Deuteronomy 4:20

New English Translation

²⁰You, however, the Lord has selected and brought from Egypt, that iron-smelting furnace,^[a] to be his special people^[b] as you are today.

Deuteronomy 4:20 tn A כּוּר (kur) was not a source of heat but a crucible (“iron-smelting furnace”) in which precious metals were melted down and their impurities burned away (see I. Cornelius, NIDOTTE 2:618-19); cf. NAB “that iron foundry, Egypt.” The term is a metaphor for intense heat. Here it refers to the oppression and suffering Israel endured in Egypt. Since a crucible was used to burn away impurities, it is possible that the metaphor views Egypt as a place of refinement to bring Israel to a place of submission to divine sovereignty.
Deuteronomy 4:20 tn Heb “to be his people of inheritance.” The Lord compares his people to valued property inherited from one’s ancestors and passed on to one’s descendants.

New English Translation (NET)