Jeremiah Learns That YHWH Has Established Him As An Assayer Of His People, As Well As Their Fortress, Although All That He Will Discover Will Be That They Are A Mixture Of Base Metal And Dross (Jeremiah 6:27-30).
YHWH declares that He has made Jeremiah both a metal assayer and a fortress to His people, in order that he may test their ways so as to discover what they are made of. And what he will discover when he does this is that they are grievous rebels (‘revolters of the revolters', rebels above all rebels) who continually indulge in slanders and deal corruptly. Rather than being silver and gold they are discovered to be merely bronze and iron, and even then all attempts at refinement could only fail, because all are wicked and it is thus not possible to separate the wicked out from among the good. Men will therefore call them ‘refuse silver', ore which has so little silver in it that it is not worth bothering about (we might say ‘fool's gold'), because they will see that YHWH has rejected them.
“A tester I have made you,
A fortress among my people,
That you may know,
And try their way.”
Jeremiah has been appointed for two purposes. On the one hand he is to test out the metal of the people, and on the other he is to be a fortress for believers. For he is called on to know their ways and to test them out. Thus God is making provision for all His supposed people. Through Jeremiah He will uphold the righteous, and through him He will sift out the wicked.
Alternately some would repoint mibtser (fortress) as mebatser (tester of metals) to produce, ‘An assayer I made you among my people, a tester of metal --.'
‘They are all grievous rebels,
Going about with slanders,
They are bronze and iron,
They all of them deal corruptly.'
And what he will discover about the vast majority is that they are grievous rebels (‘rebels of the rebels'), and that rather than being silver and gold they are bronze and iron. They are of inferior quality, something evidenced by the fact that they go about destroying other people's reputations falsely (compare Leviticus 19:16), and are unreliable in their dealings.
‘The bellows blow fiercely,
The lead is consumed by the fire,
In vain do they go on refining,
For the wicked are not plucked away.'
Indeed they are so all so evil that there is no way of refining them. The fiercely blowing bellows will heat up the furnace to such an extent that the lead being used for refining is burned up (during refining lead is placed in a crucible with the silver ore and heated, and when the lead becomes oxidized it serves as a flux to collect impurities), but even such heat will not be sufficient to refine His people because when the attempt is made the wicked are not removed, simply because all are wicked.
‘Men will call them refuse (reject) silver,
Because YHWH has rejected them.'
Thus because they have been tested and rejected by YHWH men will call them ‘refuse silver', poor quality silver ore which is thrown away because it is unrefinable. In other words it will be seen that there is no good in them.