Albert Barnes Commentary 1 Kings 7:47

"And Solomon left all the vessels [unweighed], because they were exceeding many: the weight of the brass could not be found out." — 1 Kings 7:47 (ASV)

The bronze used to make the two pillars, Jachin and Boaz, the bronze sea, and the various vessels was taken by David from two cities belonging to Hadadezer, king of Zobah (1 Chronicles 18:8).