Sifrei Bamidbar 158

(Bamidbar 31:22) "Only the gold and the silver": You say (finished) vessels. But perhaps (the verse is speaking of) golmim (unfinished, undifferentiated metal forms)? It follows (otherwise), viz.: The dead of Israel cause uncleanliness, and the slain of Midian cause uncleanliness. Just as with the dead of Israel (finished) vessels contract uncleanliness, and not golmim, so, with the slain of Midian. R. Yossi Haglili says; (The verse speaks of finished) vessels. You say vessels, but perhaps golmim? It is, therefore, written "Only," to distinguish (between the two.)

(Bamidbar 31:23) "Every thing that comes into the fire shall you pass through the fire": Knives, spits, and grills are whitened in fire. Pots, stew-pots, and kettles and boilers are to be boiled against absorption of idolatrous elements. "And every thing that does not come into the fire": such as spears, cups and flasks, are rinsed and immersed. And whence is (the need for) immersion (derived)? It follows a fortiori, viz.: If what does not require sprinkling (with the waters of the red heifer) requires immersion, how much more so that which requires sprinkling!

(Bamidbar 31:24) "And you shall wash your garments on the seventh day and you shall be clean": What is the intent of this? Since we are speaking of one slain by the sword, Scripture comes to teach us that the sword and one who touches it is tamei for seven days. Whence do we derive (the same for transfer of tumah from) vessels (i.e., appurtenances) to man to vessels (appurtenances)? From "And you shall wash your garments." "and then you shall come to the camp." Just as here, (without washing of garments and sprinkling) he is forbidden to enter the camp (of the Shechinah), so, there (vis-à-vis the red heifer, Vayikra 19:19) he is forbidden to enter the camp; and just as there, until the evening, so, here, until the evening.