Numbers 31:23

Anything placed on a fire. In which to cook something.

You must pass through fire. It must be purged the way it was used. Something used with heated water, one purges with heated water; something used for roasting, like a spit or a grill, one brings to white heat with fire.

However, it must be purified with sprinkling water. According to the plain interpretation, this purification is to cleanse it from the contamination of the dead. He said to them, ‘The utensils require purging to purify them from prohibited food, and purification, to cleanse them from ritual contamination.’ And the Rabbis extrapolate here, that even for purification from prohibited [food contamination], metal utensils require immersion, and מֵי נִדָּה written here they expound as referring to water suitable for the immersion of the “niddah.” How much is this? Forty seah.

But anything that was not placed on a fire. Anything not used with fire, such as cups [and jugs], and flasks, which are used for cold food, and, therefore, did not absorb prohibited [food].

You may pass through water. Immerse them, and it is sufficient. This is only with metal utensils.

every thing whose nature it is to abide the fire, of the pans, pots, spits, and gridirons, you shall make to pass through fire to purify them, and afterward (sprinkle them) with water such as is used to purify the unclean; but whatever will not abide the fire coverlids, cups flagons, and utensils, you shall make to pass through forty sata of water;

EVERY THING THAT MAY COME INTO THE FIRE YE SHALL PASS THROUGH THE FIRE AND IT SHALL BE CLEAN. A vessel which touched a human corpse or dead animal does not become purified by fire, since the immersion mentioned in the Torah is always in water. Therefore our Rabbis had to explain that this purification [mentioned here by Scripture] refers to remove from them [the traces of] the forbidden foods which they had absorbed whilst they were in the possession of the non-Jews. This is the true [explanation] without any doubt.

אך במי נדה, a reference to the waters containing the ash of the red heifer that would be used to cleanse them from their ritual impurity. It would also be used to purify other items, such as remnants of forbidden foods absorbed by the walls of the captured pots and pans.

וטהר, it will have become decontaminated from the negative vibes attached to it as residue of idolaters.

אך במי נדה יתחטא, even making it red hot does not automatically remove its impurity, even if it looks as if new. It requires the waters and ash of the red heifer, i.e. מי נדה

כל דבר אשר יבא באש, “any vessel that can withstand fire, etc.;” it seems strange that the Israelites had not yet been commanded concerning the need to ritually cleanse vessels that were part of a war’s booty already after they had conquered the land of Sichon and Og, lands which they had taken physical possession of, and which was considered as part of the Holy Land, its soil having been promised to the descendants of Avraham in chapter 15 of Genesis? Perhaps the reason is that those campaigns were fought in open fields, and the soldiers at the time of killing their adversaries did not need to enter the homes of their victims and come into physical contact with all their victims’ personal belongings. Now they had to be taught that such belongings of idolaters had to be treated as out of bounds until their idolatrous character had been removed from them.