Bloomery
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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 tuyères allow air to enter the furnace, either by natural draft, or forced with a bellows. In operation, the bloomery is preheated by burning charcoal, and once hot, iron ore and additional charcoal are introduced through the top.
Inside the furnace, carbon monoxide from the incomplete combustion of the charcoal reduces the iron oxides in the ore to metallic iron. 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 called a bloom. The bottom of the furnace also fills with slag, a molten mixture of impurities in 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 forged in this way is wrought iron.
Historically, bloomery use coincides with the onset of the iron age in most parts of the world. Beginning around 1100 BC in the near east and southern Greece, the iron age spread outward, reaching Egypt and Mesopotamia by 900 BC, central Europe and India by 800 BC, the Eurasian steppes by 700 BC, and sub-Saharan Africa by 400 BC. China presents somewhat of a special case: by 500 BC, metalworkers in the southern state of Wu had invented the blast furnace, and the means to decarburize the carbon-rich cast iron produced in a blast furnace to a low-carbon, wrought iron-like material. Archaeologists and historians continue to debate whether bloomery-based iron smelting was ever widely used in China.
Most bloomeries were relatively small, smelting less than 100 kg of iron with each firing. Medieval Europe saw the construction of progressively larger bloomeries. The Catalan forge, used in Visigothic Spain, was a bellows-powered bloomery capable of smelting up to 150 kg (350 lb) in each heat. In the eight and ninth centuries under the Frankish empire, large bloomeries were also built in the Rhine valley.
The use of waterwheels to power the bellows allowed the bloomery to become larger and hotter; some of the largest of this period were 5 meters (16 ft) tall and could smelt up to 350 kg (750 lb) at a time. By the early 14th century, the largest bloomeries ran hot enough to produce molten metal; that is, they had become blast furnaces. However, generally the objective with a bloomery was not to melt the iron. Cast iron resulting from melting the charge is a brittle material that is unsuitable for many uses, such as most tools, without further processing in a finery forge. Nevertheless, the ability to operate on a larger scale meant that the use of bloomeries declined.
Water power was applied to the Catalan forge in the 15th century and the process continued in use, essentially unchanged in the Pyrenees as late as 1840.
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.
Footnotes
External links
- [Rockbridge bloomery]
- [WIRG experimental bloomery]
- [Precursors of the blast furnace]
- [Roger Smith's article on bloomery construction]
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