The Torah states in conjunction with the laws of the Korban Chatas that if the meat of this Korban is cooked in an earthenware utensil, the utensil must be broken. If, however, it is cooked in a metal pot, the pot may be cleansed by means of water (ויקרא ו:כ"א). The problem is that after a certain amount of time, meat from a Korban, including any meat flavor absorbed in the walls of the pot, becomes "נותר", leftover, and may not be eaten. Moreover, a pot containing such absorbed flavor may not be used. The Torah thus tells us that we may perform הגעלה, that is, purging the forbidden flavor through the use of hot water. This is one method of what we call "Kashering." The Eliyahu Rabbah (אורח חיים סימן תכ"ח) cites the Avudraham who states that our calendar is designed so that Parshas Tzav is read prior to Pesach because many of the laws of Kashering utensils for Pesach are derived from this Parsha.
The Rambam (פרק ה' מהל' חמץ ומצה הלכה כ"ג) states that metal utensils used for hot food should be placed in a large vessel filled with water which is then heated until the Chametz flavor has been purged. This represents the normal method הגעלה. This type of הגעלה is based on the laws of ביטול, nullification of a flavor. The flavor of Chametz is purged into a vessel containing sixty times more water than purged Chametz flavor, so ביטול is effective. Consequently, הגעלה for Pesach, must take place prior to Pesach, as indicated to by the Ramo (או"ח סימן תנ"ב סעיף א'), because at this point ביטול still works. Once Pesach starts, however, and Chametz is אסור במשהו, that is, it is forbidden in even the minutest amounts, ביטול cannot work and thus הגעלה would not be effective. However, if one chooses to Kasher his utensils through ליבון, that is, with the use of fire which burns out the forbidden flavor and eradicates it entirely, the Kashering can be performed even on Pesach itself.
Our practice is to perform הגעלה only if the utensils are אינן בני יומן, not having been used for at least 24 hours prior to the הגעלה, as indicated by the Ramo (שם סעיף ב'). However, if one did not take this precaution , one can still Kasher the utensil prior to the time on Erev Pesach when one can no longer have Chametz, based on the fact Chametz before Pesach is considered היתירא בלע, meaning that it is an absorbed flavor which is allowed at that moment. The pot would then become permissible because it is a case of נ"ט בר נ"ט דהיתירא, meaning, in effect, that the absorbed flavored was a permissible one and thus is more easily purged. After the time arrives, however, when Chametz is prohibited (which is at mid-day according to the Torah and at the fifth hour of the morning according to the Rabbanan), the Chametz becomes categorized as a forbidden product and would not be subject to the above leniency.
This leniency is predicated on the assumption that Chametz before Pesach is in fact a case of היתירא בלע; such indeed is the opinion of the Rambam according to the Maggid Mishnah's comment on the above cited ruling of the Rambam. Support for this opinion is derived from the Rambam's ruling (פרק ט"ו מהל' מאכלות אסורות הלכה ט') that Chametz is אסור במשהו, forbidden in the minutest amount, meaning that the usual rules of ביטול don't apply, because it is a דבר שיש לו מתירין, that is, it will become permissible eventually anyway. If Chametz were to be classified as איסורא בלע, such that the absorbed flavor would be considered a forbidden one, this obviously would not be true.
Rav Chaim Soloveitchik offered an additional support to this premise. The Rambam (פרק ח' מהל' מעשה הקרבנות הלכה י"ד) states that utensils used for other Korbanos (besides a Korban Chatas) may be Kashered through הגעלה, even if they are earthenware utensils. The Raavad (שם) objects and says that the Rambam erred in ruling that earthenware utensils can be Kashered. Logically, the Raavad is correct because the Torah itself seems to say that earthenware utensils cannot ever be Kashered. Rav Chaim thus explains that since the absorbed flavor of the Korban is not yet נותר, meaning that it is not yet forbidden at the time of absorption, the vessel is considered to have absorbed a permitted substance (היתירא בלע) and consequently הגעלה would be effective. We thus see that the absorption of a flavor at a time when the flavor is not yet forbidden is labeled as היתירא בלע; this would then be true as well of Chametz flavor absorbed prior to the time when it may no longer be eaten.
Items which are used directly over fire, such as a שפוד, a spit, require ליבון, the use of fire; הגעלה would be insufficient. Some authorities put regular ovens into this category. However, HaRav Joseph B. Soloveitchik is of the opinion that the general principle used in Kashering utensils is "כבולעו כך פולטו," a utensil will purge a flavor in the same manner in which it absorbed it. Therefore, if one will turn on the heat of one's oven to its highest possible temperature for the maximum duration of time that it was ever used to cook or bake a Chametz food item, this would be sufficient to Kasher the oven.
When performing הגעלה on pots, we try to Kasher the handles as well by unscrewing them and placing them in hot water. If one cannot remove the handle, or immerse the handle in he larger Kashering pot, one should, בדיעבד, simply pour hot water on the handle, as indicated by the Ramo (סימן תנ"א סעיף י"ב).
Under ordinary circumstances, one should not change the use of a utensil from dairy to meat or vice-versa even though he has done הגעלה on the utensil and, until the first usage, it is technically a "neutral" utensil. The reason for this is that we are afraid that the individual will get confused and will once forget to do הגעלה on the utensil and then use it for either meat or milk without the requisite הגעלה. However, the Magen Avraham states that if one is Kashering for Pesach, one can indeed switch the designation of the utensil from one to the other.
According to the accepted Halacha, we reject the Rambam's position regarding earthenware utensils, and we never do הגעלה on them. Rav Moshe Feinstein is quoted as having ruled that if one possesses a self-cleaning oven, the heat generated in the cleaning cycle is so intense that it in effect breaks down the finish of the china, and subsequent application of this heat then renews the vessel. The oven would thus be treated as if it were broken and then repaired, following which full further use is permitted.
Glassware, according to the accepted Halacha, does not absorb flavor at all; theoretically one could thus use glasses for Chometz, Pesach, meat and dairy without any problem. The Ramo states, however, (שם סעיף כ"ו) that Ashkenazic Jewry is stringent and treats glassware as earthenware vessels. It is thus our practice that glassware used with Chametz for hot drinks should not be used for Pesach. But if they were used for cold drinks, one may, if really necessary, Kasher them by filling the glasses with water for 72 hours, changing the water every 24 hours, as indicated by the Mishnah Berurah (שם ס"ק קנ"ו). For further information governing these complex and important issues, one must consult a qualified Rabbinic authority.