It is customary not to perform hagala on a kli until 24 hours have elapsed since the last time it was used with forbidden food (i.e., until it is no longer “ben yomo”), because for 24 hours the taste that is absorbed into and stuck to the kli remains flavorful, and if the boiling water used for hagala is not sixty times larger than the volume of the walls of the kli, the hagala water may absorb the forbidden taste and cause it to be reabsorbed in the walls and re-stuck to the sides, rendering the hagala ineffective. But after 24 hours, the taste absorbed into and stuck to the kli becomes foul, and then the kli can be kashered even if the water is not sixty times its volume, because the kli releases a foul taste into the water, and such a taste will not render the kli forbidden, even if it is reabsorbed. For a kli is only rendered forbidden if the flavor it absorbs initially is good, in which case it remains forbidden even once the absorbed flavor has become foul. However, if at the time of its absorption and adhesion to the kli the taste was foul, the kli is not forbidden.
Another reason for this 24-hour delay is our concern that meat and dairy kelim will undergo hagala in the same water, release good meat and milk flavor into the hagala water, and if the hagala water is not sixty times the volume of either the meat or the dairy, the flavors will not be batel be-shishim. Rather, these flavors will mingle and then render all of the hagala water, as well as any kelim that undergo hagala in it, forbidden, as they will have absorbed the flavor of basar be-ḥalav (the forbidden mixture of milk and meat). However, once 24 hours have elapsed, the flavors of milk and of meat that are released by the kelim is foul and will not render the hagala water and the kelim that underwent hagala in it forbidden, since noten ta’am li-fgam is permitted (SA 452:2; MB ad loc.).
The common practice at public hagala stations is not to rely on the presumption that everyone waited 24 hours before performing hagala. In order to avoid problems, liquid soap is added to the hagala water, thereby immediately rendering any taste released by the kelim foul, so even if it is reabsorbed into the kelim, since it is foul, it does not render kelim forbidden.
Technically, it is possible to perform hagala on metal and glass kelim that were cleaned with soap without waiting 24 hours, since they do not release flavor. However, le-khathila the custom is to wait 24 hours or add liquid soap in the case of glass and metal kelim, like in the case of wooden kelim.
According to Issur Ve-hetter Ha-arokh, if one kli after another is inserted into the hagala water, the flavor that is released by them is batel, one by one, and we do not combine all the kelim together. However, according to Rashba, the kelim are ḥozer ve-ne’or, so if the hagala water is not sixty times the volume of the walls of all the kelim inserted into the hagala water, all of the aforementioned concerns apply. This is the accepted custom (MB ad loc. 13). The custom, le-khatḥila, is also to kasher only kelim that have not been used for 24 hours, lest the hagala water have less than sixty times the volume of the walls of the kelim (MB ad loc. 20).
All of these principles also applied to metal kelim that were not cleaned with soap, as was the standard practice until modern times. Nowadays, however, when we clean these kelim with soap, and there is no longer any concern that a layer of residual food remains on them, there is no concern that they will release flavor into the hagala water. Only in the case of wooden and earthen kelim is there concern that they will release flavor (Peninei Halakha: Kashrut 32:4-8). Nevertheless, the accepted ruling is to wait 24 hours, in accordance with time-honored custom, or to put soap in the hagala water, as is necessary for wooden and earthen kelim and for glass and metal kelim that were not cleaned properly.
Some maintain that hagala can only be performed in water, as other liquids do not adequately cause the release of flavor (Ramban, Ritva, Me’iri). Others maintain that hagala can be performed in other liquids as well (Rashba). Rema 452:5 rules that hagala should le-khatḥila be done in water, and be-di’avad, it can be done in other liquids. Based on this, several Aḥaronim wrote that le-khatḥila ash should not be mixed into the hagala water (MB, Pri Ḥadash, Ḥok Yaakov). Nevertheless, the accepted ruling nowadays is to mix liquid soap into the hagala water. Apparently there is no concern as the liquid soap does not alter the consistency of the water (so states Sidur Pesaḥ Ke-hilkhato 7:76).
Regarding whether one may perform hagala on Pesaḥ: As we learned above (7:5), according to the majority of Rishonim and SA, who maintain that even on Pesaḥ something that imparts foul flavor does not render the mixture forbidden, it is permissible to perform hagala as long as the kelim have not been used for 24 hours, for then the flavor absorbed in and adhering to the kli is foul. According to Rema, who is concerned about the view that something that imparts foul flavor renders a mixture forbidden on Pesaḥ, one may not perform hagala on Pesaḥ. Only libun, which incinerates the taste in and on the kli, may be done to kasher kelim on Pesaḥ. However, one may perform hagala on clean metal and glass kelim on Pesaḥ, since they do not release ḥametz flavor into the hagala water. (See Peninei Halakha: Kashrut 32:6-7.)