Jeremiah 6:28,29

They are all grievous revolters Obstinate and refractory; walking with slanders Making it their business to detract from thee and the other prophets. Blaney translates the clause, They are all of them the dross of revolters, passing with a fraudulent currency; an interpretation for which he assigns plausible reasons. They are brass and iron They have basely degenerated. It appears, upon trial, that they have nothing in them of the purity of silver or gold, but their impudence resembles brass, and their obstinacy iron. They are all corrupters Hebrew, משׁחיתים, corrupting, or adulterating; or, as Blaney renders it, instruments of adulteration, alluding to brass and iron, or any base metals, being used to adulterate the pure silver. The bellows are burned, &c. All methods to purify and amend them are ineffectual. All the expressions to the end of the chapter are metaphorical. The lead is consumed of the fire Before the use of quicksilver was known, the refiners used lead to separate the silver from the other substances mixed with it. Thus Pliny, Nat. Hist., lib. 31. sec. 31, “Excoqui (argentum) non potest, nisi cum plumbo nigro, aut cum vena plumbi.” The founder melteth in vain Or, as Houbigant reads it, heapeth up fire in vain. For the wicked are not plucked away Or rather, The dross of iniquity is not purged away. The word רעים, meaning here, it seems, the base ingredients among the metals; that is, the bad principles and habits, which prevailed so much, and adhered so closely, that all endeavours and pains used to purge them away and get clear of them proved ineffectual; so that, as it follows in the next verse, nothing remained but to throw them aside, as metal disallowed, and cried down by authority; counterfeiting silver, but not capable of being brought to the sterling standard. See Blaney. In other words: As base money is refused by every one, because it cannot bear the touchstone; so should these hypocrites and evil-doers be rejected both by God and man.