For the source text click/tap here: Avodah Zarah 75
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Utensils that are made by non-Jews and purchased by Jews must be dipped in a kosher mikveh prior to their use.
The source for this halakha is the passage in Sefer Bamidbar (31:23) that describes how after the war with the Midianites, all metal vessels that were taken as booty in the war needed to be washed be-mei nidda – in a kosher mikveh containing 40 se’a of water. This is not a requirement connected to the laws of kashrut; the Torah requires this even if the utensil had been made kosher by means of heat in fire beforehand.
We explore the halachic ramifications of toveling keilim.