What is the meaning of the iron and brass in Daniel 4:23?

Lou Newton, former Steel Mill Crane Designer and Physics Teacher

Daniel 4 NIV

“23 “Your Majesty saw a holy one, a messenger, coming down from heaven and saying, ‘Cut down the tree and destroy it, but leave the stump, bound with iron and bronze, in the grass of the field, while its roots remain in the ground. Let him be drenched with the dew of heaven; let him live with the wild animals, until seven times pass by for him.’ “

Iron was the strongest metal they knew of but could rust. Bronze would not rust and would remain. The tree was Nebuchadnezzar and he would be cut down but not killed, or the stump would remain. But this stump was bound with iron and bronze which one is very strong and the other will not rust. He would not be able to regain his power for seven years.

Answer requested by Chris .................