Scattering was not the only part of God’s judgment. God would also gather the people of Judah into Jerusalem for judgment.
The prophet compared this gathering to metal placed into a furnace (a very hot oven used to melt metal). A worker gathers pieces of metal and puts them into the fire to melt and refine them.
But this fire would not make the people pure. Normally, a furnace removes waste material and leaves pure metal. In this case, the people were like worthless slag (waste left over after metal is melted). Since only waste would go into the fire, only melted waste would come out.
This judgment would not cleanse or restore the people. It would destroy everything in its path.