The small particles of iron produced in this way fall to the bottom of the furnace, where they combine with molten slag, often consisting of fayalite, a compound of silicon, oxygen and iron mixed with other impurities from the ore. The mixed iron and slag cool to form a spongy mass referred to as the bloom. Because the bloom is highly porous, and its open spaces are full of slag, the bloom must later be reheated and beaten with a hammer to drive the molten slag out of it. Iron treated this way is said to be "wrought" (worked), and the resulting iron, with reduced amounts of slag, is called "wrought iron" or bar iron. It is also possible to produce blooms coated in steel by manipulating the charge of and air flow to the bloomery.