Albert Barnes Commentary Jeremiah 6:27

"I have made thee a trier [and] a fortress among my people; that thou mayest know and try their way." — Jeremiah 6:27 (ASV)

Render it:

I have set you among My people as a prover of ore,
And you shall know and test their way.
They are all of them rebels of rebels (i.e., utter rebels):
Slander-walkers, were copper and iron,
Corrupters all of them.
The bellows glow: from their fire lead only!
In vain has the smelter smelted,
And the wicked are not separated.
Refuse-silver have men called them:
For Yahweh has refused them.

The intermixture throughout of moral words and metallurgical terms is remarkable.