What is the difference between "qitr" (molten copper/brass) and "nuḥās" (copper) mentioned in the Quranic verses: "And We caused a fount of [molten] copper to flow for him" and "Bring me, that I may pour over it molten copper" and "There will be sent upon you a flame of fire and [molten] copper [to scald you]"?

The exegetes differed regarding the "qitr" (molten material) mentioned in the story of Dhu al-Qarnayn: was it copper, or iron, or brass, or lead? And it was said that Dhu al-Qarnayn melted it and poured it so that it would mix and adhere to itself, until it became a solid mountain.

Similarly, they differed regarding the "qitr" mentioned in the story of Solomon: was it copper or iron and copper? And it was said that Allah melted copper for him without fire.