Burnt bricks
From Feed The Beast Wiki
Burnt Bricks are used in the Mariculture Mod.
To make Burnt Bricks, place a Vat on the ground, use a bucket to fill it with Lava, and right click on the vat while holding Brick to place them in the Vat. You can put more than one brick in the vat by clicking on the vat again. It will take 10 seconds for the Bricks to become Burnt Bricks, which will lower the level of lava. Using Nether Brick (Item) will use half as much lava. Right clicking on the vat with an empty hand will retrieve whatever Burnt Bricks are in the vat.