Halakhah su Numeri 31:23

In Parshat Matot (Bemidbar 31:21-23) the Torah presents the basic rules of kashering: “Every object that has gone through fire, you shall pass through fire and it will become pure.” This verse teaches that every non-kosher utensil that was used directly with fire must be kashered with fire. Rashi (ad loc., based on Pesachim 30b and Avodah Zarah 75b) explains that the phrase “has gone through fire” alludes to one of two methods for kashering utensils, depending on how the utensil cooked the non-kosher food. If the non-kosher food was cooked directly on the utensil (such as on a grill), absent a liquid medium, then one kashers the utensil by heating it in a fire (libun). If, however, the non-kosher food was boiled in a pot containing hot water, then the utensil may be kashered via boiling hot water (hag’alah). The Torah also teaches that if only cold non-kosher food was placed in a utensil, one merely needs to clean the dish before using it with kosher food (Bemidbar 31:23).¹Of course, any metal or glass utensil acquired from a non-Jew must also be immersed in a mikvah (see Rashi, Bemidbar 31:23).

Nonetheless, some authorities do permit kashering metal dishwashers, but it is not entirely clear how to do so. The Torah articulates the basic guidelines of kashering in Bemidbar (31:23), “That which became not kosher through contact with fire must be kashered with fire, and that which became not kosher in a water medium, must be kashered in a water medium.” This verse establishes the principle that an item must be kashered in the same manner as its use. Dishwashers come in contact with food particles through hot water, so they should be kashered with hot water (hag’alah). Rav Moshe writes that when kashering a dishwasher, a hot brick must be placed inside the dishwasher to boost its water’s temperature to the boiling point,¹¹Dr. Joel Berman notes that, scientifically speaking, it is difficult to imagine how this brick would raise the water to the boiling point. The brick would not access all of the water, and the brick’s outside temperature would rapidly fall as the water cooled it down. based on the practice of using boiling water whenever water is required (see Taz, Yoreh Deah 94:3 and Mishnah Berurah 452:8). Hence, even though the water’s temperature never climbs higher than 190 F in dishwashers,¹²In a lecture at Yeshiva University in 1989, Rav Moshe Tendler reported that at that time the water did not get hotter than 190tF. kashering them still requires boiling water.

R. Moshe Feinstein, Iggerot Mosheh, Yoreh De'ah, II, no. 164, expresses doubt with regard to whether an airplane is subject to defilement despite the fact that it is made of metal. Iggerot Mosheh suggests that "perhaps" only the six metals specified in Numbers 31:23, viz., gold, silver, copper, iron, tin and lead, have the halakhic status of metal. Those materials, together with mercury which is actually a liquid, constitute the seven metals of antiquity. Arsenic was discovered in the thirteenth century by Albertus Magnus and a number of other metals, including zinc, were discovered only subsequently. However, most other metals were unknown until relatively modern times. Iggerot Mosheh suggests that only the enumerated metals are susceptible to defilement and that it is for that reason that they are named individually in Numbers 31:23 rather than collectively identified as metals. Iggerot Mosheh points out that, biblically, glass is not susceptible to defilement despite the fact that it is comparable to metal in the sense that it is melted, rather than destroyed, by fire. Accordingly, Iggerot Mosheh suggests that since airplanes are composed primarily of metals discovered during subsequent periods of history, i.e., aluminum and titanium, a plane cannot become defiled and hence the airplane itself may serve as an interposition preventing defilement from entering the passenger section. Although the material used for that purpose in the construction of airplanes is a mixture of one of the metals of antiquity and subsequently discovered metals, the halakhic identity of the entire mixture, asserts Iggerot Mosheh, is that of its major component. The halakhic status of modern metals will be more fully discussed in a subsequent section.