Why does it say "Nachar Mafu'ach"?
Rashi: This is a parable. When one refines silver, he blows a bellows and constantly puts lead in the furnace (Radak - lest the fire consume the silver); this helps to refine. When I came to refine you, the bellows was burned 1 from the fire and the lead was finished. Radak - this is because the smelter blew it constantly. This is a parable for the Navi's rebuke.
Rashi: Nachar is like "Charah Api" (Hoshe'a 8:5) and "v'Atzmi Charah Mini Chorev" (Iyov 30:30) (Radak - and "v'Charah Nechushtah" (Yechezkel 24:11).
Why does it say "la'Shav"?
Rashi: The refiner refined for naught. It did not help. Radak - i.e. the Navi's rebuke did not help.
What is the meaning of "v'Ra'im Lo Nitaku"?
Rashi #1: They were not uprooted from their evil way. I.e. the Nevi'im toiled to rebuke, and it did not help.
Rashi #2: The Resha'im were not uprooted.
Radak: The silver did not melt, i.e. the Resha'im did not melt; they are still hard (stubborn).
Malbim: Lead is the most common impurity in silver. Normally, after all the lead is removed, the silver is pure. Here, it was all removed, but still it is impure - it is all impurities!
