Exegetes dispute both the nature of the laws of purification of utensils in Bemidbar 31 and what was unique about the war with Midyan that led to the transmitting of these additional laws. The Hoil Moshe maintains that the commands revolve solely on cleansing from the defilement of dead bodies, and he thus claims that Moshe did the same in other wars as well and that there was nothing unusual here. Others like Shadal suggest that there was a special impurity of idolatry related to the special religious character of the war, as the Midianites had lured the Israelites into worshiping Baal Peor. Most exegetes though, following Rabbinic interpretation, explain that the verses speak of impurity of non-Kosher cooking, and Ramban, adopting this position, explains that there had been a special dispensation which permitted this in previous battles.
The utensils needed to be decontaminated because they came into contact with dead bodies.
Immediate context – The immediate context of Elazar's instructions is purification from contact with dead bodies. Both Moshe's directive in verses 19-20 and the command in verse 24 speak of the seven day purification rite after contact with a corpse, suggesting that Elazar's words in the middle must also refer to the same topic.4
Role of Elazar vs. Moshe – This position must explain why Moshe and Elazar each relayed only part of the law rather than having one of them deliver all of the instructions.5
Ibn Ezra suggests that Moshe directed the nation just in general terms, but then had Elazar, who was the expert on the red heifer procedure,6 explain the details.7
According to the Sifre, Moshe's anger at the nation caused him to forget the law.
Relationship to laws of purity in Bemidbar 19 – The content and language of the command are very similar to that used by the laws of purity in Bemidbar 19, supporting the notion that both are referring to the same topic, purity from contact with a corpse.8 These commentators must explain, though, why the laws of Bemidbar 19 do not mention the passing through fire and water.9 Hoil Moshe asserts that the laws of Bemidbar 19 are incomplete,10 and only by combining the instructions there with those mentioned in this chapter can one can get a full picture of the law.11
Why commanded specifically during the war with Midyan?
Tangential mention – Hoil Moshe maintains that the law had actually been applied after earlier battles, but the Torah did not find it necessary to mention the fact. Only in this story when the text was already discussing Moshe's anger at the nation and his ensuing speech, did it also include his words regarding the laws of purification.
First practical application – This position might alternatively assert, like Ramban below, that in the previous wars there actually was no problem of impurity since all of Israel participated in those wars12 and "communal impurity is permitted". It is questionable, though, whether this applies when there is no time bound obligation involved.13
What type of utensils? The Karaite fragment emphasizes that the verses do not speak specifically of food utensils, and the inclusion of gold and silver amidst the list of metals more likely refers to jewelry than to pots or pans.14
"כׇּל דָּבָר אֲשֶׁר יָבֹא בָאֵשׁ" – According to the Karaite fragment and Hoil Moshe, this expression includes all materials which can withstand fire, rather than being limited only to something which is regularly used with fire (like pots or pans).
"בְּמֵי נִדָּה יִתְחַטָּא" – According to these commentators, this refers to purification by the liquid mixture of the ashes of the red heifer, as is implied by the term's usage in Bemidbar 19.
"תַּעֲבִירוּ בַמָּיִם" – According to the Hoil Moshe, this is an additional directive beyond the sprinkling of "מֵי נִדָּה", but it is not clear whether it refers to immersion in boiling or cold water.15
The objects required purification since they were owned by Gentiles or used for idolatry.
Immediate context – Though Elazar's words are framed by laws dealing with purification from corpses, there are some indications which suggest that his speech might relate to a different topic. Verse 21 opens with both a new speaker (Elazar rather than Moshe) and a new audience ("אַנְשֵׁי הַצָּבָא" rather than "פְּקוּדֵי הֶחָיִל"). In addition, Elazar begins his instructions with an introductory formula, "זֹאת חֻקַּת הַתּוֹרָה אֲשֶׁר צִוָּה ה'", suggesting that this is not a direct continuation of what came beforehand.
Role of Elazar vs. Moshe – This approach could easily propose that there are two speakers because each is meant to relay a different set of instructions. However, according to the Sifre, Moshe's earlier anger at the officers caused him to forget the law.
Relationship to laws of purity in Bemidbar 19 – Bemidbar 19 focuses on the laws of purity from contact with corpses, which Moshe alludes to in his command in verses 19-20. Elazar's directive in verses 21-23, though, has no relationship to that chapter at all and refers to a different purification ritual with its own laws and purpose.20
Relationship to laws of idolatry in Devarim 7 – Devarim 7:25 is difficult for this approach, as it suggests that items used for idolatry are to be destroyed completely ("תִּשְׂרְפוּן בָּאֵשׁ"), not purified and then used.
Idols vs. accessories – This approach might explain that the items mentioned here were not actual idols but rather accessories to idolatry or simply objects owned by idolaters with no explicit religious function.
Both refer to purification by fire – Alternatively, perhaps the phrase "תִּשְׂרְפוּן בָּאֵשׁ" in Devarim 7 is equivalent to the words "תַּעֲבִירוּ בָאֵשׁ" here, and both simply refer to purifying by fire.21
Why commanded specifically during the war with Midyan?
Ad hoc law relating to Midyan – According to Shadal, the law is specific to this war and not meant for future generations. Since the Midianites lured the nation into worship of Baal Peor through these items, they were prohibited from use by the nation until they underwent a process of purification.22
Context of spoils of war – The other commentators might explain that this was not really the first application of the law, but simply the first mention of it in the text. Only in this war was there a focus on the spoils of war, and in that context, the laws regrading purifying these spoils from idolatrous use were also mentioned.23
What type of utensils? According to these commentators,24 the vessels mentioned by Elazar are not limited to cooking utensils. According to the reconstructed text from the Damascus Document, the list refers to metals that were made into actual idols, while the Karaitic fragment and Shadal assert that the gold and silver are likely the women's jewelry.25 Sifre Zuta also includes both purely decorative items and weapons of war.26
"כׇּל דָּבָר אֲשֶׁר יָבֹא בָאֵשׁ" – According to the Karaitic fragment and Shadal, this refers to all items which can withstand fire (and thus is not limited to cooking utensils, but rather includes all metals), while according to the Sifre and Sifre Zuta it refers to vessels used with fire. Other vessels, even metal ones, are to be passed instead through water.27
"בְּמֵי נִדָּה יִתְחַטָּא"
Decontamination from corpse – According to the Damascus Document, Sifre Zuta, and Shadal, this refers to the additional purification from contact with corpses via the ashes of the red heifer.28 Elazar is warning the people that they should not think that the new purification makes the other unnecessary; rather both are needed.29
Purification from Heathens – Alternatively, this position could suggest that this is another part of the process of purification from idolatry (and unconnected to corpses). Later prophets refer to the idolatrous nation as contaminating the land "כְּטֻמְאַת הַנִּדָּה" and assert that their purification will come by throwing upon them "pure water".30 This is perhaps not simply a metaphor for purification, but a description of the actual process.
"תַּעֲבִירוּ בַמָּיִם" – According to Sifre and Sifre Zuta, this directive is part of the process of purification from idolatry, though it is unclear whether it refers to immersion in cold or boiling water. The other commentators might agree,31 but could also suggest that the phrase is parallel to the earlier, "בְּמֵי נִדָּה יִתְחַטָּא", and refers to purification from contact with corpses.32 If so, Elazar was introducing a law of purification from heathen contact that applied to metals only. Thus, he clarified that the materials spoken of by Moshe, in contrast, merely needed to be decontaminated from contact with death via sprinkling with the water/ashes of the red heifer.33
The vessels needed to be purged of any residue from non-kosher foods.
Immediate context – Although the laws relating to purification from a corpse sandwich Elazar's words, this approach asserts that he is nonetheless speaking about a different issue, the laws of purging non-kosher taste from the walls of vessels. Like above, the fact that there is a new speaker and audience might support the idea that there is a change of topic as well.
Role of Elazar vs. Moshe – As above, the switch in speaker might be explained by the fact that the two are telling the nation different sets of laws. Sifre and Rashi assert that though Moshe could have taught both procedures, his anger at the way the war was conducted led him to forget the law.36
Relationship to laws of purity in Bemidbar 19 – The laws of purging residual non-kosher taste are totally distinct from the laws of purity. They thus have their own set procedure which does not overlap with that of Bemidbar 19.37
Why commanded specifically during the war with Midyan? Ramban suggests that this was the first practical application of the law. The previous battles against Sichon and Og were part of the wars of conquest,38 and as such had certain unique dispensations. Since the conquered land became part of the inheritance of Israel, all the spoils of war (even the non-kosher vessels) were permitted to them, without need for any purification.39
What type of utensils? According to these commentators,40 the verses refer only to cooking utensils. This, though, is not the simple sense of the verses which mention just the materials used to make the vessels and not their function.41
"כׇּל דָּבָר אֲשֶׁר יָבֹא בָאֵשׁ" – Most of these commentators42 assert that this refers to the way that the vessel was used. If the non-kosher taste was absorbed via fire, it needs to be purged through fire.43
"בְּמֵי נִדָּה יִתְחַטָּא" – The meaning of this phrase is a major point of dispute amongst these commentators:
Purification from contact with a corpse – Sifre, Sifre Zuta, Rashi,44 and Abarbanel45 all explain that the phrase refers to the water of the ashes of the red heifer used for purification from contact with a coprse. Elazar is telling the nation, that the kashering process alone is not enough to permit the vessels for use; they also need to be purified from contact with the dead. This preserves the connotation of the phrase "מֵי נִדָּה" in its earlier appearances in Bemidbar 19.
Immersion in a ritual bath – Bavli, Targum Yerushalmi (Yonatan), R. Yosef Bekhor Shor, and Ramban46 claim instead that this phrase is speaking of water in which a woman who is a "נִדָּה" (in a state of ritual impurity) immerses herself.47 Elazar is telling the nation that in addition to purging vessels of non-kosher taste, vessels made of metal also need to be immersed in a ritual bath before use.48 This is the source for the Rabbinic law of טבילת כלים.
Purging of non-kosher residue – R. Yosef Bekhor Shor, Ramban, and Abarbanel all maintain that this refers to the method of kashering substances that "do not go through fire". While R. Yosef Bekhor Shor and Abarbanel assert that it includes all purging done by water – either through boiling49 or by cold water,50 Ramban maintains that it only refers to cleansing in cold water.51
Immersion in ritual bath – Targum Yerushalmi (Yonatan) and Rashi, in contrast, suggest that this phrase is equivalent to the clause "בְּמֵי נִדָּה יִתְחַטָּא" found in the first half of the verse, and refers not to cleansing items from non-kosher taste,52 but to immersing them in a ritual bath.53