Improvements in Iron Processing and the Development of the Blast Furnace

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Improvements in Iron Processing and the Development of the Blast Furnace

Overview

The development of more efficient means for producing and working with iron, and the production of high-grade iron, is one the key advances in human history. Because iron is harder than bronze—the metal most commonly used before iron—it is better able to hold sharp edges, making it a superb metal for weapons and tools. Armies bearing iron weapons held a significant advantage over forces equipped with bronze weapons. This superiority gave a name to an epoch—the Iron Age—which began in approximately 1000 b.c.

Equally important, but not so keenly appreciated as stronger weapons and tools, were the technological advances required, in turn, to produce better iron. Because pure iron does not occur naturally on earth, the skills required for its extraction from raw ore included advances in chemistry, heat and temperature control, toolmaking, and mining. While forms of iron had been in use for centuries, it was the development of the blast furnace that enabled both the production of higher quality iron and the use of that iron for various implements and purposes. Particularly crucial was the fact that blast furnaces not only purified iron more completely, they also produced molten iron, which could be poured into molds, resulting in cast tools, implements, and weapons—a dramatic leap over previous methods of hammering iron into shape.

Background

Iron is, after aluminum, the second most common metal in the earth's crust, and the fourth most common of the elements (after oxygen, silicon, and aluminum.) Unfortunately for early extractors of iron, the metal does not exist independently—natural iron is always found in combination with other materials such as carbon. A purer iron, as much as 90% pure, is found in meteorites, and it is meteoritic iron that proved most useful to early civilizations. Obviously, though, meteoritic iron would always be in short supply. The use of meteoritic iron has been traced back to 3000 b.c., with extractions of iron from mined ore occurring in Persia around 2000 b.c.

While early smelting processes (the use of heat to extract metals from ores) proved useful for some metals, the processes were not so effective for the extraction of iron. Iron was too tightly bound to the other materials in the ore. The iron tended to separate into a mass called a bloom. The bloom was then removed, heated separately, and while still hot was hammered into desired shapes; the force of the hammer blows served not only to shape the iron, but also to strengthen it. Because the iron had to be worked by hand implements, the result was called "wrought" iron. Wrought iron proved superior to bronze, although too brittle and impure to serve well in most uses. The process was also inefficient, tending to produce blooms of only a few kilograms. An advance was needed in smelting techniques.

The advance arrived on several fronts. Part of the difficulty in producing a purer form of iron involved the high temperatures required to separate iron from other substances. The melting point of iron is 2,800°F (1,538°C), a temperature difficult to achieve in ancient furnaces, which were often little more than bowls carved out of hillsides. Bellows were used to pump air into some bowl furnaces, increasing the heat of the charcoal fires, but the fires still burned too cool. Even early stone-built shaft furnaces—essentially chimneys that funneled air more forcefully over the fire—failed to achieve temperatures high enough to remove all of the impurities, or slag, from the iron ore. Bellows were common in Roman shaft furnaces.

While actual blast furnaces—in which high-pressure air is forced into to the shaft, vastly increasing the temperature of the fire—did not appear in Europe until the fifteenth century, blast furnaces are recorded in China as early as 300 b.c. Though the iron that emerged from these blast furnaces remained brittle as a result of impurities, the iron was nonetheless molten: it could be poured into molds shaped to produce the desired object when the iron cooled. Blast furnaces also produced larger quantities of iron more quickly than traditional bloomeries.

By a.d. 1100 major blast furnace operations had been established in the Chinese prefecture (state) of Chizou, with smaller blast furnaces in operation throughout China. Many, if not most, of the Chinese furnaces were built into hillsides, simplifying the challenge of creating a tall stack for the furnace. The largest of the Chinese furnaces of that period provided living quarters for more than 1,000 workers, whose tasks were divided among mining and working the furnaces. Smelting sites were constructed farther and farther from the mines themselves, so as to be closer to the forests whose dwindling supplies of wood were used to create charcoal.

Deforestation was one consequence of the large quantities of charcoal required for blast furnace operation. The depletion of readily available wood may have spurred the Chinese to use coal (coke) as early as a.d. 400, although widespread use of coal did not occur until roughly 1100. Poetry of that time celebrates the use of coal to produce molten iron, the quality of the weapons cast from the iron, and, tellingly, the relief felt by the forests as charcoal gave way to coal for fueling the furnaces.

In addition to weapons, the Chinese cast coins, tools, and implements, some of which (or accounts of which) made their way to the West. Whether or not Chinese blast furnace technologies were themselves copied by Western ironworkers is not known, but the first European blast furnaces made their appearance in the mid-1300s, and reached England about a half century later. Europe did not shift from charcoal to coal/coke firing until the seventeenth century, however, and not completely for a century after that.

Progress toward the modern European blast furnace came in the form of several innovations to the shaft furnace. Larger and larger blooms were able to be produced, some weighing over 220.5 lbs (100 kg). High bloomery furnaces raised the shaft to more than 9.8 ft (3 m) and produced even larger blooms.

By 1205 at least one blast furnace was in operation in Germany. Constructed primarily of loam, the furnace, and other similar ones built over the next two centuries, had more in common with bloomeries than with subsequent blast furnaces. Among the important innovations of the early German furnaces, though, was the use of water-powered bellows to drive greater volumes of air at higher pressure.

By the mid-fourteenth century European blast furnaces were beginning to take shape. Taller shafts and mechanically driven bellows forced larger amounts of air into the furnace with a resultant increase in furnace temperature. Larger volumes of charcoal were also permitted by the larger furnace size, further raising the temperature. The higher temperature produced higher carbon molten iron.

The liquefied iron was guided into runoff channels constructed at right angles to the furnace. Because the arrangement of the channels around the furnace resembled piglets suckling at a sow, the product came to be known as pig iron. The fact that the iron was cast in molds gave it another name—cast iron.

Cast iron proved a valuable military innovation, as cannon barrels cast of iron could be larger and more durable. For other applications, cast iron was reheated, reducing the amount of carbon it contained; the heated iron was then worked with hand tools, combining the product of the blast furnace with traditional methods of working wrought iron. Since the iron was more pure, however, the products were less brittle.

While blast furnace technology was still relatively new to Europe in 1450, its importance could not be denied, nor the demand for its products stopped. The true age of iron was begun and, with its beginning the iron foundation of the Industrial Revolution, still three centuries in the future, was laid.

Impact

Iron changed the world. Better iron, the product of blast furnaces, changed the world more dramatically. Iron—and steel—would become the most important manufacturing materials of the modern world. Improvements in iron meant improvements in weapons, often to devastating effect. Iron plows offered an agricultural advance equaled only by the earliest cultivation of crops. Airtight iron vessels would permit experiments with pressure and steam that led to the steam engine. Demand for iron products led to a voracious demand for charcoal, which would result in the deforestation of much of Europe and England, forcing the development of coal mining, which in turn led to deeper and deeper mines. Those deep mines would, in the centuries ahead, require powerful engines to remove ground water. And those engines—steam engines—were themselves made possible only by advances in metalworking, advances whose origins may lie in bronze, but whose greatest accomplishments flowed from molten iron.

KEITH FERRELL