Bloomery
A
bloomery is a type of
furnace once widely used for smelting
iron from its
oxides. The bloomery was the earliest form of smelter capable of smelting iron. A bloomery's product is a porous mass of iron and
slag called a
bloom. This mix of slag and iron in the bloom is called
sponge iron, which is usually further
forged into
wrought iron.
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, or forced with a
bellows. 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 bottom.
The first step taken before the bloomery can be used is the preperation 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 recylced 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. Since the desired product of a bloomery is easily forgeable, nearly pure iron, with a low
carbon content, the temperature and ratio of charcoal to iron ore must be carefully controlled to keep the iron from abosrbing the carbon and becoming unforgeable.
Limestone could also be added to the bloomery, about 10% of the ore weight, which would act as
flux and help carry away 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 excpetion: by
500 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; Eurpoean average bloom sizes quickly rose to 300 kg, where the 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, however, as that 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 waterpower 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.
[H. R. Schubert, History of the British Iron and Steel Industry (1957). R. F. Tylecote, History of Metallurgy (1991). ]In
Adirondacks in
New York State, new bloomeries using
hot blast were built in the
19th century.
[Gordon C. Pollard, 'Experimentation in 19th century bloomery production: evidence from the Adirondacks of New York' Historical Metallurgy 32(1) (1998), 33-40. ]*
Technology and archaeology of the earliest iron smelting and smithing*
Rockbridge bloomery*
WIRG experimental bloomery*
Precursors of the blast furnace*
Roger Smith's article on bloomery construction*
Late Medieval Iron Making, the Bloomery Process*
How Stuff Works*
The Smelter's Art Experimental Iron Production at The Rockbridge Bloomery*
Early use of iron in China
