Brass and iron as in Isaiah 48:4 and Daniel 7:19 signify what is hard.

In Genesis 4:22, brass signifies natural good. (Arcana Coelestia 421)

In Numbers 21:9, this signifies the Lord's divine natural. (Apocalypse Revealed 49)

Brass also signifies the good in man's natural and the sensuous in the Lord. (Arcana Coelestia 197, Apocalypse Revealed 49, Apocalypse Explained 705)

(References: Apocalypse Explained 70)