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.
Jeremiah 11:4
2 “Hear the terms of the covenant I made with Israel and pass them on to the people of Judah and the citizens of Jerusalem. 3 Tell them that the Lord, the God of Israel, says, ‘Anyone who does not keep the terms of the covenant will be under a curse. 4 Those are the terms that I charged your ancestors to keep when I brought them out of Egypt, that place that was like an iron-smelting furnace. I said at that time, “Obey me and carry out the terms of the covenant exactly as I commanded you. If you do, you will be my people and I will be your God. 5 Then I will keep the promise I swore on oath to your ancestors to give them a land flowing with milk and honey.” That is the very land that you still live in today.’” And I responded, “Amen. Let it be so, Lord.” 6 The Lord said to me, “Announce all the following words in the towns of Judah and in the streets of Jerusalem: ‘Listen to the terms of my covenant with you and carry them out!
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