The dross, mentioned in Isaiah 1:22, signifies falsity.

(Referenties: Apocalypse Explained 887)

In Revelation 21:20, 'an unclean thing' signifies spiritual whoring, which is the adulteration of good, and falsification of truth of the Word. This is uncleanness and impurity itself, because the Word is cleanness and purity itself, and it is defiled by evils and falsities when it is perverted.

(Referenties: Apocalypse Revealed 924)