Deuteronomy 4:20
18 anything that crawls on the ground, or any fish in the deep waters under the earth. 19 When you look up to the sky and see the sun, moon, and stars—the whole heavenly creation—you must not be seduced to worship and serve them, for the Lord your God has assigned them to all the people of the world. 20 You, however, the Lord has selected and brought from Egypt, that iron-smelting furnace, to be his special people as you are today. 21 But the Lord became angry with me because of you and vowed that I would never cross the Jordan nor enter the good land that he is about to give you. 22 So I must die here in this land; I will not cross the Jordan. But you are going over and will possess that good land.
1 Kings 8:51
49 then listen from your heavenly dwelling place to their prayers for help and vindicate them. 50 Forgive all the rebellious acts of your sinful people and cause their captors to have mercy on them. 51 After all, they are your people and your special possession whom you brought out of Egypt, from the middle of the iron-smelting furnace. 52 “May you be attentive to your servant’s and your people Israel’s requests for help and may you respond to all their prayers to you. 53 After all, you picked them out of all the nations of the earth to be your special possession, just as you, O Sovereign Lord, announced through your servant Moses when you brought our ancestors out of Egypt.”
Notes and References
"... The question here is what this phrase means in Deuteronomy 4:20. Is it a furnace made out of iron? Alternatively: Does the phrase rather refer to an iron-smelting furnace? The next question is, whether a reference to an iron-smelting device carry any meaning at all in the historical context into which Deuteronomy and Deuteronomy 4 in particular apparently speak. The final question to be addressed is: What particular message does this metaphor convey? This peculiar expression also occurs in two other places in the Hebrew Bible, 1 Kings 8:51 and Jeremiah 11:4. The phraseology is identical in these three verses, it appears in hiphil with Yahweh as subject followed by our phrase ..."
Vang, Carsten Israel in the Iron-Smelting Furnace? Towards a New Understanding of the כּוּר הַבַּרזֶל in Deut 4:20 (pp. 25-34) HIPHIL Novum, Vol. 1, Issue 1, 2014