From Technic Wiki
The Casting Basin is a way to cast Molten Ores into solid Blocks of Ore, which can be placed or broken into 9 Ingots. It can also be used to make Brownstone.
Place the Casting Basin under a Seared Faucet. Use the faucet to pour Molten Ore into the Basin, or over a block of Gravel to make Brownstone. The Molten Ore will only form a Block of Ore once there is enough in the basin.