Fire bricks are used extensively as insulators and liners in all kinds of wood-fire heaters, small furnaces and even industrial-sized furnaces. Refractory bricks, as fire bricks are sometimes called, are used in blast furnaces because they have low thermal conductivity and insulate the heat from the furnace, providing the double benefit of making the furnace hotter and more efficient, as well as protecting the outside of the furnace from excess heat