Introduction
Before one may use a utensil that had been used in the preparation of non-kosher food, it must be properly cleansed from any remains of the non-kosher food. This cleansing process is referred to as hechsher keilim (הֶכְשֵׁר כֵּלִים) or, colloquially, kashering. The process of hechsher keilim finds its source in the Torah: when the Jews defeated the Midianites and took their utensils as spoils of war, the Torah (Bamidbar 31:22-23) instructed that, regarding any utensils used for food, they must be purged of any forbidden substance they may contain before they can be used by a Jew.38 A non-kosher utensil that was not kashered, and then was subsequently used for kosher food, will impart non-kosher flavor into the kosher food, rendering it non-kosher as well.
As we will learn, the way that a utensil is kashered depends on the way it was used for non-kosher food. The most common forms of hechsher keilim are libbun (לִיבּוּן), in which direct fire or intense heat is applied to the utensil; hagalah (הַגְעָלָה), in which the utensil is immersed in boiling water; and irui (עֵרוּי), in which boiling water is poured upon a non-kosher utensil or surface. Other methods of kashering will be discussed below.
The halachos of hechsher keilim become especially relevant before Pesach. Throughout the rest of the year, one generally uses their kitchenware only for permitted foods. There usually is not a problem regarding meat and dairy utensils, as they are generally kept separate from one another. So, throughout the year, kashering becomes relevant only when an occasional mistake occurs, or where one wishes to use a utensil that had belonged to someone who does not keep kosher. On Pesach, however, utensils that are used for chametz throughout the year suddenly become forbidden. Thus, a person may have many utensils that they wish to use on Pesach which, before Pesach, all must be kashered from any chametz they may contain.39 This must be done before the time that that chametz becomes forbidden: a chametz utensil should not be kashered after chatzos hayom (midday) on erev Pesach,40 and certainly not on Pesach itself. It is advised that one kasher their utensils well before Pesach, and not wait until the last minute.41
Since the purpose of hechsher keilim is to cleanse a utensil of residue and absorbed flavors of non-kosher foods, new utensils (which have never been used) need not be kashered.42
Section I
Cleaning and Purging
Kashering a utensil involves two stages. First, the utensil’s surface must be cleaned well. This removes any actual food remnants and greasy residue that may be stuck to it, in order that these remnants will not touch the kosher food for which the utensil will be used. The second step of hechsher keilim is based on the halachic principle of absorptions (bleeyos, בְּלִיעוֹת) that, under many circumstances, a utensil will absorb flavor from foods with which it comes into contact. Before being used for kosher food, the utensil must therefore be “cleaned out,” or purged of any flavor from any forbidden foods which it may have absorbed. This is to ensure that these absorptions imbued within the utensil will not subsequently be transferred to the kosher food.
The same way that a utensil was used with a non-kosher food — and thus absorbed its flavor — is the same way it must be kashered and purged of the absorbed flavor. This is based on the principle of kebolo kach polto (כְּבוֹלְעוֹ כׇּךְ פּוֹלְטוֹ), “it purges in the manner that it absorbed.” (See Pesachim 30b; Avodah Zarah 75b-76b; Shulchan Aruch, Orach Chaim, Chapter 451, Passages 4—5; and Yoreh Deah Chapter 121, Passage 1.) Thus, utensils which are used over a fire with direct contact with hot solid food (for example, rotisserie spits, grill grates, or oven pans) can only be purged of these absorptions by kashering them with intense, direct heat. Any lesser kashering method is ineffective for removing the absorbed non-kosher flavor. If a utensil was used for cooking in a manner that involves less intense heat, and less direct contact with the non-kosher food — such as through the medium of boiling liquid, or soaking, or other ways — the absorbed flavors can successfully be removed with a method of kashering that matches the original use, though less intense than direct heat.
38 Hechsher keilim is one obligation, whereas tevilas keilim is another, separate obligation. Tevilas keilim, discussed in Shulchan Aruch, Yoreh Deah 120, means that even unused utensils obtained from a non-Jew must be immersed in a mikveh before using them for food.
39 It should be noted that that hechsher keilim is not a part of the bedikas chametz requirement of searching for chametz and ridding it from our homes. Bedikas chametz only requires one to search for actual pieces of chametz, and not for chametz residue or chametz flavor absorbed in utensils. Hechsher keilim which, as discussed above, purges these traces of chametz from utensils, is only necessary to allow a utensil to be used on Pesach so it will not imbue chametz flavor into Pesach food. A person is thus not required to kasher the utensils that he does not plan on using on Pesach. These utensils need only be washed and stored in a closed-off area so that they will not inadvertently be used (Shulchan Aruch and Rema 451:1).
40 Hagalah may ideally be done until chatzos hayom on erev Pesach and no later, out of concern for the halachic opinion that chametz on the afternoon of erev Pesach carries the same severity regarding its not being able to be nullified as does chametz on Pesach itself (Chayei Adam 125:35; Sha’ar Hatziyun 452:13). In the pressing situation of someone who, on the afternoon of erev Pesach, realized that he has eino ben yomo utensils that he needs for Yom Tov, and that he neglected to kasher them, he may kasher them during the afternoon hours, before sunset when Pesach begins (see Chayei Adam ibid.).
41 This is especially important regarding hechsher keilim for, as will be discussed below, a ben yomo utensil may not be kashered; thus, one must properly plan before kashering to make sure that the utensil has not been used during the previous twenty-four hours. A halachic authority should be consulted in circumstances where a utensil must be kashered while still ben yomo.
42 Historically, some would kasher new utensils before use, out of concern that they were coated by a non-kosher substance, such as animal fats. Other authorities reasoned that this was unnecessary, for these substances were only applied in a way, after being processed, that rendered it nosein ta’am lifgam, “imparts detrimental taste” (discussed in Section II below).
In modern times utensils are coated during production with synthetic materials that are much cheaper than natural, non-kosher, alternatives. Thus, there is halachically no need to kasher new utensils in our times.