A bloomery consists of a pit or chimney with heat-resistant walls made of earth, clay, or stone. Near the bottom, one or more clay pipes enter through the side walls. These pipes, called tuyères, allow air to enter the furnace, either by natural draft, 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 refining process. The ore is broken into small pieces and 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, slag from previous blooms can 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 become unforgeable. Limestone can also be added to the charge (about 10% of the ore weight) as a flux, to remove impurities.
The small particles of iron produced in this way fall to the bottom of the furnace and become welded together to form a 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.
Iron appears to have been smelted in the west as early as 3000 BC, but bronze smiths, not being familiar with iron, did not put it to use until much later. In the west, iron began to be used around 1200 BC, presumably as a replacement for bronze, which was becoming harder to come by due to shortages in copper and tin.
The onset of the Iron Age in most parts of the world coincides with the first widespread use of the bloomery. While earlier examples of iron are found, their high nickel content indicates that this is meteoric iron. Other early samples of iron may have been produced by accidental introduction of iron ore in bronze smelting operations.
China has long been considered the exception: by 5th century BC, metalworkers in the southern state of Wu had invented the blast furnace, and the means to both cast iron and to decarburize the carbon-rich pig iron produced in a blast furnace to a low-carbon, wrought iron-like material. It was thought that the Chinese skipped the bloomery process completely, starting with the blast furnace and the finery forge to get wrought iron. Recent evidence, however, shows that bloomeries were used earlier in China, migrating in from the west as early as 800 BC, before being supplanted by the locally developed blast furnace.
Early bloomeries were relatively small, smelting less than 1 kg of iron with each firing. Medieval Europe saw the construction of progressively larger bloomeries, leveling off at around 15 kg on average, though exceptions did exist. The use of waterwheels to power the bellows allowed the bloomery to become larger and hotter; European average bloom sizes quickly rose to 300 kg, where they leveled off through the demise of the bloomery. Water powered bellows and larger bloomeries also increased the heat to the point where the iron could melt; this was not considered desirable because it allowed carbon to diffuse into the molten iron, producing unworkable pig iron. Molten iron was not desirable until the advent of the blast furnace.
In England and Wales, despite the arrival of the blast furnace in the Weald in about 1491, bloomery forges (probably using water-power for a hammer as well as the bellows) were operating in the west Midland region beyond 1580. In Furness and Cumberland, they operated into the early 17th century and the last one in England (near Garstang) did not close until about 1770. It survived in Spain and southern France as Catalan Forges to the mid 19th century, in Austria as the stuckofen to 1775. In Adirondacks, New York, new bloomeries using the hot blast technique were built in the 19th century.