A common analogy is dross being removed from gold in a furnace.

In Catholic doctrine, purgatory refers to the final cleansing of those who died in the State of Grace, and leaves in them only "the holiness necessary to enter the joy of heaven"; it is entirely different from the punishment of the damned and is not related to the forgiveness of sins for salvation. A forgiven person can be freed from their "unhealthy attachment to creatures"