Jamieson, Fausset and Brown on Jeremiah 15:12

12. steel--rather, brass or copper, which mixed with "iron" (by the Chalybes near the Euxine Pontus, far north of Palestine), formed the hardest metal, like our steel. Can the Jews, hardy like common iron though they be, break the still hardier Chaldees of the north (Jr 1:14), who resemble the Chalybian iron hardened with copper? Certainly not [Calvin]. Henderson translates. "Can one break iron, (even) the northern iron, and brass," on the ground that English Version makes ordinary iron not so hard as brass. But it is not brass, but a particular mixture of iron and brass, which is represented as harder than common iron, which was probably then of inferior texture, owing to ignorance of modern modes of preparation.