James Richard Fromm
The Iron Age began around 1200 BC in the ancient Near East, which was about the time of the Trojan war. Iron is not found in nature as the free metal and can only be obtained from the smelting of its oxide ores, which is not an easy process. These oxide ores are wuestite (FeO),
hematite (Fe2O3), and magnetite (Fe3O4), of which the red-brown hematite is the most common. The direct reduction of iron (III) oxide with carbon is sufficiently endothermic that it does not proceed, but reduction with carbon monoxide is possible:
2Fe2O3 + 3C 3CO2(g) + 4Fe, DG0 = +301.3 kJ/mole
Fe2O3 + 3CO(g) 3CO2(g) + 2Fe, DG0 = -29.4 kJ/mole
By about 750 - 800 BC, when Homer actually wrote about the siege of Troy, iron smelting had spread to Europe. One of its major centres was in modern Austria (ancient Noricum); it had spread even to Spain by 400 BC.
During this time the technology of iron and steel were developing. The actual smelting process of iron was similar to that of copper, with higher temperatures and a greater airblast required. One of the early differences was the deliberate slagging of the ore. Material such as limestone, CaCO3, was added as a flux for siliceous (SiO2-containing) gangue. The reaction which takes place is actually with the CaO left after CO2 is driven off:
CaO + SiO2 CaSiO3(l).
In some cases the smelting was done without a flux, in which case much of the iron remained behind in the slag as FeSiO3 rather than being replaced by the calcium:
FeO + SiO2(l) FeSiO3(l)
FeSiO3 + CaO CaSiO3(l) + FeO
Either the straight or fluxed smelting of iron produced a bloom, or tough spongy agglomeration of metallic iron and slag, at the bottom of the furnace. This had to be repeatedly heated and hammered out while hot to force out the slag and cinders and consolidate the globules of iron. This hammering process, which required the development of tongs, hammer, and anvil, produces a tough but somewhat soft material known as wrought iron.
Iron can be converted to the much more useful material steel, which is an alloy of iron and perhaps other minor metals containing a significant small percentage of carbon. In the ancient world, steel was generally made by a process called carburization, or cementation, which is a repeated heating and hammering of iron in contact with charcoal, thus converting at least the casing of the metal to steel. This process was known by about 1200 BC, in Egypt. At Noricum, smelting yielded steel directly because iron carbonate ore such as that found there can produce it; the ore naturally contained some manganese so an alloy steel, and a good one, was obtained. A modern alloy steel, stainless steel, contains 0.4% carbon, 18% chromium, and 1% nickel. Noricum had 3 to 6-foot shaft furnaces with forced draft and some control of the degree of carbonization. The slags there from the ancient works total about 100 Gg (100,000 metric tons) which would correspond to about 30 Gg of iron or steel actually produced.
The steel produced by this process is somewhat variable in hardness, or ability to take and hold a sharp edge. The process of quenching, which is the preservation of the high-temperature crystal structure of steel by suddenly cooling it in water or oil, gives a much harder material, though somewhat more brittle. This process was known, at least in Egypt, by 900 - 700 BC.
Steel can be improved further by the process of tempering, in which the steel is heated for some time at a moderate temperature and then cooled slowly rather than quickly. Some hardness is sacrificed but tempering gives a better toughness. Tempering did not come into use until Roman times; in Egypt, it can be dated to about 200 BC.
By the period of classical history, about 500 BC, history speaks of the Greek city-states such as Athens and Sparta, the Roman Republic, and the Persian Empire of Cyrus. Iron weapons are now in extensive use, generally consisting of a steel casing on a wrought-iron core, produced by the cementation method. Weaponry is not entirely iron; the classical hoplite or heavy infantryman of Greece was armored not in iron but in bronze. He carried a heavy elliptical or circular bronze-faced shield and wore a bronze helmut with nose and cheek-pieces, a bronze breastplate, and bronze greaves or leg-armor. As a secondary weapon he carried a short iron sword no more than 18 inches long much like a modern bayonet; his principal weapon was a nine-foot spear equipped with a nine-inch to twelve-inch iron head. The major military force of the Greek city-states was made up of a phalanx or tight formation of these men, some eight or more ranks deep. Other forces, such as the light infantry or peltasts who wore no armor and carried a light leather shield with a thin bronze face and an iron-tipped javelin or throwing-spear, the cavalry if any, the archers, and the slingers, were posted on the flanks of the phalanx to protect them and were strictly auxiliary to it. Bows were not, for the Greeks or Romans, serious weapons of war because they were not effective against a line of armored and shielded spearmen. Iron armor came in with the Roman Empire, but even then only for legionary soldiers.
In classical history, the period of iron weapons is well known to us. Some of its events were the empire of Alexander the Great, 340 - 320 BC; the rise of the Roman Republic from 300 - 50 BC until overthrown by Julius Caesar, 48 BC; and the birth of Christ, 4 BC. By 50 AD, Roman rule was extensive; under Claudius, the Romans were conquering Britain, while the Christians were being persecuted under Nero. There was an initial collapse of Roman rule in civil war around 200 - 300 AD but the empire was re-established by Diocletian around 300 AD; shortly thereafter, Constantine made Christianity the official religion of the Empire and established Constantinople (now Istanbul, Turkey) as the Imperial capital.
By 375 AD, the final collapse of the Roman Empire had begun in the West, although it lingered much longer in the East. Metallurgical output, or metal production, began to decrease. No new mines were opened, although old mines continued with declining production. There was considerable reworking of metals to weapons, and for the same reason less decline in iron production than in the production of all other metals. By 476 AD, when the last puppet Western Roman Emperor was deposed by the barbarians, European metal production was very low. It continued to be low for the ensuing 500 years, the so-called Dark Ages. During this time few techniques were totally lost to the world although many fell into at least temporary disuse.
Metallurgical output in Europe began to increase again by about 900 AD, and some new techniques were developed; for example, cast bronze church bells started to come into use. Metal production continued to increase, rapidly surpassing that of classical days. By 1200 AD, developments included water-powered bellows for air supply. By 1305 AD, casting of bronze and even iron cannon was being done and water-powered heavy forging hammers of 600 - 1200 kg were in use. But also, around 1350, the Black Death plague swept across Europe; this was also the time of the Hundred Years' War which did not end until about 1453. Metal production declined some 25 - 50% due to the loss of labor resulting from these natural and man-made disasters before recovering.
Modern methods of iron and steel production differ significantly in technology, though not in chemistry, from those mentioned above.
Up to about 1750, all iron production had been based on the use of charcoal, a porous form of carbon obtained by strong heating of wood in the absence of an adequate supply of air. With increasing metal production, deforestation became a significant problem throughout Europe. Attempts were made to use coal, which was already replacing wood for household heating and cooking, as a replacement for charcoal in metal smelting as well. Direct replacement of charcoal with coal in metal smelting does not work well because the impurities (especially sulfur) in the coal are transferred to the metal. Its properties are then vastly inferior to those of iron obtained using charcoal reduction.
When bituminous coal is heated to about 1000oC in the absence of air (pyrolysis), nearly all of the volatile materials distill out of it. The organic materials come out as gases (coal gas) or liquids (coal oil and coal tar); ammonia and some sulfur compounds are also evolved. The remaining product, coke, is a porous and brittle material which, like charcoal, is nearly pure carbon. Coke, unlike coal, can be used effectively for the smelting of metals and has virtually completely replaced charcoal for this purpose.