4. When—that is, After.
washed—(Zec 13:1).
filth—moral (Isa 1:21-25).
daughters of Zion—same as in Isa 3:16.
purged—purified by judgments; destroying the ungodly, correcting and refining the godly.
blood—(Isa 1:15).
spirit—Whatever God does in the universe, He does by His Spirit, "without the hand" of man (Job 34:20; Ps 104:30). Here He is represented using His power as Judge.
burning—(Mt 3:11, 12). The same Holy Ghost, who sanctifies believers by the fire of affliction (Mal 3:2, 3), dooms unbelievers to the fire of perdition (1Co 3:13-15).