7 The Iron Age Historians claim that the Iron Age began between 1500 and 1000 B.C.(at least in some parts of the world).This does not mean that iron was unknown to man before that time;quite the contrary is the case.Meteoric iron(which has a large nickel content)must have been used by prehistoric people as early as 4000 B.C.They made tools and weapons from it by shaping and hammering.It is thus quite understandable that in some ancient languages the word for iron meant "metal from the sky".Naturally,the supply of meteoric iron was limited.Thus,stone,copper,and bronze were the materials of choice at least until the second millennium B.C.There were,however,some important uses for iron ores dur- ing the Bronze Age and also during the Chalcolithic period.As explained already in Chapter 1,copper needs a fluxing agent for the smelting process when using malachite.For this,iron oxide was utilized,which was known to react during smelting with the unwanted sand particles that are part of malachite.Eventually, a slag was formed which could be easily separated from the cop- per after the melt had cooled down. It has been frequently debated and asked by scholars in which way early man might have produced iron,utilizing terrestrial sources in particular,since the melting point of iron is 1538C. This temperature was essentially unachievable during that pe- riod,at least in the western (or middle eastern)part of the world. The answer can probably be found by considering the above- mentioned slag,large amounts of which have been found in ar- eas at which major copper smelting operations were conducted. This slag was observed to contain some reduced iron,but in a porous condition,which is today known by the name of sponge iron or bloom.When bloom is repeatedly hammered at high temperatures,the slag can be eventually removed and the iron is compacted.In this way nearly pure iron is obtained.The end product is known among today's metallurgists by the name of wrought iron.Therefore,it can be reasonably assumed that iron
7 Historians claim that the Iron Age began between 1500 and 1000 B.C. (at least in some parts of the world). This does not mean that iron was unknown to man before that time; quite the contrary is the case. Meteoric iron (which has a large nickel content) must have been used by prehistoric people as early as 4000 B.C. They made tools and weapons from it by shaping and hammering. It is thus quite understandable that in some ancient languages the word for iron meant “metal from the sky”. Naturally, the supply of meteoric iron was limited. Thus, stone, copper, and bronze were the materials of choice at least until the second millennium B.C. There were, however, some important uses for iron ores during the Bronze Age and also during the Chalcolithic period. As explained already in Chapter 1, copper needs a fluxing agent for the smelting process when using malachite. For this, iron oxide was utilized, which was known to react during smelting with the unwanted sand particles that are part of malachite. Eventually, a slag was formed which could be easily separated from the copper after the melt had cooled down. It has been frequently debated and asked by scholars in which way early man might have produced iron, utilizing terrestrial sources in particular, since the melting point of iron is 1538°C. This temperature was essentially unachievable during that period, at least in the western (or middle eastern) part of the world. The answer can probably be found by considering the abovementioned slag, large amounts of which have been found in areas at which major copper smelting operations were conducted. This slag was observed to contain some reduced iron, but in a porous condition, which is today known by the name of sponge iron or bloom. When bloom is repeatedly hammered at high temperatures, the slag can be eventually removed and the iron is compacted. In this way nearly pure iron is obtained. The end product is known among today’s metallurgists by the name of wrought iron. Therefore, it can be reasonably assumed that iron The Iron Age
126 7·The Iron Age production took its way from reducing iron ore into spongy bloom,which is a process that needs a lower temperature(about 1000C)than melting pure iron.In other words,the temperature was never high enough to yield a liquid product.Bloom was then eventually hammered into wrought iron.However,pure iron is quite soft;actually,it is softer than bronze,as can be seen from Figure 7.1.Additionally,pure iron corrodes readily when exposed to air of high humidity.As a consequence,pure iron must have been of little interest to early man,at least until a major discovery was made to produce "good iron",as it was named in old records. The discovery of good iron is credited by many archaeo- metallurgists to the Hittites,or probably to subjects of the Hit- tites(called the Chalybes)who lived for some time in the Ana- tolia-Mesopotamia region which is today Turkey.The Hittites conquered large areas of the Mediterranean,such as Assyria, Babylon,and Northern Palestine.They challenged the Egyptians and the Syrians.The system of government of the Hittites is said to have been more advanced than that of many of their neigh- bors,and their legal system emphasized compensation for wrong- doings rather than punishment.The Hittite language (belonging to the indogermanic languages)was recorded in hieroglyphics or in cuneiform(a system of syllabic notations,borrowed from the Mesopotamians),and their international correspondence was written in the Akkadian tongue.Legend has it that their suc- cessful weapons consisted of swords,spears,and arrows made of iron which pierced through the bronze shields of their ene- mies.But their light and fast chariots certainly must have like- Mass %C Fe 0.2 0.4 0.6 0.8 1.0 50 90% Steel80% 400 300 90% 80% 200 Bronze 100 Work-hardened FIGURE 7.1.Hardness of various steels and (%) bronzes as a function of composition and degree of work hardening.The work 0 1 hardening is given in percent reduction Cu 2 4 6 8 10 of area (see also Table 2.1). Mass Sn
production took its way from reducing iron ore into spongy bloom, which is a process that needs a lower temperature (about 1000°C) than melting pure iron. In other words, the temperature was never high enough to yield a liquid product. Bloom was then eventually hammered into wrought iron. However, pure iron is quite soft; actually, it is softer than bronze, as can be seen from Figure 7.1. Additionally, pure iron corrodes readily when exposed to air of high humidity. As a consequence, pure iron must have been of little interest to early man, at least until a major discovery was made to produce “good iron”, as it was named in old records. The discovery of good iron is credited by many archaeometallurgists to the Hittites, or probably to subjects of the Hittites (called the Chalybes) who lived for some time in the Anatolia-Mesopotamia region which is today Turkey. The Hittites conquered large areas of the Mediterranean, such as Assyria, Babylon, and Northern Palestine. They challenged the Egyptians and the Syrians. The system of government of the Hittites is said to have been more advanced than that of many of their neighbors, and their legal system emphasized compensation for wrongdoings rather than punishment. The Hittite language (belonging to the indogermanic languages) was recorded in hieroglyphics or in cuneiform (a system of syllabic notations, borrowed from the Mesopotamians), and their international correspondence was written in the Akkadian tongue. Legend has it that their successful weapons consisted of swords, spears, and arrows made of iron which pierced through the bronze shields of their enemies. But their light and fast chariots certainly must have like- 126 7 • The Iron Age Fe 0.2 0.4 0.6 0.8 1.0 Mass % C Cu 2 4 6 8 10 Mass % Sn 500 400 300 200 100 0 Bronze Work-hardened (%) 80% 80% 90% 90% Steel Hardness (Vickers) FIGURE 7.1. Hardness of various steels and bronzes as a function of composition and degree of work hardening. The work hardening is given in percent reduction of area (see also Table 2.1)
7·The Iron Age 127 wise contributed to their victories.The secret of making good iron is said to have been kept by the Hittites for two hundred years,that is,from about 1400 B.C.to about 1200 B.C. Good iron was produced by applying repeated cycles of heat- ing a piece of bloom in a charcoal furnace near 1200C for soft- ening purposes and by subsequently hammering to remove the slag and to compact it.During the heat treatment,the bloom and eventually the iron was frequently exposed to the carbon monox- ide gas of the burning charcoal.As has been explained in Chap- ter 6,this procedure is supportive of carbon diffusion into the surface of iron.As a result,an iron-carbon alloy is formed(called steel)which is substantially harder than bronze (e.g.,Cu-10% Sn),even if the carbon content is only about 0.5%as seen in Fig- ure 7.1.(Steel is defined to be iron that contains up to 2.11 mass percent carbon.)The steel of antiquity had about 0.3 to 0.6%car- bon.Cold-working adds additional strength(Figure 7.1).Iron Age man must have also observed that limiting the carbonization to only the surface(such as the edge of a blade or the tip of a tool) combined high hardness of the surface with good ductility in the interior.Examples of "selective steeling"whose end product is iron with 1.5%carbon on the surface have been found from time periods as early as 1200 B.C.This process corresponds essentially to modern case hardened iron. The role of carbon on the hardness of iron and steel was,how- ever,not recognized for a long time.Indeed,the Greek philoso- pher and scientist Aristotle(384-322 B.C.),among others,believed (contrary to the truth)that steel was a purer form of iron due to the "purifying effect of charcoal fire."It was not before 1774, when S.Rinman,a Swedish metallurgist,discovered a "graphite- like residue"when cast iron was dissolved in acid.Seven years later,Bergman and Gadolin finally reported the different amounts of carbon in various irons and steels. There were two more discoveries which were probably made during the first millennium B.C.that improved the quality of car- bonized iron even further.One of them (interestingly enough,de- scribed in Homer's Odyssey)involves quenching,that is,a rapid cooling of a red-hot piece of carbonized iron into cold water.This procedure hardens the work piece considerably more,sometimes even to the extent of brittleness.As a result,quenched swords, tools,and other utensils may have cracked or even shattered. The other discovery which was made during the end of the first millennium B.C.entailed a short-time reheating of a previously quenched piece of steel to about 600C.This procedure,which is known today as tempering,restores some ductility and relieves the brittleness at the expense of some loss in hardness.We shall pro- vide the scientific explanations for all these processes in Chapter 8
wise contributed to their victories. The secret of making good iron is said to have been kept by the Hittites for two hundred years, that is, from about 1400 B.C. to about 1200 B.C. Good iron was produced by applying repeated cycles of heating a piece of bloom in a charcoal furnace near 1200°C for softening purposes and by subsequently hammering to remove the slag and to compact it. During the heat treatment, the bloom and eventually the iron was frequently exposed to the carbon monoxide gas of the burning charcoal. As has been explained in Chapter 6, this procedure is supportive of carbon diffusion into the surface of iron. As a result, an iron–carbon alloy is formed (called steel) which is substantially harder than bronze (e.g., Cu–10% Sn), even if the carbon content is only about 0.5% as seen in Figure 7.1. (Steel is defined to be iron that contains up to 2.11 mass percent carbon.) The steel of antiquity had about 0.3 to 0.6% carbon. Cold-working adds additional strength (Figure 7.1). Iron Age man must have also observed that limiting the carbonization to only the surface (such as the edge of a blade or the tip of a tool) combined high hardness of the surface with good ductility in the interior. Examples of “selective steeling” whose end product is iron with 1.5% carbon on the surface have been found from time periods as early as 1200 B.C. This process corresponds essentially to modern case hardened iron. The role of carbon on the hardness of iron and steel was, however, not recognized for a long time. Indeed, the Greek philosopher and scientist Aristotle (384–322 B.C.), among others, believed (contrary to the truth) that steel was a purer form of iron due to the “purifying effect of charcoal fire.” It was not before 1774, when S. Rinman, a Swedish metallurgist, discovered a “graphitelike residue” when cast iron was dissolved in acid. Seven years later, Bergman and Gadolin finally reported the different amounts of carbon in various irons and steels. There were two more discoveries which were probably made during the first millennium B.C. that improved the quality of carbonized iron even further. One of them (interestingly enough, described in Homer’s Odyssey) involves quenching, that is, a rapid cooling of a red-hot piece of carbonized iron into cold water. This procedure hardens the work piece considerably more, sometimes even to the extent of brittleness. As a result, quenched swords, tools, and other utensils may have cracked or even shattered. The other discovery which was made during the end of the first millennium B.C. entailed a short-time reheating of a previously quenched piece of steel to about 600°C. This procedure, which is known today as tempering, restores some ductility and relieves the brittleness at the expense of some loss in hardness. We shall provide the scientific explanations for all these processes in Chapter 8. 7 • The Iron Age 127
128 7·The Iron Age After a relatively short time of Hittite dominance over the Mediterranean region (from about 1900 B.C.),some European tribes,vaguely called in the literature the "Sea Peoples,"overran the Middle East in about 1200 B.C.,destroying almost everything on their way.This caused the Hittites to vanish almost into obliv- ion.The destruction of the Hittite empire probably caused the scattering of their subjects and with them their iron-making skills.It can be observed that,after this dispersal of Hittite metal artisans,iron production was eventually conducted almost every- where in the western part of the world.For example,iron mak- ing was practiced by the Celtic tribes(from about 500 B.C.),who lived in Europe between the Mediterranean and the Baltic Seas and from the Atlantic Ocean to the Black Sea.They put iron rims on the wheels of their chariots,fitted their horses with shoes, produced iron plowshares,and invented the chain armor (see Plate 7.5).Despite their achievements,the distribution of iron utensils was not widely spread and was probably limited to the upper class,mostly because of its labor-intensive production from iron bloom. Before our attention is directed to iron smelting in the Far East, we need to discuss the important question concerning the rea- sons why the Bronze Age people abandoned their well-established technology and turned to a new material,that is,to iron.Cer- tainly,iron ore was more abundant than copper or tin.Actually, 5%of the earth's crust consists of iron whereas the abundance of copper and tin on the earth's crust is only 50 and 3 parts per million,respectively.Additionally,iron was often available on the surface of the earth,which did not necessitate underground min- ing.But,as discussed above,the high melting temperature of iron was virtually inaccessible in the Mediterranean basin dur- ing the second millennium B.C.The apparent reason for turning away from bronze was something much graver,namely,the in- terruption of trade routes probably by the above-mentioned Sea Peoples.As a consequence,the supply of tin,wherever it may have come from,was cut.When new bronze articles were wanted one had to rely on "recycled"bronze,that is,on bronze which was obtained by melting down earlier goods.Thus,2000 years of bronze technology came to a halt in the Middle East in a rel- atively short span of time for lack of raw materials. The same conditions naturally did not apply to the Far East. Thus,the Bronze Age lasted there somewhat longer.Eventually, however,iron making came to China most likely from the West between 1000 and 650 B.C.as a result of the above-mentioned dispersion of Hittite metal artisans and their know-how.At the beginning,the Chinese most probably applied the Western tech-
After a relatively short time of Hittite dominance over the Mediterranean region (from about 1900 B.C.), some European tribes, vaguely called in the literature the “Sea Peoples,” overran the Middle East in about 1200 B.C., destroying almost everything on their way. This caused the Hittites to vanish almost into oblivion. The destruction of the Hittite empire probably caused the scattering of their subjects and with them their iron-making skills. It can be observed that, after this dispersal of Hittite metal artisans, iron production was eventually conducted almost everywhere in the western part of the world. For example, iron making was practiced by the Celtic tribes (from about 500 B.C.), who lived in Europe between the Mediterranean and the Baltic Seas and from the Atlantic Ocean to the Black Sea. They put iron rims on the wheels of their chariots, fitted their horses with shoes, produced iron plowshares, and invented the chain armor (see Plate 7.5). Despite their achievements, the distribution of iron utensils was not widely spread and was probably limited to the upper class, mostly because of its labor-intensive production from iron bloom. Before our attention is directed to iron smelting in the Far East, we need to discuss the important question concerning the reasons why the Bronze Age people abandoned their well-established technology and turned to a new material, that is, to iron. Certainly, iron ore was more abundant than copper or tin. Actually, 5% of the earth’s crust consists of iron whereas the abundance of copper and tin on the earth’s crust is only 50 and 3 parts per million, respectively. Additionally, iron was often available on the surface of the earth, which did not necessitate underground mining. But, as discussed above, the high melting temperature of iron was virtually inaccessible in the Mediterranean basin during the second millennium B.C. The apparent reason for turning away from bronze was something much graver, namely, the interruption of trade routes probably by the above-mentioned Sea Peoples. As a consequence, the supply of tin, wherever it may have come from, was cut. When new bronze articles were wanted one had to rely on “recycled” bronze, that is, on bronze which was obtained by melting down earlier goods. Thus, 2000 years of bronze technology came to a halt in the Middle East in a relatively short span of time for lack of raw materials. The same conditions naturally did not apply to the Far East. Thus, the Bronze Age lasted there somewhat longer. Eventually, however, iron making came to China most likely from the West between 1000 and 650 B.C. as a result of the above-mentioned dispersion of Hittite metal artisans and their know-how. At the beginning, the Chinese most probably applied the Western tech- 128 7 • The Iron Age
7·The Iron Age 129 nique of converting bloom into wrought iron with subsequent carbonizing and possibly quenching and tempering.Very soon, however,Chinese iron makers went their own way by utilizing much larger and more powerful,horizontally operated,double- acting box bellows.They were driven by animals or water wheels and probably also by several humans.(This application of large amounts of forced air was reinvented in the West in the fifteenth century and was then called the blast furnace.)Most important, however,the Chinese also increased their carbon monoxide con- tent by enlarging their furnaces and substantially increasing the amount of charcoal fuel.As a result of both improvements,sig- nificant amounts of carbon diffused into the iron.This,in turn, decreased the melting point of the resulting charge to as low as 1148C.We have learned already in Chapter 5 that the melting temperature of metals is often reduced by alloying,that is,by adding a second constituent to a substance.(In the present case, the lowest melting point is obtained for the eutectic composition, which involves iron with 4.3%C,as we shall see in Chapter 8.) As a result of this new technology,iron could be cast similarly as bronze.Today,crude cast iron taken directly from the furnace is called pig iron because a row of parallel molds is said to re- semble little piglets drinking on their mother. Iron that contains large amounts of carbon is quite hard,but it is also brittle.The material is therefore almost worthless for tools and weapons because it cracks or shatters easily when a blow is applied to it.Thus,cast iron requires an additional treatment.This new treatment was probably introduced by the Chinese at about 500 B.C.It consisted of removing some of the excess carbon from the surface of high carbon iron.This eventually yielded a steel jacket that has similar properties as the steel that western people had produced when carbonizing wrought iron.To accomplish the reduction of carbon a piece made of cast iron was heated at tem- peratures between 800 and 900C in the presence of air.The oxy- gen in the air combines with some of the carbon and forms car- bon monoxide gas,which is allowed to escape.In essence,both the Chinese and the Mediterranean people eventually achieved a similar product but arrived at it from opposite directions.The main advantage of the Chinese technology was,however,that the Chinese could shape their products by casting,which allowed easy mass production,whereas the Western world had to shape and carbonize their goods individually by hammering. Not enough.There was still another development that arose from China in the first century A.D.It involved the stirring of car- bon-rich iron to allow the oxygen from the air to react with the carbon of the melt.As a consequence,the carbon content was
nique of converting bloom into wrought iron with subsequent carbonizing and possibly quenching and tempering. Very soon, however, Chinese iron makers went their own way by utilizing much larger and more powerful, horizontally operated, doubleacting box bellows. They were driven by animals or water wheels and probably also by several humans. (This application of large amounts of forced air was reinvented in the West in the fifteenth century and was then called the blast furnace.) Most important, however, the Chinese also increased their carbon monoxide content by enlarging their furnaces and substantially increasing the amount of charcoal fuel. As a result of both improvements, significant amounts of carbon diffused into the iron. This, in turn, decreased the melting point of the resulting charge to as low as 1148°C. We have learned already in Chapter 5 that the melting temperature of metals is often reduced by alloying, that is, by adding a second constituent to a substance. (In the present case, the lowest melting point is obtained for the eutectic composition, which involves iron with 4.3% C, as we shall see in Chapter 8.) As a result of this new technology, iron could be cast similarly as bronze. Today, crude cast iron taken directly from the furnace is called pig iron because a row of parallel molds is said to resemble little piglets drinking on their mother. Iron that contains large amounts of carbon is quite hard, but it is also brittle. The material is therefore almost worthless for tools and weapons because it cracks or shatters easily when a blow is applied to it. Thus, cast iron requires an additional treatment. This new treatment was probably introduced by the Chinese at about 500 B.C. It consisted of removing some of the excess carbon from the surface of high carbon iron. This eventually yielded a steel jacket that has similar properties as the steel that western people had produced when carbonizing wrought iron. To accomplish the reduction of carbon a piece made of cast iron was heated at temperatures between 800 and 900°C in the presence of air. The oxygen in the air combines with some of the carbon and forms carbon monoxide gas, which is allowed to escape. In essence, both the Chinese and the Mediterranean people eventually achieved a similar product but arrived at it from opposite directions. The main advantage of the Chinese technology was, however, that the Chinese could shape their products by casting, which allowed easy mass production, whereas the Western world had to shape and carbonize their goods individually by hammering. Not enough. There was still another development that arose from China in the first century A.D. It involved the stirring of carbon-rich iron to allow the oxygen from the air to react with the carbon of the melt. As a consequence, the carbon content was 7 • The Iron Age 129