מִלּוּי וְעֵרוּי / Milui Ve’irui (Filling and Pouring Out)
Regarding utensils made of glass or earthenware, such as cups and glasses: if they became imbued with non-kosher taste through the process of kavush (כָּבוּשׁ, soaking)61 — meaning that cold liquid, which is either chametz or non-kosher, remained in these utensils for a twenty-four hour period — they may sometimes be kashered via the process of milui ve’irui, literally “filling and pouring out.”62 The utensils are filled with water and left to sit for at least twenty-four hours. The utensil is then emptied and refilled with different water, which is once again left to sit in the utensil for twenty-four hours. The process is then repeated a third time. Although the utensils must be soaked for (at least) a total of three days, these three days need not be consecutive.63
The process of milui ve’irui, however, is only relevant for certain materials and under certain circumstances.64 Therefore, a halachic authority should be consulted before kashering utensils with this method.
שִׁפְשׁוּף וַהֲדָחָה / Shifshuf Vehadachah (Scrubbing and Rinsing)
Each kashering method that has been discussed so far is done to remove flavor from a utensil with non-kosher absorptions, which have been imbued either through heat or kavush. However, as mentioned above in the Introduction, it is also necessary for the utensil’s surface area to be cleaned from actual food remnants and greasy residue that may be stuck to it. Otherwise, when the utensil is subsequently used with kosher food, these remnants will touch the kosher food and become mixed into it. This requirement of scrubbing and rinsing applies even if a utensil is used only for cold foods. Although it did not absorb any non-kosher taste, it must be cleansed from any residue on its surface area. After a utensil is scrubbed well,65 the water used for scrubbing must be poured out, as it contains the non-kosher residue, and the utensil is then rinsed with fresh, clean water.66
Furthermore, if a utensil is not entirely clean, then hagalah — or other methods of kashering67 — will be ineffective, for the residue on the utensil’s surface will prevent the hot water from reaching inside. A utensil must therefore be thoroughly scrubbed and rinsed before it is kashered by other means.
If a utensil cannot be thoroughly cleaned — for example, if it has cracks and crevices (which may have particles of food stuck inside them), or a bottle whose opening is too narrow to scrub inside68 — then it cannot be kashered via hagalah or irui, and will remain forbidden for use.69
נְעִיצָה בְּקַרְקַע / Neitzah Bekarka (Thrusting a Knife in the Ground)
As a rule, if a utensil is used for foods that are cold, non-charif, and non-greasy, then the utensil does not absorb any flavor. If a utensil was used with such non-kosher foods, it may be used for kosher food simply after being scrubbed and rinsed. Knives, however, are an exception to this rule. This is because, while the knife cuts through a food then, in turn, that food presses against the knife, often leaving a thin film of residue that cannot be removed with regular scrubbing.70 The Gemara (Avodah Zarah 76b) states that this residue can be removed by thrusting the knife into hard earth ten times, in ten different places. The hard dirt continually rubbing against the knife serves to thoroughly clean its surface.
Some Poskim are reluctant to rely on this method of kashering for several reasons, among them there is uncertainty as to how hard the earth must be be for neitzah to be effective,71 and there is uncertainty as to whether neitzah bekarka cleans out the crevices of scratched or serrated knives.72 They advise, instead, that hagalah be done to the knife after scrubbing it with an abrasive material, and all agree this is effective.73 Alternatively, the knife may be momentarily placed into a flame, which will also burn off any residue. Some Poskim suggest that, instead of neitzah bekarka, the knife should be vigorously rubbed with steel wool,74 which cleans it out just as well.7561 Yoreh Deah 105:9. A sharp (davar charif) liquid, such as brine or vinegar, will become imbued in a container in which it is soaking after remaining there for only eighteen minutes or, according to some opinions, as quickly as six minutes (Yoreh Deah 105:1). Likewise, the presence of a leavening agent such as sourdough in the utensil quickens the kavush process. See Shulchan Aruch Chapter 451, Passage 22, with Mishnah Berurah note 126.
62 Shulchan Aruch, Chapter 451, Passage 21.
63 Mishnah Berurah, Passage 21, note 118, based on Shulchan Aruch, Yoreh Deah Chapter 135.
64 See Mishnah Berurah 451:118 citing Shach. See also Mishnah Berurah 451:156.
65 Mishnah Berurah, Chapter 451, note 49 based on Shulchan Aruch, Yoreh Deah Chapter 121, Passage 1.
66 Chelkas Binyamin, Yoreh Deah 121:9.
67 Other than libun, for the heat of libun will penetrate the layer of residue. See discussion regarding libun kal, above, from Rema 451:4-5.
68 Mishnah Berurah 451:156.
69 Libun kal, though, remains effective to purge the non-kosher residue from cracks. It is not effective to cleanse bottlenecks.
70 See Yoreh Deah 121:7l and Rema, Yoreh Deah 89:4, and 94:7.
71 Tevu’os Shor 10:16.
72 Chut Shani, Pesachim, p. 133.
73 This approach is mentioned in Sefer Davar Charif 8:10; and in Hilchos Basar Bechalav, Rabbi Zev Hofstetter, Chapter 9 fn 63, citing R’ Moshe Shternbuch.
74 For example, R’ Yaakov Kamenetsky, cited in Emes L’Yaakov Al Daled Chelkei Shulchan Aruch, Yoreh Deah 89:4.
75 Many Poskim, for example R’ Shlomo Zalman Aurebach, cited in Halichos Shlomo, Pesach, Chapter 3 Devar Halachah 14, disagree. See Hilchos Basar Bechalav, R’ Zev Hofstetter, Chapter 9 fn. 61, for discussion of the topic, as well for wider discussion of potential concerns that may prevent ne’itzah from being effective.