Charles Spurgeon Commentary 2 Chronicles 12:9-11

"So Shishak king of Egypt came up against Jerusalem, and took away the treasures of the house of Jehovah, and the treasures of the king`s house: he took all away: he took away also the shields of gold which Solomon had made. And king Rehoboam made in their stead shields of brass, and committed them to the hands of the captains of the guard, that kept the door of the king`s house. And it was so, that, as oft as the king entered into the house of Jehovah, the guard came and bare them, and brought them back into the guard-chamber." — 2 Chronicles 12:9-11 (ASV)

He carried away also the shields of gold which Solomon had made. Instead of which king Rehoboam made shields of brass, and committed them to the hands of the chief of the guard that kept the entrance of the king's house. And when the king entered into the house of the LORD, the guard came and fetched them, and brought them again into the guard chamber.

That was a come-down indeed, from shields of gold to shields of copper; that is, I suppose, what is meant here by the brass. This is what the king suffered at the hands of Shishak; and it was an emblem of the condition of his people. The golden kingdom had become a brazen one.