A kli must be cleaned before undergoing hagala, for although boiling water extracts the flavor absorbed in and stuck to the kli, it does not clean the kli of residual food stuck to it. If one kashers a kli via hagala without first cleaning it, he must clean it and then redo the hagala (SA 451:3).
If the kli has cracks and crevices from which it is hard to remove food residue, it must be cleaned with a lot of soap, until it is certain that the residual food on it is not fit for a dog to eat, and then hagala may be performed. Likewise, if a pot has handles, one must clean around the handles with a lot of soap so that the food residue that may have been left there is completely befouled.
Handles of kelim must also be kashered because when metal vessels are heated during cooking, the heat spreads to the handles, and if they reach the temperature of yad soledet bo, the entire kli, including its handles, is considered as a kli in which a forbidden food was cooked, and the entire kli must be kashered. Its handles can be kashered through irui from a kli rishon, because their use and absorption are not as intense as a kli rishon over fire (Rema 451:12). The handles of wooden kelim also must be kashered, even though they do not get so hot, because sometimes the cooking food drips or sprays onto the handles, and its flavor becomes absorbed in and stuck to the handles. Therefore, both the kli and its handles must undergo hagala (SA 451:12; MB ad loc. 68).
Kelim that are likely to be damaged by the kashering process may not be kashered. This is why earthen kelim cannot be kashered; since earthenware absorbs a lot of flavor, hagala cannot cause the release of all the flavor it absorbed. Libun could kasher an earthenware kli, as libun incinerates all the flavor that the kli absorbed, but since libun is likely to make an earthenware kli shatter, there is concern that libun will not be performed properly. Therefore, the established halakha is that there is no way to kasher an earthenware kli. The only way to kasher an earthenware vessel is to return it to the kiln, where it is impossible to protect it from the full force of the kiln’s fire. Then, if the kli survives, it is as though it has been made anew (SA 451:1; MB ad loc. 13, 14).
Likewise, baking trays, Teflon skillets, and Wonder Pots (an Israeli invention used for baking cakes on stovetops), since they absorb taste through fire, cannot be kashered, as the prevailing custom accords with the stringent view (in section 5) that requires a temperature of 350-400oC to kasher them, and at such a temperature, they will likely become warped to the extent that most people would prefer to simply throw them out. However, as we learned (ibid.), in times of need one may rely on the lenient view that kelim used in fire can be kashered at the temperature they were used to bake or roast, and at such a temperature, they will not be ruined.
. The accepted ruling is that baking trays must be kashered by means of heavy libun, and since this is likely to ruin them, they should not be kashered for Pesaḥ (Or Le-Tziyon 3:10:2; Be-ohalah Shel Torah 1:18; Sidur Pesaḥ Ke-hilkhato 8:40; Hagalat Kelim 5:6; 13:315). The same applies to Wonder Pots and Teflon skillets. However, in times of need, one may be lenient and kasher them at the temperature at which they absorbed flavor, as we learned in section 5 that in times of need one may rely on the lenient view that kashering temperature hinges on the temperature at which the absorption by fire took place. We also learned in section 3 that one may factor in the view that ḥametz is considered “heteira bala” and can be kashered by means of hagala. This leniency is reinforced by the opinion of Shulḥan Gavo’ah 451:31 and Responsa Tiferet Adam, OḤ 16, that when one coats a baking tray with oil, the tray has the status of a pot that absorbed through liquid and is kashered by means of hagala. Ḥazon Ovadia: Pesaḥ, p. 134, states that one who wishes to rely on this view may do so. It seems that they mean to say that because of this oil, the pastry does not stick to the kli, and it can therefore be removed easily from the tray. Therefore, its absorption is not considered to be by means of fire. There is room to say that in a case that one is careful to place a baking sheet between the pastry and the tray, according to this view, there is no absorption through fire, and so it can be kashered by means of hagala. In the case of baking sheets, perhaps other poskim would concur with this view.