Iron is a common element in the earth's crust, but it almost always occurs as a compound: it is very rarely to find it naturally as metallic iron. Occasionally an iron meteorite will fall from space, to provide a "magical" source of the metal. Such rare occurrences allowed a few smiths to discover the valuable properties of iron, because meteoritic iron was used to make swords and daggers (including Tutankhamen's). Aztec chiefs had iron knives made from meteoritic iron, again prized more highly than gold.
It is much more difficult to smelt iron than copper. In copper smelting, the copper flows as a liquid to the bottom of the furnace. The slag of waste material accumulates on top of the copper, and can be poured off to leave behind the copper. The copper is easily identified as copper, and the process is intuitive. But the physical chemistry of smelting iron from oxide or sulfide ores differs from that of copper, which may well have made iron smelting difficult to recognize even when it had happened. In principle, iron can be smelted from magnetite or hematite, which are comparatively common ores, but iron does not melt at the temperatures that are reached in a primitive furnace: iron is still solid when copper and bronze are molten. Let us suppose that some iron ore, say FeCO3, siderite or "ironstone", is loaded into a furnace along with malachite, and fired with charcoal. (Siderite is a grey-green mineral whan it is fresh, though it weathers to a brown rusty iron oxide mineral at the surface. It often occurs with malachite, copper carbonate.) Inside the furnace, the first reaction of the siderite is like that in copper smelting: the carbonate breaks down to form an iron oxide:
Then, as the charcoal burns to form hot carbon monoxide (CO), metallic iron is produced:
The problem is that even when the reaction is complete, the iron that is produced is not molten: it has reacted as a solid. The furnace would produce molten copper, from the malachite, and a liquid slag. But the iron is left unmelted, as a dense spongy mass of metal. There are always impurities that form a slag, but the iron will hold at least some of the slag in the small holes in its spongy texture. If the smelter is allowed to cool after the slag and the copper have been tapped off, then the iron would be left behind as an ugly solid mass that could easily be discarded as worthless.
Probably some curious smelter investigated the lump (hit it with a hammer). An astute or experienced smith might have recognized the metal as iron, but with so much slag frozen into it that it could not be used. The breakthrough would have come when the decision was made to hammer the lump hot, when the slag was still molten. Hammered on an anvil, a smith can force the liquid slag to squirt out, leaving behind fairly pure iron. Obviously, this was a a very difficult, dangerous activity, and took extra time, energy, and fuel. The spongy mass (the bloom) might have to be reheated several times to re-melt the slag until the last of it had gone. However, a smith could eventually produce a block of fairly pure iron, called wrought iron.
The smith's job is easier if the piece is small, because then the slag does not have to be squeezed very far. This is the only way I can envisage a smith even discovering how to produce wrought iron, working a small piece from the smelting furnace. However, there is no easier way to produce wrought iron with the equipment available. Each small piece had to be hand-hammered. The labor-intensive preparation meant that the first uses of iron were confined to fairly small objects, and, more important, wrought iron had to be clearly superior to bronze before it could be produced on a large scale. Smiths could join pieces of wrought iron together by heating and hammering, but it was not always easy to achieve uniform quality along the piece, and re-shaping also required even more investment of time, energy, and fuel.
Even when smiths produced wrought iron, it was not always better than bronze. Ancient smiths had to learn the metallurgy of iron and its alloys, which again is non-intuitive, much more difficult than that of copper and bronze. They solved the problems empirically, of course, but the chemistry and metallurgy involved were not understood at all until modern times. Most important, the smithing of iron demanded much more luck or skill than the well-understood smithing of copper and bronze. Some smiths made tools and armor and weapons that were significantly better than others. The mastery of iron and steel truly was a rather magical process, and it is at this point that legends begin to feature magic swords, produced by smiths who are also wizards.
Bronze Age heroes did not have magic weapons. The heroes of Troy had to use tricks for victory, even when the gods were interfering. (I imagine that the gods would have provided their favorite mortals with magic weapons if they had been available: certainly the Iron Age Norse gods did.) However, Iron Age soldiers had swords that were forged one at a time, and Iron Age gods and heroes often had magic swords such as Fafnir and Excalibur. Legendary Iron Age smiths such as Wayland Smith traditionally possessed great strength for hand-forging weapons, as well as great skill. Ironically, Wayland was lame, but only because he was maimed deliberately by the King: is this a faint echo of the Bronze Age?
Like copper, iron can be alloyed with other metals such as nickel and titanium to yield different properties, and the value of such alloys was appreciated even by smiths who could not see a chemical analysis of the alloy. In particular, natural iron from meteorites is nickel-rich, and the smiths dealing with it were able to make good tools and weapons because of the high quality of nickel-iron alloy. But the most important iron alloy is unexpected, was invisible to smiths, and could not have been made deliberately when it was first produced.
Unlike any other common metal, iron can be made to alloy with carbon at high temperatures, to form steel. In addition, the properties of iron-carbon alloys vary widely and non-intuitively, and had to be learned empirically by bitter experience by ancient smiths. The reactions were not understood at all until the 19th century, because carbon was not deliberately added or subtracted in the way that tin was added to copper to form bronze. The carbon came from the fuel used in working the iron (the charcoal of the forge) and could be burned and/or hammered into or out of the alloy invisibly.
Newly smelted wrought iron always has small amounts of impurities from the ore. Even with the best modern smelting methods, "iron" contains perhaps 2-3% of phosphorus, copper, nickel, and manganese, depending what ore and/or scrap was fed into the smelter. "Iron" thus has slightly variable properties, even today, and the impurity load and variability would have been considerably greater in ancient "iron." However, we can assume that ancient wrought iron is at least 90% iron, but is also practically carbon-free. It is rather soft, and can be worked by hot hammering. In modern times, smiths choose wrought iron for pieces that are to be ornamental, or pieces that are easily modified (gates, horseshoes) and pieces that require weight but won't easily chip (sledgehammers).
Iron that has a tiny amount of carbon in it (about 0.1%, only 1 part in 1000) has properties that are significantly different from ordinary iron. Most important, it is harder, so will take a better edge. As a smith hammers a piece of wrought iron, to shape it, it must be maintained at a relatively high temperature so that it is soft. In charcoal-fired forges, working with relatively high-quality smelted iron ingots, carbon and carbon monoxide are in contact with iron at relatively high temperatures. In these conditions, with a smith hammering away at the wrought iron, the immediate surface of the iron combines with a small percentage of the carbon (the iron is carburized) to form a new iron-carbon alloy, steel. If the smith hammers away at it until there is 1% of carbon in the iron, he makes a mild steel that is stronger than the best bronze.
If the smith is shaping a thick object from a slab of wrought iron (say a hammer head) he will not need much forging time, so any steel he produces will make only a thin hard jacket over the object. But suppose that a smith is making a sword. He is hammering pieces of wrought iron to form a long, thin shape. This will need a lot of hammering, and re-heating, and more hammering. The forging makes a skin of steel over the entire sword. But as the smith forms the edges of the sword, the entire thickness of the edge is "steeled." Suppose the smith is making a sword for an officer: he is likely to forge it for longer, to make a more perfect shape and a sharper edge. The extra forging would also make a better steel than a sword that was finished more crudely. Chemically, the smith does not know what he has done, but in practical terms, he can recognize that longer forging makes a better sword. And an experienced officer would certainly know a superior sword when he wielded one!
What's more, longer forging makes a sword that is not only sharper, but harder, less likely to bend in a battle. (When the Romans invaded Britain, they found that the Britons would sometimes have to step back out of the battle line to straighten their swords!) Smiths could expect that if they forged swords for a long time, they would produce even better weapons, though at extra cost in time, energy, and fuel. At first this was true: an alloy containing between 0.2% and 1% carbon is the hardest of the steels. Wayland Smith's most famous sword Fafnir was made by forging it three times, breaking it into pieces between each stage.
So far, this is intuitive: whatever it takes to add carbon to iron, more of it adds hardness. But now the trouble begins. If iron is alloyed with more than 1% carbon, properties change yet again. High-carbon iron is harder still than steel, but it also becomes brittle. One can imagine ancient smiths finding this out with some relief: there came a point where more forging was counter-productive. Even so, one hears more about swords breaking in battle than one does about them bending: clearly, at some point in sword-making, smiths erred toward strength and hardness at the cost of making swords too brittle (and I shall return to this problem later). As a result, the forging process in the Western World did not lead to the discovery of iron alloys with much higher carbon.
At more than 2% carbon, iron alloys change in a completely unexpected way: they melt at lower temperatures than pure iron. So the first iron alloy to be used for pouring out into molds was a high-carbon alloy. It became known as cast iron or pig iron (from the traditional shape of the molds). Cast iron is very strong, but brittle. This traditional terminology is confusing ("cast iron" has more carbon than "steel"), and the products are not now (and never were) manufactured in sequence of increasing or decreasing carbon. I shall return to cast iron later in the chapter.
We do not know where ironworking began, nor do we know where smiths first learned about steel. However, both tradition and fragmentary archaeological evidence suggest that ironworking developed in or near the long-time metallurgical heartland of the Bronze Age, the plateau of Anatolia. Ancient historians and surviving contemporary documents associate iron-working with the Hittites, who flourished in Anatolia from about 1450-1200 BC. The name of the Biblical/mythological smith Tubal Cain is based on the name of a northern Anatolian tribe. The Hittites never used iron extensively, but they produced limited quantities of iron daggers and swords before their empire collapsed around 1200 BC.
However, iron-working is much older than the Hittites. Several objects of smelted iron are older than 2000 BC, all of them in a context that suggests they were treated as ornamental objects of great value. One in particular stands out, because the regional setting accords with the traditional origin of iron working. It is a dagger with an smelted iron blade and a bronze handle, found in a Hattic royal tomb dated about 2500 BC, at Alaca Höyük in northern Anatolia. The Hattic people preceded the Hittites, and were already working bronze in a sophisticated way at this time.
Although I described a possible discovery of iron smelting from siderite (iron carbonate), it should be clear that any iron oxide in a copper smelter could have worked just as well, because the smelting process begins with the oxide step. It turns out that there are geological reasons why iron-working may have developed in Anatolia, and why the Hittites came to be associated with it. Black sands rich in the iron oxide mineral magnetite are found all along the south coast of the Black Sea, the north shore of present-day Turkey. If Theophrastus (writing around 300 BC) is correct, the sands were used deliberately for producing iron in ancient times. He describes beach sands being washed to concentrate the iron-bearing grains, after which they were smelted in stone furnaces. He says the iron is rust-resistant (does that mean it is steel?). His account was written many centuries after the first introduction of iron into Anatolia, but it may reflect a much older tradition.
I have already described how a flux can make smelting easier, by lowering the temperature at which a slag melts. Iron oxides are among the fluxes useful in copper smelting, and others help the slag to drain from iron ores in a smelter. By a quirk of fate, some of the iron ores along the Black Sea coast have enough natural fluxing material with them that they are self-fluxing (the smelter does not need to deliberately add anything) and molten iron can be produced at temperatures around 900° C, well within the reach of copper smelters. Perhaps the first smelted iron was discovered as some of these iron ores were used as fluxes in copper extraction. If enough iron melted in the process (as would happen with these particular ores at these temperatures) the slag would contain enough reasonably pure iron to be hammered with curiosity by the coppersmiths. Maybe the properties of wrought iron were investigated at this time, and maybe much of early iron metallurgy was mastered here. Because of this coincidence of the chemistry of local iron ores, occurring in a region dense in skilled smelters and smiths, much higher-grade iron may have been available to Hattic smiths than to other late Bronze Age people, possibly encouraging them to experiment with iron more than smiths of other regions. If the Hatti or the Hittites were the first people to smelt iron ores deliberately to form iron, rather than using the iron that turned up in copper smelting, then they were truly the first Iron Age smiths.
In this period iron was made into ornaments, signifying its rarity. Apart from Tutankhamen's dagger, we have a battle-axe from Ugarit, in Syria, dated about 1450-1350 BC. It has a bronze hilt decorated with gold, and an iron blade that contained nickel. The iron may have been alloyed with nickel deliberately, or it may have been meteoritic iron. But the blade had been forged into mild steel, with 0.4% carbon. These items show a great deal of metallurgical sophistication, but at the same time they reflect the great value attributed to well-worked iron objects. About 1700 BC the King of Carchemish sent a royal gift to the city-state of Mari, an iron bracelet. Homer, composing much later a historical story about Bronze Age traditions, says that iron was awarded as a prize in athletic competitions, along with gold and women!
Hittite sites don't have a great quantity of iron objects, and on the face of it it looks as if there was no significant iron and steel industry in Anatolia. A letter from the Hittite King Hassitulis III, written about 1250 BC, makes excuses for not delivering a shipment of iron to Shalmeneser I of Assyria, and presents him with an iron dagger blade. This has sometimes been taken as evidence that the Hittites had an extensive iron production, but if a single dagger blade could appease the King of Assyria, it is unlikely that any quantities of iron were available at all, no matter what the payment or threat from Assyria. The Hittites gave an iron throne to the earlier Middle Bronze Age monarch, King Anitta of Kanesh, and a tub, small figurines, and ornamental jewelry are also mentioned as being made of iron, again underlining the rarity and therefore the high value of the metal. Hittite texts make the distinction between meteoritic iron (referred to as black iron of heaven) and terrestrial (smelted) iron. If the relatively tiny quantities of meteoritic iron were worth mentioning at all, it again shows the small quantity of smelted iron available. Perhaps the Hittites had secrets of iron or steel technology, but if so, they were obviously related to quality rather than quantity of output.
Iron was prized very highly even toward the end of the 2nd millenium BC. The Old Assyrian letters that discuss the tin trade between Assur and Kültepe also refer to two metals called amutum and assi'um. Amutum is generally identified as iron. It could be bought, though its price was 40 times that of silver, and 400 times more expensive than tin. Head office in Assur continually urged the branches in Kültepe to look out for reasonably-priced amutum.
The material the Assyrians called asi'um is even more interesting. Its export from Kültepe across the Taurus Mountains was forbidden, so the Assyrian merchants did not even discuss its price. A Persian merchant is recorded as being jailed for attempting to smuggle it. It is tempting to identify as'um with steel.
No iron objects have been found outside the area bounded by Anatolia, Mesopotamia, Egypt, and Crete before about 1600 BC. Iron reached mainland Greece about 1500 BC, and about 1350 BC iron objects had found their way to the eastern Danube. An iron-hilted bronze dagger at Ganovce on the eastern Danube, is interesting: the use of iron for the hilt and bronze for the blade of the dagger implies either that the properties of iron were not appreciated locally, or that local smiths had not yet produced iron that was harder than good-quality bronze, or that iron was so rare that it was used ornamentally in display, or that the dagger was a gift that was meant to be ornamental rather than useful.
We have a good insight into iron technology from the Egyptian copper-mining operations at Timna, which were at their peak at the end of the Bronze Age, about 1300-1200 BC. Copper and iron objects from this period were found at a temple dedicated to Hathor, at Timna. The iron objects contained a small amount of copper, and the copper objects contained a small amount of iron, enough to show that all the objects had passed through a smelter. Possibly much of the early supply of iron outside Anatolia was obtained in this way, as a by-product rather than by deliberate smelting of iron ore specifically to obtain iron. There is no specific commitment to an "Iron Age" here, only to the necessarily smale-scale processing of iron that resulted as a by-product of the bronze industry. Not all copper ores require fluxing with iron oxide, and some smelters would not have made the by-product at all.
Therefore, despite early technological successes, iron and steel did not become widespread materials at this time. The technology of the Mediterranean world could not yet reach the 1537° C necessary for melting iron on any large scale. The available techniques for producing wrought iron and then carburizing it into steel required a great deal of hammering and forging. Wrought iron was usually softer than well-manufactured bronze, and rusted quickly. Only the highest-quality wrought or steeled iron surpassed bronze in physical performance, and it was prized accordingly, for jewelry, ornaments, and ornamental weapons.
Iron-working technology was therefore developing during most of the "Bronze Age," but the beginning of the Iron Age should properly be drawn at the time when everyday objects were usually made from iron rather than copper, bronze, stone, or wood. For example, although an iron industry developed in Southeast Europe between 1100 BC and 800 BC, bronze weapons remained dominant, presumably because supplies were plentiful from an established industry, and possibly because iron-smiths could not yet produce weapons as good as bronze. Furthermore, the production of iron still required more labor and more fuel than bronze.
It's clear from the Iliad and the Odyssey that Homer lived in the Iron Age but told his stories about the Bronze Age. This helps to date both the composition of the stories and the events related. Sometime around 1100 BC, smiths discovered that "quenching" a forged steel blade by plunging it quickly into liquid dramatically improved its hardness: Homer describes this process in the Odyssey, but he does not seem to understand it fully. It is not clear whether this is Homer's own lack of understanding, or whether no-one understood just how it worked. What is important, however, is that quenching improves only steel; it has no effect on wrought iron. Homer is describing, therefore, one of the vital steps in producing steel weaponry.
Quench-hardened steel was still a brittle metal, however, and it was even later that smiths discovered that tempering (reheating quenched steel to moderate heat and allowing it to re-cool slowly) cures the brittleness at only a small cost in hardness. This lengthy series of steps could now be used to make swords and other weapons and tools that combined the best of hardness, strength, and flexibility, and from this time on the superiority of iron and steel over bronze was never challenged.
The dates are not yet clear. But the transition began about 1200 BC in the Eastern Mediterranean, and coincided with the collapse of almost all the flourishing Bronze Age civilizations in the region: the Hittites, the Myceneans, the Egyptian New Kingdom, and the kingdoms of Ugarit and Alalakh in north Syria. Iron weapons are at first scarce, then more abundant, but they increased uniformly over the entire region, suggesting that no nation had a monopoly of iron technology. Widespread unrest and large-scale movements of populations took place in what must have been very troubled times, and by the time events settled down, 1000 BC, iron tools and weapons were as abundant as those of bronze, and increased in proportion even more as time went on. Probably warfare spurred technology in the 12th century BC as much as it spurred the technology in the 20th century AD: look at the history of automobiles, aircraft, electronics, and space flight.
Cyprus and Greece may have played an important role in the transition to iron. In Cyprus, the balance tipped very quickly toward iron in only the 150 years of "Late Cypriot III" after 1200 BC. By 1050 BC Cyprus and southern Greece were fully in the Iron Age, and 80% of the working metal of Athens was iron. There is less precise evidence from Crete and from mainland Greece as far north as Macedonia, but they too entered the Iron Age at about this time.
Smaller tools such as knives were the first to be made of iron, but gradually larger iron tools and implements were introduced as smiths learned the techniques of forging larger and larger pieces. The earliest quenched-steel object is a knife from Cyprus, dated about 1100 BC. At about the same time, we see a rapid evolution of larger weapons, especially swords. The first iron swords were copies of existing patterns from Bronze Age Cyprus, but over the next few hundred years the iron sword evolved on mainland Greece into a much improved weapon.
Several scholars have argued that the early transition into the Iron Age in the region from Cyprus to Greece was not begun or encouraged simply by the superiority of iron and steel for making tools and weapons. After all, the relative value of the metals had been known for centuries. Instead, the transition may have been favored by a drastic bronze shortage, forcing people to use iron whether they liked it or not. Other scholars have suggested that the transition was prompted by a drastic fuel shortage which led to iron production as a cheap by-product of smelting copper or lead. As poorer and poorer copper ores were smelted, and as furnaces reached higher temperatures, there would have been more and more iron-rich slag running to waste at smelters, and perhaps more and more "blooms" of iron ore. Iron could therefore be produced with only a small additional investment of fuel.
The complete transition into the Iron Age took place progressively both eastward and westward from Greece. Bronze dominated along the eastern Mediterranean coast until about 950 BC. A steel pick is known from Israel by 1000 BC. The transition was completed slowly in the Middle East, however, maybe because there was still a large bronze supply there.
Mesopotamia seems to have been about 100 years behind the Greeks in the full deployment of the Iron Age, but the change was rapid when it came. In Mesopotamia, Assyrian documents written about 800 BC use phrases such as putting enemies "to the iron dagger," implying that their armies were equipped with iron weapons. By 720 BC, Sargon II of Assyria was using iron lavishly, and nearly 150 tons of unworked iron bars were found in his palace, presumably as some sort of strategic reserve. There is good indirect evidence that the Assyrians did not smelt their iron themselves, but imported it (perhaps from Anatolia) as ready-made bars for their smiths to work into weapons or tools.
Egypt is interesting. From about 900 BC onward, Egyptian smiths produced axes that had been steeled, quenched, and probably tempered as well, showing that technologically they were well up to the innovations of the smiths elsewhere in the eastern Mediterranean. But the Egyptians did not adopt iron into everyday life until after Egypt was conquered by the Assyrians in 663 BC.
In all of these dates, there is clear evidence that the rise and fall of nations did not depend on their possession of bronze or iron weapons. It is also clear that iron-working was militarily important. In the Roman Province of Dacia, a frontier area that is now Rumania, the procurator ferrariarum, the Superintendent of Ironworks, was a high-ranking military officer.
On the northwest frontier, Julius Caesar's raid into southern Britain in 54 BC confirmed the presence of a small-scale iron industry in the Weald, a hilly area south of London. As the Romans tightened their grip on Gaul they invaded Britain in strength in 43 AD and took over the area. They established London as their provincial capital, built roads into and across the Weald, and expanded the iron industry at least ten-fold. It became a major source for military hardware. Apparently the northwestern units of the Roman Army and Navy got all their nails, tools, and miscellaneous building iron from at least 67 iron-making sites in the Weald. (The roof tiles at some of the sites are stamped with CL BR, [Classis Britannica, "Fleet of Britain"] just to remind us that acronyms are not entirely a product of the modern military mind.
The size of the Wealden slag heaps suggests a production of between 100 and 750 tons of iron a year while the Romans were in full control. Production on this scale probably means that the Romans shipped iron from the Weald to units the northwestern provinces of the Roman Empire, including the legions on the German frontier. They had strategic stockpiles. When the Romans abandoned an exposed frontier fort at Inchtuthil, far north in Scotland, they left behind a million iron nails, 7 tons of them. This is a full two thousand years into iron production. Iron was certainly available, and cannot have been too expensive.
Some indirect comparisons are amazing. In the 19th century BC the Assyrian merchants of Kültepe were willing to pay 40 times its weight in silver for amutum or iron. By the 7th century BC in Greece, a silver drachma would buy an iron ingot 2000 times it own weight. In other words, iron had become 80,000 times cheaper in relation to silver in those 1200 years, and the process undoubtedly continued as the Romans developed large-scale production and distribution.
People at the time realized the importance of the transition from bronze to iron. Iron required long and arduous forging, and was not worked and ornamented as easily as bronze. Iron tools and weapons were plain, ugly, utilitarian, and crudely efficient, where bronze was often beautiful if increasingly impractical. The curmudgeonly Greek poet Hesiod looked back to the Bronze Age with nostalgia, and rightly or wrongly regarded it as having been a happier time: "And I wish that I were not part of [this] generation of men, but had died before it came, or been born afterward. For here now is the age of iron." One can understand Hesiod's attitude, in a century where we have increasingly turned from wood, ceramics, and metal to molded plastic for our everyday goods.
Sometime after 1000 BC, knowledge of iron-forging techniques reached China from the West. The Chinese then applied their superior furnace technology to take iron-working to new levels of expertise. They were the first to cast iron into useful objects, because they could routinely melt iron on a large scale. Some Chinese smelter must have reached such a high temperature (around 1150° C) that the iron, instead of remaining as a bloom that could be hammered into wrought iron ("ripe iron" or shu thieh), combined with the carbon and carbon monoxide in the furnace to produce an iron-carbon alloy with more than 2% carbon. No doubt to the astonishment and dismay of the discoverer, this promptly melted into a liquid that solidified to cast iron ("raw iron", or shêng thieh. This could be tapped off into molds (a process that was already completely mastered by bronze-smiths) and a new industry could be built around it. The Chinese iron industry grew quickly. By 512 BC the Chinese were casting all kinds of iron objects, including large cauldrons.
The Chinese invented sophisticated bellows for their iron furnaces before 100 BC, so that a single continuous stream of air entered the furnace, rather than intermittent puffs. The process uses a lot of fuel, but gives higher temperatures. This invention is, for practical purposes, a blast furnace; by 100 AD the Chinese were driving blast furnace bellows with water wheels. This technology was not invented in (or transmitted to) the West until the 15th century.
The superiority of iron did not lie simply in the volume of production, because bronze objects could have been cast just as easily. Large copper mines from Zhou times show that bronze supplies were at least adequate in China: the mine at Tonglushan is very large. The fact is that Chinese iron was stronger than the best contemporary bronze alloys by about 450 BC, and thereafter iron became the dominant metal, even for everyday tools. Bronze was used only for ornament and ceremony after about 100 AD. The Chinese still despised iron, as the Greeks before them had done: iron was "the ugly metal" as opposed to bronze, "the lovely metal," but aesthetic preference apparently did not affect the transition very much.
As I mentioned earlier, cast iron is too brittle for everyday use that involves impact, especially for armor and weapons. It is too high in carbon. About 300400 BC, the Chinese learned that if a cast iron object is reheated to 800° or 900° in air, it is decarburized, that is, it essentially has some of the carbon burned out of the surface layer. This process forms a skin of lower-carbon iron (steel) over the cast iron core. The finished tool is hard and wear-resistant, and for most uses is comparable with the end product of Western forging, in which a skin of steel is formed over a core of wrought iron by forging. But the Chinese technology was far more efficient. The Chinese cast objects already had the precise shape required, whereas Western smiths had to produce the right shape by hammering wrought iron on a forge. The Chinese could effectively mass-produce cast steel-jacketed tools of all kinds, while Western smiths had to make them one at a time. By about 250 BC, one iron works in Szechuan employed 1000 people, and the Chinese were producing much more iron by casting than by forging.
The Chinese still forged steel weapons, where (one assumes) the extra quality was worth the extra labor. But they may have made their finest weapons in a different way. They may have begun by pouring flat sheets of cast iron into molds. By further heating, say under hot ashes for several days, they could have decarburized these cast iron sheets to make sheets of steel directly, which could then be forged into tools and weapons. Some of the famous Chinese "hundred refined" steel swords could have been made this way, although this is conjecture.
Somewhere after 1 AD, the Han dynasty built a series of major blast furnaces in Henan Province, in the central Huang-Ho valley (not far from Anyang). Hundreds of farm tools (adzes and shovels) were made from cast iron, each tool bearing the imprint "Henan # 1." This particular blast furnace, Henan # 1, was already known from written records, but it has now been discovered and partially excavated. Production was on an enormous scale. The clay floor of a single furnace measured 4 m by 3 m (13 feet by 10), and was 3 m thick. The furnace wall was fire-resistant brick, three meters high. The furnace could hold 50 tons of fuel, ore, and flux, and tuyeres 26 cm (nearly a foot) in diameter admitted forced air from the bellows. A furnace like this could produce several tons of iron a day, and was only one of a series. Remnants of production runs show that the Henan cast iron compared in quality to the pig-iron produced in a modern blast furnace.
This industrial complex yielded scores of clay and iron molds for making plowshares, farm tools, and gears and wheel bearings for carts. There were 14 pottery kilns on the site for producing the molds, the tuyeres for the furnaces, and the firebricks for the furnace walls. The molds were arranged in stacks or cascades, so that scores of objects could be cast in one pour.
"Henan # 3" represented an even greater technological breakthrough. In this complex the Chinese did not reheat cast iron to make a steel jacket, but melted cast iron and "puddled" it. The molten iron was stirred to allow air to come into contact with it, turning the whole melt directly into steel, or into wrought iron (shu thieh, or ripe iron). The process was not re-invented in the West until the 18th century.
Probably a little later, the Chinese began to use blast furnaces for making steel in an even more simple way, by melting together wrought iron and cast iron. This means, in effect, that they realized the chemistry of steel was intermediate between that of cast iron and wrought iron. Legend connects this discovery with the making of the first Han Emperor's sword by melting rather than forging, in 230 BC, but even if that is legend, the story was written down in 370 AD, showing that the process was fully understood by then, at least in practical terms. A cryptic saying of about 270 AD speaks of "the harmony of the hard and the soft" in terms of alloying. This process too was re-invented much later in the West as the Siemens-Martin open-hearth process of steel-making.
The Chinese used the new tools to tremendous effect. Steel tools allowed the Han dynasty (206 BC to 220 AD) to develop intensive agriculture and major irrigation and drainage projects, with a concomitant increase in population and wealth. The Han seem to have used their iron and steel technology to dominate the "barbarians" north and south of them: imperial edicts forbade the export of iron tools and weapons. Salt and iron were declared to be state monopolies in 117 BC.
The fuel consumption of the Chinese iron industry must have been enormous. It seems that some iron foundries were deliberately constructed in peripheral areas that would have had (at least for a while) the forest resources that could maintain them. In particular, even by late Zhou times, the northern state of Yan, in present-day Manchuria, was an important iron-producing region, possibly because of its relatively luxuriant forests. Nevertheless, the iron industry stayed healthy in China for many centuries. The Chinese were building cast iron suspension bridges from the 6th century, 1200 years before the Europeans. The iron and steel production of the Sung Dynasty was greater than early industrial Europe. In 1061 AD the Chinese used 53 tons of cast iron in building the 13-story pagoda of the Yü-chhüan Ssu temple at Tang-yang in Hopei.
In India, on the other hand, the forging tradition was dominant. In the 4th century AD, the Gupta king Chandra II erected a gigantic iron pillar to honor Garuda, Vishnu's representative. The pillar still stands in a courtyard outside Delhi. It is 7 m (22 feet) tall, weighs six tons, and is made of iron so pure (99.72%) that it has not rusted in the 1600 years it has been exposed to the weather. This is the largest piece of forged iron that has survived from ancient times, and must have been made by heating and hammering together hundreds of iron ingots.
Page last updated April 1999.
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