General Rules of Kashering Utensils

When the soldiers returned from the war against Midian with abundant spoils, including metal eating utensils, Elazar the kohen instructed them to kasher the eating utensils from the non-kosher foods the Midianites had eaten in them, as it states: “This is the ritual law that God has commanded Moshe: Gold and silver, copper, iron, tin, and lead – any article that can withstand fire, you shall pass through fire and it shall be clean, except that it must be cleansed with water of lustration; and anything that cannot withstand fire you must pass through water” (Bamidbar 31:21-23). From here the Sages learned that utensils are kashered in the same manner in which they absorbed – ke-bol’o kakh polto (“as it absorbs, so it expels”): utensils used directly on fire, such as skewers and baking pans, are kashered through libun (heating to incandescence) in fire; utensils used for cooking are kashered through hag’ala (purging) in boiling water; and utensils used with cold food are kashered through hadaḥa (rinsing) with water (Avoda Zara 75b). Thus there are three types of kashering: libun, hag’ala, and hadaḥa (above 31:1 we learned that in order to kasher utensils acquired from gentiles, one must also immerse them).

Let us explain further: when a utensil was used with food that is not hot, since the food remnants cling to it only loosely, they can be removed by washing with cold water; this is hadaḥa. But if it was used for cooking, the flavors of the cooked food stick to the utensil and are absorbed in it, and only boiling water can purge them; this is hag’ala. And if it was used with ur, that is, fire, as in the case of skewers or baking pans, since there are no liquids there to soften the adhesion of the food to the walls, the flavors are absorbed in the utensil with great force. Therefore boiling water does not have the power to purge them from the walls of the utensil; rather, it must be kashered in the manner in which it absorbed – with ur, namely through libun.

It appears that when the Sages spoke of flavors absorbed in a utensil, they also meant flavors that stuck so firmly to its walls through cooking that they were considered absorbed in it. For in the past, when there were no pipes carrying water to every home, and no method had yet been found for mass-producing soap that effectively removes grease and dirt from utensils, and the surfaces of utensils were rough – without hag’ala in boiling water to dissolve the fats that had stuck to them, remnants of the foods cooked in them would generally remain stuck to their walls (below, n. 6). It emerges, then, that the instruction of the mitzva to kasher metal utensils in the manner of their absorption is directed also at the flavors that stuck to their walls. And although even after hag’ala a small amount of the previous food may still remain on the walls of the utensil, since the utensil was purged as the Torah commands, and this hag’ala removed the bulk of the flavors that had stuck to the utensil to the point that their taste could no longer be detected, they are considered nullified and no longer contain any prohibition.^¹