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)

(Reference: Apocalypse Explained 70)

421. Verse 22 And Zillah also, she gave birth to Tubal-cain, teacher of every craftsman in bronze and iron. And Tubal-cain's sister was Naamah. 'Zillah', as has been stated, means the mother of the natural features of the new Church. 'Tubal-cain, a teacher of every craftsman in bronze and iron' means the doctrine of natural good and truth; 'bronze' means natural good, 'iron' natural truth. 'Tubal-cain's sister was Naamah' means another Church or doctrine of natural good and truth, which though similar was not an integral part of that Church.