Let us further explain the principle of “as it absorbs, so it releases” (“ke-bole’o kakh polto”; Pesaḥim 30b). If a kli was used for food that is not hot, since the residual food adheres loosely to the kli, it can be removed easily, by rinsing it in cold water. This is called hadaḥa (rinsing).
If the kli was used with hot liquids, for instance, a pot in which a ḥametz dish was cooked or a ladle or spoon inserted into a pot in which ḥametz was cooked, the taste of the food adheres to and is absorbed by the kli, and only boiling water can cause its release. This is hagala.
If a kli was used directly on a fire – as in the case of roasting spits or baking trays – since there is no liquid to soften the adhesion of the food to the walls, the taste becomes absorbed strongly into the kli, and boiling water is not powerful enough to cause the release of the taste from the walls of the kli. They must be kashered using the method by which they absorbed the taste – through fire, namely, libun.
We must add that absorption through fire takes place only when the food is directly on the fire, for example, in the case of a knife used to cut a pastry that is on the fire. However, if the pastry was removed from the fire and then cut with the knife, even though the pastry is dry, since it is no longer on the fire, the knife’s absorption is deemed weaker, and it can be kashered by means of hagala (Issur Ve-hetter Ha-arokh 58:27; Erekh Ha-shulḥan 94:13).
Before kashering with hagala, it is necessary to clean the kelim, since the boiling water is not powerful enough to remove all the food residue stuck to the walls of the kli. With respect to libun, however, there is a dispute among the Rishonim: Some say that one must clean the kelim before libun as well, because fire, too, is not powerful enough to remove all the food residue stuck to the walls of the kli. Others say that fire is powerful enough to destroy anything stuck to the kli, so it is not necessary to clean kelim that will be kashered by libun. This is the accepted halakhic ruling.