Copper Bars are made from 2 Copper Ores. They make Copper Tools, Bullets, Clocks, and Copper Walls. That is generally all they do, but for the first week or so of the game, they are required to make casing for Bullets or Iron Bullets. Iron is needed to make the casings for Gold, and Gold is needed for Diamond. A Clock is advised to make. Copper Tools are that much sturdier than Stone, and Copper Walls aren't really that much more useful than any other block. Copper Bars can also be acquired from a Dragon's loot.