He will sit as a refiner and purifier of silver; he will purify the Levites and refine them like gold and silver. Then the LORD will have men who will bring offerings in righteousness, (Malachi 3:3 NIV)
In ancient days, a metalsmith would follow a process to refine precious metals. He would put the metal in a crucible and put it in the furnace to heat it until it became liquid. All the while he sat near the fire keeping an eye on his precious metal testing the temperature of the fire to make sure it was just right. Eventually, when the fire got hot enough, the dross (impurities) would appear on the surface of the metal. He would remove the crucible from the fire and gently scrape the dross from the top and then put the metal back in the furnace. He continued this process multiple times until the metal was completely refined. How did he know the metal was ready? He could see his reflection in the surface.
It’s the same with us. As we go through our ‘fires’ God is sitting next to us making sure the fire doesn’t get too hot. He is also watching for our impurities to bubble to the top. When he sees them surface, he gently removes them. This process continues throughout our lives. When will the process end? When God looks at us and can clearly see his reflection.