God speaks of punishing his people as if they were silver and he was boiling the silver in a very hot fire. The one boiling the metal was working very hard.
"The fire is so hot that it scorches the bellows" or "The punishment is so severe that it is like a fire that even burns the bellows". This very hot fire represents God's judgment.
"they are like silver that is so full of impurities that the lead cannot remove them all". This represents the people of Israel being so full of sin, that they continue to sin even after God punishes them.