It's the stuff left over from smelting, or purifying ore. Most metallurgic processes involve heating ore to certain temperatures, wherein the oxides and silica that metals are bonded to in the natural world are caused to separate...leaving that which is desired apart from that which is not wanted or needed. Small wonder that the prophecies and visions relating to the purification of the People of God and the Churches would make use of that metaphor. In the same way that Jesus' teachings make use of horticulture images, and that the psalms ascribed to David made use pastoral ones, the image of the refiner's fire on base ore would not be wasted on those listening.
But in a relatively post-industrial world where most of us are far removed from the organic processes that form our human world, do these images still work? They do to a certain extent when we are willing to do the research, or visit a farm, a foundry or a mill. Those images are available to us, but they don't always work when we stand at a distance.
At least today, I can confess that I do "get" the impact of the images that relate to the trials of Israel and the Churches to the refiner's fires. I grew up in communities closely linked with ore processing, ceramic manufacture and mining. It's hard to convey the sense of awe you get when passing a steel mill at night, when the flames, sparks and heat/light are pouring out of structures open to the sky for ventilation. It's an intense challenge to be likened to the slag drawn off after the furnaces cool. My friends and I used to play on the slag heaps, and the feel of that dark, glass-like waste underfoot is still with me.
What comes from these processes? The metals we appreciate as "fine." Where do they come from, the hard work of those who labor to take what is base into refinement.
In a world that has been buffered from what it physically and spiritually costs to form things that we appreciate and use, we are closer to those the prophets are targeting than from those who can sit on the sidelines and click their tongues. The gifts to us today is the hope that continues to express itself over and against these dark images of the crucibles God has placed us in....that there will be a time when the base is drawn away, and that we will stand before God refined and revealed.