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

(Referencias: Apocalypse Explained 887)

5059. When those who have lived in ways that are the opposite of conjugial love - that is, in adulterous ways - have drawn near me, they have introduced pain into my loins, the severity of which has depended on the nature of the adulterous life led by them. That influx also demonstrated that the loins corresponded to conjugial love. Furthermore their hell is situated beneath the area behind the loins, under the buttocks, where they live in filth and excrement. These are also the things that give them delight, for such filth and excrement correspond to the pleasure they gain in the spiritual world from such things. But in the Lord's Divine mercy more will be said about these spirits when the hells are described as a whole and individually.