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

(Referencias: Apocalypse Explained 887)

773. 'They' means in general the member of the Church or everything that belonged to that Church. This is clear from the fact that 'they' refers to the characters who have just been mentioned by name, Noah, Shem, Ham, and Japheth. Although there are four of them, they nevertheless together constitute a single whole. For Noah, who is used in general to mean the Ancient Church, includes, as the parent or seed, the Churches that sprang from it. Hence 'they' means the Ancient Church. All of the Churches which are called Shem, Ham, and Japheth, together constitute the Church called the Ancient Church.