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
130 7·The Iron Age already reduced in the melt to the extent that it yielded steel.This process,which is called puddling today,was rediscovered in Eng- land in 1784.During the latter part of the Han dynasty (202 B.C.-A.D.220),immense industrial complexes were operating near Zheng-Zhou and other places containing several huge furnaces (4 X 3 m in area and about 3 m high)which might have pro- duced several tons of iron per day.Moreover,the Chinese already used the technique of stack casting,that is,they arranged several molds on top of and next to each other,which enabled them to cast up to 120 articles at the same time.The large-scale produc- tion of plowshares,hoes,cart bearings,and harness buckles made the output quite cost-effective.It thus allowed a wide distribu- tion of tools for working the fields and digging irrigation chan- nels,which in turn might have led to a larger production of agri- cultural crops and probably even to an increase in population. In other words,the change that was brought about by the intro- duction of iron and steel slowly revolutionized the way the Chi- nese and probably other peoples lived and worked.Nomads have no need for agricultural tools,but the availability of these tools probably contributed to the settlement of some nomads.And fi- nally,iron paved the way from agriculture to industry.The po- tential for China to become a world-wide industrial power al- ready 2000 years ago was laid by these inventions and by their large-scale exploitation.But Chinese bureaucracy upheld by Con- fucian-trained civil servants apparently stifled new ideas and the expansion of major trade beyond the boundaries of China. The same revolutionizing developments that were caused by the use of iron were eventually also seen in other parts of the world.Goods made of iron were traded virtually everywhere.Iron axes reduced the amount of forests to provide fuel and to clear land for feeding more people.Weapons made of iron or steel (see for example Plate 7.2)unfortunately provided the means to con- quer and often destroy other civilizations.Knights wore armors of iron.Indeed,iron was and still is,in many cultures,the sym- bol for strength,power,and will. One particular amazing demonstration of iron workmanship is the famous iron pillar next to the Qutub Minar tower on the outskirts of Delhi in India.The pillar made of forged iron is seven meters tall and has a purity of about 99.2%,containing only small amounts of sulphur (0.08%),phosphorus (0.11%),silicon (0.46%),and carbon (0.08%).It dates back to the fourth century A.D.and probably was manufactured by heating and hammering together a large number of small iron pieces.Most amazingly, however,the iron pillar has not experienced any corrosion dur- ing the 1500 years in which it has been exposed to air.It is spec- ulated that the lack of rusting is due to a combination of climatic
already reduced in the melt to the extent that it yielded steel. This process, which is called puddling today, was rediscovered in England in 1784. During the latter part of the Han dynasty (202 B.C.–A.D. 220), immense industrial complexes were operating near Zheng-Zhou and other places containing several huge furnaces (4 3 m in area and about 3 m high) which might have produced several tons of iron per day. Moreover, the Chinese already used the technique of stack casting, that is, they arranged several molds on top of and next to each other, which enabled them to cast up to 120 articles at the same time. The large-scale production of plowshares, hoes, cart bearings, and harness buckles made the output quite cost-effective. It thus allowed a wide distribution of tools for working the fields and digging irrigation channels, which in turn might have led to a larger production of agricultural crops and probably even to an increase in population. In other words, the change that was brought about by the introduction of iron and steel slowly revolutionized the way the Chinese and probably other peoples lived and worked. Nomads have no need for agricultural tools, but the availability of these tools probably contributed to the settlement of some nomads. And finally, iron paved the way from agriculture to industry. The potential for China to become a world-wide industrial power already 2000 years ago was laid by these inventions and by their large-scale exploitation. But Chinese bureaucracy upheld by Confucian-trained civil servants apparently stifled new ideas and the expansion of major trade beyond the boundaries of China. The same revolutionizing developments that were caused by the use of iron were eventually also seen in other parts of the world. Goods made of iron were traded virtually everywhere. Iron axes reduced the amount of forests to provide fuel and to clear land for feeding more people. Weapons made of iron or steel (see for example Plate 7.2) unfortunately provided the means to conquer and often destroy other civilizations. Knights wore armors of iron. Indeed, iron was and still is, in many cultures, the symbol for strength, power, and will. One particular amazing demonstration of iron workmanship is the famous iron pillar next to the Qutub Minar tower on the outskirts of Delhi in India. The pillar made of forged iron is seven meters tall and has a purity of about 99.2%, containing only small amounts of sulphur (0.08%), phosphorus (0.11%), silicon (0.46%), and carbon (0.08%). It dates back to the fourth century A.D. and probably was manufactured by heating and hammering together a large number of small iron pieces. Most amazingly, however, the iron pillar has not experienced any corrosion during the 1500 years in which it has been exposed to air. It is speculated that the lack of rusting is due to a combination of climatic 130 7 • The Iron Age
7·The Iron Age 131 factors,high P and S contents,and a large heat capacity.Another, larger iron pillar was found at Sarnath,which was produced be- tween 300 and 200 B.C.and before it broke was almost 14 m tall. The time at which iron was first smelted in India is not exactly known.However,iron production is mentioned in the Rigveda, which is the oldest known Hindu religious book.Conservative es- timates place its origin around 1200 B.C.Other sources claim that iron smelting in India did not commence before 600 B.C. Another specialty of India that was produced and sold virtu- ally across the entire continent from the first millennium B.C.un- til the middle ages was the so-called wootz steel which was later named Damascus steel.It is said that it was the raw material for the best swords and daggers of that time (see Plates 7.3 and 7.4).Wootz steel was made by placing small pieces of wrought iron or sponge iron (see above)together with some wood chips and leaves in small clay crucibles which were sealed with a clay lid and then heated in air-blast-enhanced fires.This enabled the carbon from the plant material to evenly penetrate the iron,thus providing an essentially homogeneous iron-carbon steel.(In con- trast to this,the Mediterranean or Chinese technologies allowed only a portion of the work piece to be steel,as described above. The other parts were either wrought iron or cast iron.)The In- dians succeeded in keeping their technique a secret until the sev- enth century A.D.,after which the Syrians near Damascus and the Spaniards near Toledo came up with their own versions.The Damascus swords of later times were produced by joining and folding through hammer-welding alternate bars of iron and steel. Application of a dilute acid colored the steel sections leaving the iron relatively bright.Wootz steel was also reinvented in Eng- land during the eighteenth century. When reporting about the advances which major civilizations contributed to the art of iron and steel making,it is often over- looked that less known peoples were also able to prepare iron goods.Their contributions might not have been as spectacular as those described above,but they still had some local impact on the lives in certain societies.Among them were the Haya people, who lived near the shores of Lake Victoria,which is part of to- day's Tanzania in East Africa.Their folktales are full of stories about iron making,and the vocabulary with which they are told is enriched with reproductive symbolism.(A PBS documentary entitled,"The Tree of Iron,"witnesses to this effect.) During the Middle Ages the knowledge of metallurgy in gen- eral and iron and steel making in particular precipitated into written documents.Among them were books like De re metallica by the German extractive metallurgist and miner Georgius Agri- cola,who summarized in 1556 the then-available knowledge on
factors, high P and S contents, and a large heat capacity. Another, larger iron pillar was found at Sarnath, which was produced between 300 and 200 B.C. and before it broke was almost 14 m tall. The time at which iron was first smelted in India is not exactly known. However, iron production is mentioned in the Rigveda, which is the oldest known Hindu religious book. Conservative estimates place its origin around 1200 B.C. Other sources claim that iron smelting in India did not commence before 600 B.C. Another specialty of India that was produced and sold virtually across the entire continent from the first millennium B.C. until the middle ages was the so-called wootz steel which was later named Damascus steel. It is said that it was the raw material for the best swords and daggers of that time (see Plates 7.3 and 7.4). Wootz steel was made by placing small pieces of wrought iron or sponge iron (see above) together with some wood chips and leaves in small clay crucibles which were sealed with a clay lid and then heated in air-blast–enhanced fires. This enabled the carbon from the plant material to evenly penetrate the iron, thus providing an essentially homogeneous iron–carbon steel. (In contrast to this, the Mediterranean or Chinese technologies allowed only a portion of the work piece to be steel, as described above. The other parts were either wrought iron or cast iron.) The Indians succeeded in keeping their technique a secret until the seventh century A.D., after which the Syrians near Damascus and the Spaniards near Toledo came up with their own versions. The Damascus swords of later times were produced by joining and folding through hammer-welding alternate bars of iron and steel. Application of a dilute acid colored the steel sections leaving the iron relatively bright. Wootz steel was also reinvented in England during the eighteenth century. When reporting about the advances which major civilizations contributed to the art of iron and steel making, it is often overlooked that less known peoples were also able to prepare iron goods. Their contributions might not have been as spectacular as those described above, but they still had some local impact on the lives in certain societies. Among them were the Haya people, who lived near the shores of Lake Victoria, which is part of today’s Tanzania in East Africa. Their folktales are full of stories about iron making, and the vocabulary with which they are told is enriched with reproductive symbolism. (A PBS documentary entitled, “The Tree of Iron,” witnesses to this effect.) During the Middle Ages the knowledge of metallurgy in general and iron and steel making in particular precipitated into written documents. Among them were books like De re metallica by the German extractive metallurgist and miner Georgius Agricola, who summarized in 1556 the then-available knowledge on 7 • The Iron Age 131
132 7·The Iron Age smelting,refining,and analytical methods as well as on prospect- ing and concentration of ores.Another book by the Italian met- alworker Vannoccio Biringuccio,entitled De la pirotechnia,re- ported in much detail on smelting,analytical methods,casting, molding,core making,and foundry practices.Not all documents of that time were,however,of the same authority as the two works just mentioned.For example,the book Von Stahel und Ey- sen (On Steel and Iron),which was published in Nuremberg(Ger- many)in 1532 by an alchemist,is quite an entertaining read in the twentieth century.A few samples may illustrate this.One finds the following recipe in this volume under the heading "How iron is to be hardened and some of the hardness drawn again": Take the stems and leaves of vervain,crush them,and press the juice through a cloth.Pour the juice into a glass vessel and lay it aside.When you wish to harden a piece of iron,add an equal amount of a man's urine and some of the juice obtained from the little worms known as cockchafer grubs.Do not let the iron become too hot but only moderately so;thrust it into the mixture as far as it is to be hardened.Let the heat dissipate by itself until the iron shows gold-colored flecks,then cool it completely in the aforesaid water.If it becomes very blue,it is still too soft. A good procedure for tempering might have been: Take clarified honey,fresh urine of a he-goat,alum,borax,olive oil, and salt;mix everything well together and quench therein. Actually,the nitrogen in urea (H2NCONH2)probably led to ni- trated,"case-hardened"iron.Here is another useful recipe: How To Draw The Hardness of Iron:Let the human blood stand un- til water forms on top.Strain off this water and keep it.Then hold the hardened tools over a fire until they have become hot and sub- sequently brush them with a feather soaked in this water;they will devour the water and become soft. A splendid method of hardening could have been: Take varnish,dragon's blood,horn scrapings,half as much salt,juice made from earthworms,radish juice,tallow,and vervain and quench therein.It is also very advantageous in hardening if a piece that is to be hardened is first thoroughly cleaned and well polished. (The latter recipe must have had restricted use because of the limited availability of dragon's blood.)Those who will not have instant success with these instructions may take comfort when reading in that book:"If fault should be found with some of the recipes,pray do not reject the whole book.Perhaps the fault lies in the user himself,because he did not follow the instructions correctly.All arts require practice and long experience,and their mastery is only gradually acquired
smelting, refining, and analytical methods as well as on prospecting and concentration of ores. Another book by the Italian metalworker Vannoccio Biringuccio, entitled De la pirotechnia, reported in much detail on smelting, analytical methods, casting, molding, core making, and foundry practices. Not all documents of that time were, however, of the same authority as the two works just mentioned. For example, the book Von Stahel und Eysen (On Steel and Iron), which was published in Nuremberg (Germany) in 1532 by an alchemist, is quite an entertaining read in the twentieth century. A few samples may illustrate this. One finds the following recipe in this volume under the heading “How iron is to be hardened and some of the hardness drawn again”: Take the stems and leaves of vervain, crush them, and press the juice through a cloth. Pour the juice into a glass vessel and lay it aside. When you wish to harden a piece of iron, add an equal amount of a man’s urine and some of the juice obtained from the little worms known as cockchafer grubs. Do not let the iron become too hot but only moderately so; thrust it into the mixture as far as it is to be hardened. Let the heat dissipate by itself until the iron shows gold-colored flecks, then cool it completely in the aforesaid water. If it becomes very blue, it is still too soft. A good procedure for tempering might have been: Take clarified honey, fresh urine of a he-goat, alum, borax, olive oil, and salt; mix everything well together and quench therein. Actually, the nitrogen in urea (H2NCONH2) probably led to nitrated, “case-hardened” iron. Here is another useful recipe: How To Draw The Hardness of Iron: Let the human blood stand until water forms on top. Strain off this water and keep it. Then hold the hardened tools over a fire until they have become hot and subsequently brush them with a feather soaked in this water; they will devour the water and become soft. A splendid method of hardening could have been: Take varnish, dragon’s blood, horn scrapings, half as much salt, juice made from earthworms, radish juice, tallow, and vervain and quench therein. It is also very advantageous in hardening if a piece that is to be hardened is first thoroughly cleaned and well polished. (The latter recipe must have had restricted use because of the limited availability of dragon’s blood.) Those who will not have instant success with these instructions may take comfort when reading in that book: “If fault should be found with some of the recipes, pray do not reject the whole book. Perhaps the fault lies in the user himself, because he did not follow the instructions correctly. All arts require practice and long experience, and their mastery is only gradually acquired.” 132 7 • The Iron Age
7·The Iron Age 133 Legend tells us about a medieval,germanic weapon smith,called Wieland,whose swords were unsurpassed in strength and sharp- ness.His secret recipe supposedly involved the filing of a forged piece of iron into a coarse powder that was fed to his chicken.He then separated the iron from the feces with a magnet.After seven passes,a superb sword was eventually forged from this material, which won a critical contest by slicing a competitor,who was wear- ing his armor,in half.A metallurgist working for a German com- pany tried this procedure in the 1930s and found that the chick- ens reduced the carbon content of the iron in their digestive system while enriching the iron with nitrogen.This made the steel stronger,as scientific studies have demonstrated. Today,iron is reduced from its ore in massive blast furnaces, up to 30 meters high,in which the preheated air is blast-injected through nozzles (tuyeres)near the bottom of the furnace to ob- tain the necessary high temperatures.They are fueled by the more efficient coke (derived from coal)and fluxed by limestone.One blast furnace yields in excess of 4000 tons of pig iron per day. Elimination of very fine particles in the raw material(called bur- den),which may restrict the flow of gas,has resulted in produc- tivity increases of as much as 100%.In countries with cheap elec- tricity combined with easy access to raw materials,electric furnace smelting is used to produce pig iron.Another technique, called direct reduction processes,involves rotary kilns in which a mixture of pure,dry iron ore,a reducing substance,and lime- stone (as flux)is heated to about 1000C.Reducing agents are, for example,coal,coke,graphite,fuel oil,or hydrogen.The process of making low-carbon-iron (or steel)directly from ore is quite attractive compared to a two-stage process in which high- carbon pig iron is produced first and then later is purified to steel.However,for the direct process the ore must be very rich, finely divided,and intimately mixed with the reducing agent in correct proportions.Thus,only 2%of the iron and steel produced in the world today are made in this way.The end product is nearly pure sponge iron or bloom as historically known for mil- lennia.High purity sponge iron is also used in the chemical in- dustry as a strong reducing agent and for powder metallurgy. For modern steel making from pig iron mainly three procedures are utilized.Briefly,the basic oxygen process(BOP),which was invented in 1952 in Austria,involves blowing oxygen into the molten iron (which may contain up to 25%scrap steel)either from the top by means of a retractable lance or from the bottom through tuyeres.The oxygen combines with the carbon and other impurities,thus reducing the carbon content to form steel (i.e., iron with less than 2.11%C).The BOP techniques are fast and cost-effective but do not provide an exact chemical composition
Legend tells us about a medieval, germanic weapon smith, called Wieland, whose swords were unsurpassed in strength and sharpness. His secret recipe supposedly involved the filing of a forged piece of iron into a coarse powder that was fed to his chicken. He then separated the iron from the feces with a magnet. After seven passes, a superb sword was eventually forged from this material, which won a critical contest by slicing a competitor, who was wearing his armor, in half. A metallurgist working for a German company tried this procedure in the 1930s and found that the chickens reduced the carbon content of the iron in their digestive system while enriching the iron with nitrogen. This made the steel stronger, as scientific studies have demonstrated. Today, iron is reduced from its ore in massive blast furnaces, up to 30 meters high, in which the preheated air is blast-injected through nozzles (tuyères) near the bottom of the furnace to obtain the necessary high temperatures. They are fueled by the more efficient coke (derived from coal) and fluxed by limestone. One blast furnace yields in excess of 4000 tons of pig iron per day. Elimination of very fine particles in the raw material (called burden), which may restrict the flow of gas, has resulted in productivity increases of as much as 100%. In countries with cheap electricity combined with easy access to raw materials, electric furnace smelting is used to produce pig iron. Another technique, called direct reduction processes, involves rotary kilns in which a mixture of pure, dry iron ore, a reducing substance, and limestone (as flux) is heated to about 1000°C. Reducing agents are, for example, coal, coke, graphite, fuel oil, or hydrogen. The process of making low-carbon–iron (or steel) directly from ore is quite attractive compared to a two-stage process in which highcarbon pig iron is produced first and then later is purified to steel. However, for the direct process the ore must be very rich, finely divided, and intimately mixed with the reducing agent in correct proportions. Thus, only 2% of the iron and steel produced in the world today are made in this way. The end product is nearly pure sponge iron or bloom as historically known for millennia. High purity sponge iron is also used in the chemical industry as a strong reducing agent and for powder metallurgy. For modern steel making from pig iron mainly three procedures are utilized. Briefly, the basic oxygen process (BOP), which was invented in 1952 in Austria, involves blowing oxygen into the molten iron (which may contain up to 25% scrap steel) either from the top by means of a retractable lance or from the bottom through tuyères. The oxygen combines with the carbon and other impurities, thus reducing the carbon content to form steel (i.e., iron with less than 2.11% C). The BOP techniques are fast and cost-effective but do not provide an exact chemical composition 7 • The Iron Age 133
1347·The Iron Age of the steel.Further,the possibility for recycling of scrap steel is limited. The open-hearth process uses a shallow,swimming pool- shaped (about 27 m X 9 m),fire brick-lined furnace in which air (heated essentially by oil burners)is blown horizontally over the surface of the melt.An oxygen lance from the top speeds up the carbon reduction.Since this technique is slow and utilizes oil for heating the air,and further,since it produces large volumes of polluting waste gases (carbon monoxide),its application has steadily declined to about 5%in the United States.On the posi- tive side,considerable amounts of scrap iron (usually up to 50%) can be used.The principle features of this process were devel- oped in 1864/68 by Siemens and Martin. For melting and refining primarily steel scraps,electric arc furnaces are common and cost-effective.Again,oxygen is injected during the process.The heat is generated involving principally the electrical resistance between carbon electrodes and the ingot. The Bessemer converter,which came into wide use in the middle of the nineteenth century,has been virtually replaced by the above-mentioned steel making techniques because of serious disadvantages of its products.Specifically,it has been found that the lack of ductility and workability of Bessemer steel was due to small percentages of nitrogen (about 0.015%!)that prevented its deep drawing into sheet metal.The Bessemer process involved air that was blown upward through molten pig iron held in a re- fractory-lined,pear-shaped vessel.Two types of linings were used.They consisted either of basic bricks for phosphorus-rich and silicon-poor pig iron (fluxed with limestone to remove the phosphorus which makes the iron brittle and thus useless)or acid lining for Si-rich/P-poor raw iron. The large consumption of charcoal as fuel and as a reducing agent for iron smelting eventually took its toll on the forests of industrialized countries.Charcoal production involves the in- complete burning of wood which is stacked in a pile and covered with earth and leaves.This process drives off the volatile con- stituents in the wood that would otherwise contaminate the iron and thus may compromise the properties of steel.In order to pre- serve her forests and ship-building capability,England's Parlia- ment in 1584 severely restricted the cutting of timber for char- coal production.To alleviate the resulting charcoal shortage,an alternative reducing agent and fuel was sought and eventually found in the seventeenth century.This substance is coke,which is obtained by heating soft coal powder in an airtight oven for the purpose of driving off the volatile impurities of coal.The re- sult is 88%carbon. The long history of iron and steel making is characterized by
of the steel. Further, the possibility for recycling of scrap steel is limited. The open-hearth process uses a shallow, swimming pool– shaped (about 27 m 9 m), fire brick-lined furnace in which air (heated essentially by oil burners) is blown horizontally over the surface of the melt. An oxygen lance from the top speeds up the carbon reduction. Since this technique is slow and utilizes oil for heating the air, and further, since it produces large volumes of polluting waste gases (carbon monoxide), its application has steadily declined to about 5% in the United States. On the positive side, considerable amounts of scrap iron (usually up to 50%) can be used. The principle features of this process were developed in 1864/68 by Siemens and Martin. For melting and refining primarily steel scraps, electric arc furnaces are common and cost-effective. Again, oxygen is injected during the process. The heat is generated involving principally the electrical resistance between carbon electrodes and the ingot. The Bessemer converter, which came into wide use in the middle of the nineteenth century, has been virtually replaced by the above-mentioned steel making techniques because of serious disadvantages of its products. Specifically, it has been found that the lack of ductility and workability of Bessemer steel was due to small percentages of nitrogen (about 0.015%!) that prevented its deep drawing into sheet metal. The Bessemer process involved air that was blown upward through molten pig iron held in a refractory-lined, pear-shaped vessel. Two types of linings were used. They consisted either of basic bricks for phosphorus-rich and silicon- poor pig iron (fluxed with limestone to remove the phosphorus which makes the iron brittle and thus useless) or acid lining for Si-rich/P-poor raw iron. The large consumption of charcoal as fuel and as a reducing agent for iron smelting eventually took its toll on the forests of industrialized countries. Charcoal production involves the incomplete burning of wood which is stacked in a pile and covered with earth and leaves. This process drives off the volatile constituents in the wood that would otherwise contaminate the iron and thus may compromise the properties of steel. In order to preserve her forests and ship-building capability, England’s Parliament in 1584 severely restricted the cutting of timber for charcoal production. To alleviate the resulting charcoal shortage, an alternative reducing agent and fuel was sought and eventually found in the seventeenth century. This substance is coke, which is obtained by heating soft coal powder in an airtight oven for the purpose of driving off the volatile impurities of coal. The result is 88% carbon. The long history of iron and steel making is characterized by 134 7 • The Iron Age