The Mitzva of Immersing Utensils
מצוות טבילת כלים
A Jew who bought or received from a gentile a food utensil made of metal or glass is commanded to immerse it in a mikveh to purify it, similar to a convert who immerses at the time of his conversion. Even if the utensil was produced in a factory belonging to a Jew, if a gentile bought it, the utensil became his, and a Jew who buys the utensil from him must immerse it. If the gentile used the utensil for forbidden foods, it must first be kashered in accordance with the manner of its use, in order to remove from it the forbidden flavors that were absorbed in it or stuck to it, and afterward it must be immersed for the sake of purity.
We learned these two mitzvot – kashering utensils and immersing them – from what is stated in the Torah regarding the spoils that the Israelites took from the Midianites (Bamidbar 31:21-23): "This is the statute of the Torah which God commanded Moshe: Only the gold and the silver, the copper, the iron, the tin, and the lead – everything that may go through fire, you shall pass through fire and it shall be clean; nevertheless it shall be purified with the water of nidda; and all that may not go through fire you shall pass through water." Passing through fire or boiling water is meant for utensils in which forbidden foods were cooked or roasted; in the manner in which they were used for forbidden food, so must they be kashered (below 32:1). In addition, it is stated: "and it shall be clean; nevertheless it shall be purified with the water of nidda," to teach us that one must also purify the utensil by immersing it in a mikveh, in which a nidda immerses for her purification (Avoda Zara 75b; Ramban). There is no connection between the two mitzvot; rather, if the gentile used the utensil for forbidden foods, it must be both kashered and immersed, and if he did not use it for forbidden foods, it need only be immersed.
The aim of the mitzva of immersion is to elevate the utensil from the level of a utensil meant for ordinary eating, which is accompanied by desire and human weaknesses, to the level of a utensil meant for preparing foods through which Jews can connect to the values of the Torah. This mitzva is one of a system of mitzvot meant to distinguish the eating of Jews as eating that contains faith, holiness, and blessing. Therefore, Jews were commanded to recite blessings before and after eating; not to eat forbidden foods, which express other aims; to separate from their foods gifts for the kohanim, the Levites, and the poor; and to bring some of them up to Jerusalem and eat them there in holiness. The mitzva of immersing utensils is meant to purify the utensil from the impurity that clings to ordinary eating, which is meant to sustain a person during his days in this world, so that through his eating a Jew can connect more to eternal values and the vision of tikkun olam (repairing the world).