Fire Brick
From Vintage Story Wiki
Fire bricks are needed items to make a Bloomery
You will need to gather fire clay, then form a 2x2 square in your crafting grid to obtain Fire clay brick (raw).
To fire the brick, place into a Firepit with charcoal or other high temperature fuel that can achieve the desired temperature of 1100°C. Please note that cooking Fire Bricks require a lot of fuel and time to create, because every brick needs to be heated from ambient temperature to the firing temperature.