I have learned from Wikipedia that the smelting of ore is not merely heating up an ore rock until the metal melts out of it. There also needs to be a reducing agent such as coke or charcoal to extract the oxygen from the mineral and leave the base metal behind. Providentially, carbon-heavy agents like that are also really good fuel and can get the smelting furnace up to the required temperatures at the same time. Ores are generally not very pure, though, so even after smelting one will have a mixture of different metals at the bottom of the crucible. Unless they have alloyed together, which only happens in certain combinations and certain ratios, one has to separate the dross from the majority metal.
The Lord tells Israel that they are no longer silver, but the dross, the useless mixture of tin, bronze, lead, and other compounds that have to be removed. (For the curious, the analogy is to silver and not gold because most of the gold found is already metallic and not bound up in a different chemical compound. No smelting needed.) This refining process, wherein the Lord pours judgement out on His people to purify them and extract out the righteous remnant, has been used several times in Israel’s history.
However, the land is not cleansed yet. The false prophets’ visions encourage iniquity, violence, and greed. The priests do not do their duty to teach the people how the Lord wants them to live because they do not follow His ways either. The aristocracy pursue profit through the killing of their fellow countrymen while the prophets wink and nod, excusing their behavior with false visions. Injustice is rampant among those who have no power.
Again the image of a man to stand in the breach of the wall against God’s wrath is invoked, but this time the Lord can not find anyone to fill the gap. Therefore His indignation will be brought against the people who have done all these things.
All Your deeds are purposeful; You have saved a righteous remnant for Yourself; the same fires that burn up the dross also purifies the silver.