Halakhah sobre Números 31:21

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).