for chometz cannot be kashered, neither by boiling it nor by purging it in fire; ovens and stones that are made from stones or bricks can be kashered by fire. The practice was not to kasher the stoves that people used to heat their houses in the winter (because people would put chometz on it year-round - Mishnah Brurah 451:134). If a person wanted to stand a pot on such a stove on Pesach, he would have to first put a metal tray on the stove (or coat it with clay a finger-width thick - MB 451:136). This method also works for an earthenware oven.