Albert Barnes Commentary Jeremiah 15:12

"Can one break iron, even iron from the north, and brass?" — Jeremiah 15:12 (ASV)

The steel - “brass,” that is, bronze. By the “iron” is meant Jeremiah’s intercession; but this cannot alter the divine purpose to send Judah into exile, which is firm as steel and brass. For “brass” see (Exodus 25:3) note. The alloy of copper and zinc now called brass was entirely unknown to the ancients.