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)

198. Verses 2-3 And the woman said to the serpent, Of the fruit from the tree of the garden we are to eat; but of the fruit from the tree which is in the middle of the garden God has said, You shall not eat of it nor touch it, or else you will die. 'The fruit from the tree of the garden' is the good and truth revealed to them from the Most Ancient Church. 'The fruit from the tree which was in the middle of the garden, of which they were not to eat' is the good and truth of faith, which they were forbidden to learn from themselves. 'Not to touch it' means that they were forbidden to think about the good and truth of faith from themselves, that is, from sensory perception and factual knowledge. 'Or else they would die' means that faith, that is, all wisdom and intelligence, would in this way perish.