"Behold, I have refined thee, but not with silver; I have chosen thee in the furnace of affliction." Isaiah 48:10
When silver is being refined, the jeweler takes a small amount and puts it in a crucible. Then he puts it over the fire and heats it up. But he doesn't leave it. He stands there watching it the whole time, because it has to get hot enough to burn out the impurities but not too hot, or it will burn the silver. The way the jeweler knows that the silver is pure, is when he can see his reflection in the silver, then he can turn the fire down. But this verse says that the Lord didn't refine Israel as silver, but in a furnace of fire. To heat a furnace, you have to shut the door...
We want to be refined as silver; held over the heat, but with the Lord standing there until He sees Himself reflected in us. We don't want to have to be refined in the furnace, because the door will be shut and He won't be standing right there with us.