Eleazar the priest said to the troops who had taken part in the fighting, “This is the ritual law that YHVH has enjoined upon Moses: Gold and silver, copper, iron, tin and lead — any article that can withstand fire — these you shall pass through fire and they shall be pure, except that they must be purified with water of lustration [mei niddah]; and anything that cannot withstand fire you must pass through water.
–Numbers/Bamidbar 31:21-23
translation, The Torah: A Women’s Commentary
Although the passage here deals with purifying spoils of war before use, this passage is used to explain the koshering of any metal utensil acquired from a non-Jew and the purifying all newly acquired metal utensils. See, e.g., Avodah Zarah [Foreign Worship] 75b.
“Ask Moses” offers a straightforward explanation. Additional details about preparing a kosher kitchen are available, e.g., through My Jewish Learning.
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