Note the strong wording: The Lord has to cleanse the filth of the daughters of Zion and purge the blood on the hands of his people. The people are “washed” as with the fuller’s “soap” and purged by “burning” as with the refiner’s “fire.” Purification comes by the spirit of burning or, as we sing, “The Spirit of God like a Fire Is Burning”; the Spirit cleanses and burns away evil waste.