Rashi on Numbers 31:23/רש"י על במדבר ל״א:כ״ג

1כל דבר אשר יבא באש ANYTHING THAT COMES ON THE FIRE, to boil something in it,

2תעבירו באש YE SHALL PASS THROUGH THE FIRE — according to the manner in which it is used so is the manner in which it is cleansed from what has been absorbed: that vessel the use of which is by means of hot water he shall cleanse with hot water, and that which is used for roasted meat, as for instance, a spit and a grill, he shall make it white hot in the flame (Avodah Zarah 75b).

3אך מי נדה יתחטא NEVERTHELESS IT SHALL BE PURIFIED WITH THE WATERS OF SPRINKLING — According to its plain sense this purification is to cleanse it from the uncleanness caused by a corpse. He said to them: the vessels require גיעול (the ceremony by which whatever has been absorbed will be given up) to cleanse them from the effect of having contained forbidden food, and also purification to cleanse them from the uncleanness caused by a corpse. But our Rabbis derived from here the law that in order to make them fit for use (to cleanse them entirely) from being forbidden, it (Scripture) made immersion in a ritual bath a requirement for metal vessels, and they explained the words מי נדה which are written here as denoting “waters that are proper for a נדה to immerse herself in”. And how much is that? Forty Seahs (Avodah Zarah 75b).

4וכל אשר לא יבא באש AND ALL THAT DOTH NOT COME IN THE FIRE — i.e., everything which is not used on fire, as for instance, cups and flasks which are used with cold articles, and which, therefore, absorb no forbidden food,

5תעבירו במים YE SHALL PASS THROUGH WATER — He immerses them and that is sufficient. This refers only to metal utensils (Avodah Zarah 75b).