Besides the way a utensil is used, there are several other principles that determine the proper method for kashering a utensil.
Rov Tashmisho (A Utensil’s Primary Function)
We discussed above that the way a utensil was used determines the way it needs to be kashered. The Rishonim and subsequent Poskim discuss whether the required kashering method is based on the object’s primary use (termed rov tashmisho, רֹב תַּשְׁמִישׁוֹ, lit. the majority of its use), or whether it is based on the maximum heat a utensil had been used with, even though it may have been an unusual, one-time usage. As an example of this question, Shulchan Aruch (Chapter 451, Passage 6) discusses the case of a metal bowl which is generally used by pouring hot chametz liquid into it. Based on the principle of kebolo kach polto, this bowl would normally be kashered through the method of irui. However, the bowl is occasionally used as a kli rishon directly on the fire, which, as Mishnah Berurah explains (note 46), is something that should require hagalah, a more intense level of kashering.
Shulchan Aruch (see also ibid. Passage 25) rules leniently, stating that one may kasher a utensil based on the way it was primarily used; one need not be concerned for an unusual manner of use.78 Commenting, Rema (ibid.) notes that some rule stringently — stating that one must be concerned for the occasional use, and that the common practice is, in fact, to kasher in a manner that uses the highest degree of heat that the utensil had ever been used with (when it may have absorbed non-kosher flavor). Mishnah Berurah (ibid. note 47) notes that Rema’s stringency is the correct practice lechatchilah. However, in a pressing situation, one may rely on the Shulchan Aruch’s more lenient ruling.79
Note, that Shulchan Aruch agrees that — if the occasional type of usage took place during the previous twenty-four-hours (ben yomo)80 — then the stronger method of kashering is necessary.
Heteira Bala
Another leniency that may allow one to rely upon a less intensive method of kashering is the principle of heteira bala (הֶתֵּירָא בָּלַע), “absorbed permitted substance.” This term refers to kashering a utensil from absorptions that became imbued into the utensil when they were considered kosher and, therefore, non-problematic. Common examples of heteira bala are kashering a dairy (or meat) roasting pan with the intention of making it pareve,81 or kashering a meat utensil which was accidentally used with dairy — provided that the meat utensil was not used with meat in the past twenty-four hours.82 (The same is true in the reverse scenario, of a dairy utensil that was accidently used with meat). Since dairy is of course a permitted substance, the principle of heteira bala allows the absorptions to be removed with a less intensive form of kashering. In this case, hagalah would be sufficient instead of libbun, which would normally be required if one were kashering a non-kosher utensil (such as a utensil purchased from a non-Jew who presumably used it for non-kosher foods).
Some Rishonim83 state that a utensil that absorbed chametz before Pesach is considered heteira bala, for chametz before Pesach is a substance that is permitted. Thus, the leniency of heteira bala — to allow a less intensive method of kashering —would apply when kashering chametz utensils. However, other Rishonim84 argue with this opinion. They understand that, regarding “heteira bala,” a “permitted substance” is specifically an item which begins as permitted but undergoes a qualitative change and, as a result of this change, the item gains a different halachic status and becomes prohibited (for example, cooking milk with meat). Chametz before Pesach, however, is different. It is true that, before Pesach, chametz is completely permitted. Nevertheless it is already, and qualitatively, “chametz,” and therefore does not fall into the category of heteira bala.
Shulchan Aruch (Chapter 451, Passage 4) rules that chametz utensils should be kashered with the regular principles of hechsher keilim, and as Mishnah Berurah notes, he does not mention this leniency of heteira bala. Mishnah Berurah adds, however, that if a scenario includes other mitigating factors then, in combination with these other factors, one may rely on the opinion that chametz (before Pesach) is considered heteira bala, and issue a lenient ruling.85
78 For example, a metal serving bowl that is occasionally placed directly on the fire may be kashered with irui and does not require libun. However, if something may sometimes happen through an item’s normal usage, Shulchan Aruch agrees that it must be taken into account when kashering. Thus, Shulchan Aruch (451:20) rules that a tabletop must be kashered via irui because hot foods sometimes spill on the table. Although these spills do not happen every day, they happen through the table’s normal function of being used for meals, when some food items will invariably spill. See Megillas Sefer, Inyanim I, 20:2 ד”ה עוד; Mibei Dina, Pesach, p. 183 ד”ה ואולי; Levushei Oz, p. 609; and Ma’adanei Asher, Issur V’heter, Yoreh Deah p. 328, for discussion.
79 Rashba, Responsa I:372, and She’eilos Uteshuvos Rama Mipano 96 explain that the grounds to rule leniently — not be concerned for occasional use, even though the utensil certainly imbued these absorptions — is because any absorptions that the utensil may contain are in any event nosein ta’am lifgam and halachically benign. Therefore, Chazal’s requirement that an eino ben yomo utensil be kashered before use (which is to ensure that it would not inadvertently be used when still ben yomo) may thus be fulfilled by kashering the utensil based on its primary use. See Igros Moshe, Orach Chaim, V:31 for further elaboration. Note Rashi cited in Sefer Ha’orah I:81 and II:19; Shibbulei Haleket 207; Rashba, Responsa I:372; Tosafos, Chullin 8a ד”ה שלבנה citing Rabbeinu Tam, who indicate an alternate reason for leniency.
80 Mishnah Berurah 451:46. It appears from Mishnah Berurah that, if one is uncertain whether a utensil was used during the previous twenty-four hour period, then one may be lenient. Cf. Chazon Ish 122:4.
81 Kashering a meat (or dairy) utensil to make it pareve is not included in the minhag of not kashering from dairy to meat or vice versa, discussed in Not Kashering Between Meat and Dairy, below (Shei’eilos Uteshuvos Maharsham 2:241; Darkei Teshuvah 121:59; Shei’eilos Uteshuvos Tzitz Eliezer 9:38). Cf. Chut Shani, Hilchos Pesach, p. 119.
82 The heteira bala leniency is relevant in this scenario because no actual basar bechalav has been created, for the utensil was already eino ben yomo when it absorbed the wrong type of food. The utensil still must be kashered, however, due to the rabbinic obligation to kasher eino ben yomo, and heteira bala may be applied to permit kashering via hagalah. This issue is discussed in She’eilos Uteshuvos Chasam Sofer, Yoreh De’ah 110; and Lehoros Nosson I:33:11. However, see, Meiri to Avodah Zarah 76b ד”ה גדולי הדורות.
83 See Raavad, cited in Ran, Pesachim 30b; and Rabbeinu Tam cited in Tosafos, Chullin 8a ד”ה שלבנה. Ohr Letzion, vol. 3, Dinei Hagalas Keilim, notes that the Rambam’s omission of any mention of libbun in his guidelines regarding kashering for Pesach in Hilchos Chametz Umatzah 5:3, indicates that Rambam concurs with this ruling as well.
84 Primarily Ramban (Pesachim 30b); Rosh and Rabbeinu Yerucham cited in Beis Yosef Chapter 451.
85 Mishnah Berurah Chapter 451, note 28.