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)

(Referencias: Apocalypse Explained 70)

197. With the most ancient people, who were celestial, a serpent meant being completely watchful, and so also meant their sensory perception by means of which they were to show vigilance to avoid being harmed by the evil. This is clear from the Lord's words to His disciples,

Behold, I send you out as sheep in the midst of wolves; so be shrewd as serpents and simple as doves. Matthew 10:16.

And also 'the bronze serpent' which was raised up in the wilderness and which meant the Lord's Sensory Perception. He alone is Celestial Man, and He alone is completely watchful and provides for everybody. Whoever beheld it therefore was delivered.