Bloomery - Process

Process

A bloomery consists of a pit or chimney with heat-resistant walls made of earth, clay, or stone. Near the bottom, one or more pipes (made of clay or metal) enter through the side walls. These pipes, called tuyères, allow air to enter the furnace, either by natural draft, or forced with bellows or a trompe. An opening at the bottom of the bloomery may be used to remove the bloom, or the bloomery can be tipped over and the bloom removed from the top.

The first step taken before the bloomery can be used is the preparation of the charcoal and the iron ore. The charcoal is produced by heating wood to produce the nearly pure carbon fuel needed for the smelting process. The ore is broken into small pieces and usually roasted in a fire to remove any moisture in the ore. Any large impurities in the ore can be crushed and removed. Since slag from previous blooms may have a high iron content, it can also be broken up and recycled into the bloomery with the new ore.

In operation, the bloomery is preheated by burning charcoal, and once hot, iron ore and additional charcoal are introduced through the top, in a roughly one to one ratio. Inside the furnace, carbon monoxide from the incomplete combustion of the charcoal reduces the iron oxides in the ore to metallic iron, without melting the ore; this allows the bloomery to operate at lower temperatures than the melting temperature of the ore. As the desired product of a bloomery is iron which is easily forgeable, nearly pure, and with a low carbon content, the temperature and ratio of charcoal to iron ore must be carefully controlled to keep the iron from absorbing too much carbon and thus becoming unforgeable. Cast iron occurs when the iron melts and absorbs 2% to 4% carbon. Because the bloomery is self-fluxing the addition of limestone is not required to form a slag.

The small particles of iron produced in this way fall to the bottom of the furnace and become welded together to form the spongy mass of the bloom. The bottom of the furnace also fills with molten slag, often consisting of fayalite, a compound of silicon, oxygen and iron mixed with other impurities from the ore. 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, and the resulting nearly pure iron 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 .