The last two hundred years of the Chou dynasty was marked by such turmoil that it was called the ‘Warring States Era’. During this time period, the larger kingdoms began conquering the smaller ones. This was a marked contrast from the prior Spring and Autumn Era, when the Chou empire was splintering into many smaller states.
Because iron was such an important factor in the unfolding events of the Warring States Era, let us take a few pages to talk about this element and why it was so crucial.
Iron is the fourth most abundant element in the earth’s crust. While iron is much more plentiful than copper or tin, the ingredients of bronze, it is much more difficult to extract from its ores. As such, the ability to produce and cast bronze preceded iron production by almost 2000 years.
“Although iron is a commoner metal than copper or tin, the technique of iron smelting is more complicated than that with the other ores, requiring repeated hammering at red heat to expel slag impurities (primarily stone fragments) before wrought iron can be produced.” 1
Iron has been known from the earliest times, probably retrieved in pure form from meteors. However, the ability to remove it from ores wasn’t perfected until late in the 2nd millennium BCE. The exact dating of its development is difficult because it seemed that the Hittites of Anatolia, who might have discovered the process, kept it to themselves as a military secret. Their defeat and dispersion marked the spread of iron technology.
“True iron metallurgy began among the Hittites in eastern Anatolia at some time between 1900 and 1400 BC. The art of iron smelting was perfected by the time of the fall of the Hittite empire (c.1200 BC), and by 1000 BC iron objects and the knowledge of iron metallurgy had spread throughout the Near East and the Mediterranean and westward into Europe. This development marked the end of the Near Eastern Bronze Age, although bronze working was still in use for various ritual or prestige objects.”2
The production of iron from its ores marked the beginning of the Iron Age. Because iron, in its various compounds is so readily available, while copper and tin are relatively scarce, the military technology shifted to iron.
“The Iron Age marks the period of the development of technology, when the working of iron came into general use, replacing bronze as the basic material for implements and weapons. It is the last stage of the archaeological sequence known as the three-age system (Stone Age, Bronze Age, and Iron Age).”3
The three-age system mentioned above was an attempt to organize prehistoric discoveries by archaeologists. Bronze Age Europe was definitely prehistoric. Although its beginnings were prehistoric, Bronze Age China was definitely historic, as it occurred much later. Although the Hittite empire is quite well documented with some written remains, it was prehistoric when the remains were discovered. Iron technology appeared relatively late in China, well into the historical period. Consequently most historians do not refer to China’s Iron Age, although, as mentioned, the production of iron was related to the Chinese agricultural revolution that transformed and centralized Chinese culture.
“The Iron Age appeared in China by about 600 BC, spreading widely during the course of the Warring States period (403-222 BC). The Chinese developed superior blast furnaces and technical apparatuses with which to produce cast iron, techniques not employed in Europe until the Middle Ages. Early iron artifacts in China included swords and other weapons as well as implements of common use, such as axes, adzes, sickles, hoes, and other equipment that revolutionized Chinese agriculture.”4
Probably the iron technology was spread from the Hittite culture of Anatolia the same way that the bronze military technology was diffused, i.e. through the interaction of military cultures in the Eurasia-African land mass. As mentioned, the bronze military technology was probably spread from the nomadic cultures north of China. It is well documented that the military technology of horseman and archer also diffused to China from the steppes to the north. It is probable that iron technology spread in similar fashion.
Iron ores are plentiful. The smelting of copper and tin ores had been going on for centuries. Why did it take so long to use these iron ores?
Pure iron is soft5, hence unacceptable for weaponry. Thus the first step was to find an ingredient which when mixed with iron would produce a harder metal. Bronze was created from two metals. Probably much of the early research on iron concerned mixing different metals with iron, unsuccessfully. Finally after perhaps centuries of research, the Hittites discovered that pounding the gooey mess left over after smelting iron ore with charcoal produced a harder metal. They had inadvertently mixed carbon, a byproduct of burning charcoal, with the iron to produce a stronger alloy. At ideal proportions, this alloy is called steel.
There was another problem with iron. Mixing purified tin with purified copper yields purified bronze, which is suitable for casting. However mixing lumps of iron ore with charcoal yielded a gooey mess called sponge iron. While charcoal was plentiful and used as the fuel for the metal firing process as well, it produced an unwanted by-product called slag when mixed with iron. The smelting of iron ores with charcoal produced an impure mixture.
In order to purify this substance, it was necessary to hammer it repeatedly. This process expelled the impurities.
“Iron is made by refining iron ore to a point where it reaches 90 to 95 percent purity. Wrought iron, the earliest form of manufactured iron, was made by heating lumps of iron ore with charcoal. This produced sponge iron, a pasty mix of iron with a great deal of slag, the unwanted residue of the ore-refining process. The iron-slag mixture was hammered into a semifinished bar (hammering expelled some of the slag) and then further worked into finished products. Later, furnaces were devised that could produce enough heat to smelt ore into liquid iron, which was then cast rather than wrought.”6
Once this process was discovered, the Iron Age had begun in earnest. Smelting iron with charcoal and then pounding it was not an obvious process to discover. Once it was discovered, the next step was to find the ideal mixture of iron and charcoal to make the perfect alloy. Just as there is an ideal proportion of tin and copper to make bronze, there is also an ideal proportion of iron and charcoal. Pure iron, like copper and gold, is relatively soft. Thus from prehistoric times they would mix other ingredients with these primary metals to create alloys that were stronger and more resistant to corrosion. To copper they added tin to make bronze, an alloy of copper. To iron they added carbon to make steel, a harder alloy of iron.
The tensile strength of steel is greatest when it contains 2% carbon. Steel is iron that contains between 0.25% and 2% carbon. Low carbon steels are easily fabricated, while high carbon steels are extremely hard, but brittle. Hardness is a desirable feature for weaponry, while brittleness is not. While iron isn’t brittle, it is also not that hard. Thus early iron swords were extremely hard, but brittle.
Wrought iron has less carbon than steel, and hence is most malleable. While steel, iron mixed with 2% carbon, is hardest7, it is difficult to work with. Thus the first iron to be worked with was wrought iron. Because of its low carbon content, wrought iron was less brittle and more malleable.8 These early smiths discovered significant properties of the metal working with wrought iron.
Metals were first fabricated into their desired shape by pounding techniques. To further fabricate these metals, they heated them slowly. Both of these techniques had positive secondary effects, especially on steel. The pounding increased the purity and hardness, while the heating increased the toughness. As the heats were gradually increased, the fabrication of metals was eventually replaced by casting molten metal in molds. But fabrication preceded casting especially with the iron alloys, which were hard to work with. Cast iron, a late entry, has 3 to 4% carbon, and is very hard and brittle.
They pounded wrought iron with hammers to both remove impurities and to shape the object. Also they discovered that these metals were much easier to shape when they were hot. We can imagine them heating up the metal mixture, pounding it into the desired shape until it cooled down, then heating it up again when the metal became difficult to work. This simple process yielded some unexpected results.
First, pounding made the metal harder and more brittle. Second, the metals that had been reheated seemed to be tougher, i.e. less brittle while still a little harder, than those that weren’t. Third, as the metal cooled while it was being pounded, it would reach a point where it couldn’t be worked anymore without breaking or cracking. This last process is called work hardening.
These three results were not deductive. They derived from practical experience. It took modern science to understand the whys behind the process. However, these ancient smiths had been performing these operations for thousands of years before it was discovered why they worked.
While steel, an iron alloy, was harder than iron, the early smiths found that “the hardness of steel may be substantially increased by heating the metal until it is red hot and then quickly cooling it, a process known as quench hardening.”9 Thus two techniques were known to increase the hardness of the iron alloy, pounding and ‘quenching hardening’. While both of these processes increase the hardness of the metal, they both introduce unwanted brittleness.
They discovered the solution to brittleness in the reheating process they used to reshape the metal. It is called annealing10 when the metal is reheated to be reshaped. It was found that annealing decreased the brittleness of the metal, without losing too much hardness. Soon it was discovered that reheating the metal without reworking it even increased the strength more. This process of reheating and slowly cooling the finished product was called tempering11.
The application of these techniques to make steel harder and stronger, i.e. less resistant to breaking, occurred quite early in the Iron Age.
“The Iron Age dates from about 1500 BC, when iron ore is first known to have been smelted. During this era iron was used mainly for making cooking utensils and implements of war. The cementation process for making steel and the art of quenching steel for hardening and tempering of weapons were discovered early in the Iron Age.” 12
We must remember that these early metal smiths were quite sophisticated.
“By the late 4th millennium BC, smiths were remarkably sophisticated in a practical way regarding the individual phenomena of metallurgy. They knew the effects on metals of hammering, annealing, oxidation, melting, and alloying; they were aware of the phenomena of simple decomposition of ores, their reduction, double decomposition, and exchange of impurities.”13
The metal techniques that were to be used on steel were probably all in place over a thousand years before the discovery of the smelting of iron ores with charcoal.
What are the scientific processes behind these metallic properties, of hammering, annealing, cold-working, work hardening, and tempering?
Simply put, a metal is composed of many crystals, which in turn are composed of atoms arranged in regular patterns. The planes of the crystals are aligned in planes, called slip planes.
“As force is applied to a single grain of metal, the grain distorts along the slip planes. Metals in this condition are said to be ductile and easily worked.”
Unworked metal is malleable because its atomic structure is aligned in planes, which slide over each other when force is applied. While the metal is soft and malleable, it is generally not hard enough to assume a shape that is resistant to change.
Cold-work is the working of metals at room temperature. Normally this process utilizes hammering or pounding.
“As cold-work is applied to the metal, however, the crystalline structure changes. The original grain slips, distorts, and reorients so that instead of having a single grain with all the slip planes parallel, the structure is now composed of grain fragments. The slip planes within all these grain fragments are oriented in different directions, destroying the continuity of the individual slip planes in the original grain. The fragmented or broken grain structure increases the hardness of the metal and its resistance to further working.”14
Hence the more metal is cold worked, the more disorganized its structure, making it harder and less resistant to change. The same phenomenon occurs when a metal is heated and then quickly cooled, i.e. quench hardened. The normal slip planes are frozen out of alignment, increasing hardness because of the disorganization of the grain of the metal.
While the metal is harder because the alignment of its atomic grain has been fragmented, it is also more brittle because the metal has no internal integrity to hold it together. While the slip planes slide over each other, they simultaneously hold the metal together. Destroying the natural crystalline alignment creates a resistance to change that becomes hardness or permanence. However with this increasing resistance to change comes an inflexibility, which translates into brittleness.
An additional property of iron is its tendency to become magnetic15. When the slip planes are aligned and are in the presence of a magnetic field, they may transmit magnetism and even retain magnetism after the field has been removed. The magnetism concerns the alignment of the electric fields of all the metallic crystals. When a magnetic metal is pounded or heated it loses its magnetism16. This is because of the aforementioned disorganization of the slip planes on the atomic level.
As mentioned, hardening entails pounding or quenching, which disorganizes the atomic crystals of the metal. At this point, the metal is stressed, because its natural atomic structure, which in its optimum state has the potential for magnetization, is out of alignment. To realign these crystals in their natural slip planes, the metal is reheated close to the level where it melts and then slowly cooled. Remember when the metal is reworked as it is cooled, it is called annealing. When the metal is just heated and then cooled, it is called tempering.
The reheating increases metal ductility by allowing the metallic grain to realign itself and to re-crystallize. This creates a stronger metal product, which is less brittle, if not quite as hard. Thus the reheating reverses the process of work hardening, which disorganizes the crystals, by allowing the crystals to reorganize themselves along their natural magnetic grain. Further the metallic grain, which was shattered into fragments by pounding or quenching, is allowed to reform17. Tempering re-integrates the atomic structure by allowing the metal to re-granulate. This process has the effect of softening18 the metal as well as making it less brittle.
Metal has two polarities that are being balanced – the polarity of rigid and flexible and the polarity of soft and hard. Metallically, this translates into brittle or ductile and malleable and hard, tensile. The balance between these two polarities, whose optimum condition would be called toughness, is achieved by two consecutive processes. First quenching or pounding hardens the metal. Then easy heating and cooling temper the metal – decreasing brittleness and increasing ductility. The hardening process destroys the atomic integrity of the metal, while the tempering process reconstitutes this atomic integrity. The metal changes a little under external pressure, but holds itself together. Of course this is ideal for the sword.
One other element of metallurgy must be mentioned, metal fatigue.
“Metal fatigue is the tendency for a metal to break under the action of repeated cyclic stresses. Fatigue may occur for values of cyclic stress considerably less than the ultimate tensile strength of the material. This phenomenon applies to certain fractures in metals that are caused by repeated stresses of a low enough value that a single application of the stress apparently does nothing detrimental to the structure. When enough of these seemingly harmless stresses are applied in a cyclic manner, however, they bring about a small crack that grows with continued loadings until complete fracture takes place. Since the small cracks may not be noticed, the metal may fracture with a suddenness that can be dangerous, as in fast-moving vehicles or high-speed machinery. Special inspection techniques have been developed to spot small cracks before the material fails.
Fatigue failures are due to the repeated application of tensile stresses or shear stresses, which tend to pull the material apart. However, a cycle that consists of alternating equal stresses in tension and compression, called a fully reversed cycle, is usually used to obtain the endurance limits of a particular material.”19
In simplified terms, when metal undergoes repeated regular stress well under its limits, these stresses build up and will eventually produce cracks which turn into a break in the metal. This mechanism is called metal fatigue. If the stresses are relieved by counter-action, called a fully reversed cycle, then the stresses are relieved preventing fracture and breakage.
In summary, technological advances enabled humans to first extract iron from ore and then to effectively combine it with carbon to produce steel. Farm implements and weaponry made from steel advanced both the agricultural and military technology of the day. These advances ended the Bronze Age and initiated the Iron Age. These metallurgical processes were first developed and employed in Western Eurasian in the 2nd millennium BCE and then gradually spread to China in the middle of the 1st millennium BCE. This was the beginning China’s Iron Age. The utilization of iron by the Ch’in state initiated to the Warring States Era. Their military dominance destabilized the traditional Chinese society of the Chou dynasty. The employment of this plentiful metal was also a major factor in the centralization of Chinese society that inevitably led to an empire. As we shall see in the next chapter, iron was and is used as a martial metaphor for the body due to its remarkable properties.
1 Grolier Multimedia 1997: Iron Age
2 Grolier Multimedia 1997: Iron Age
3 Grolier Multimedia 1997: Iron Age
4 Grolier Multimedia 1997: Iron Age
5 Grolier Multimedia 1997: Iron, “In its pure form iron is rather soft and is malleable and ductile at room temperature.”
6 Grolier Multimedia 1997: Steel and Iron Industry: Iron Making
7 Grolier Multimedia 1997: Hardening: “Hardening is especially important in steel processing, where the maximum hardness is dependent almost entirely upon the carbon content.”
8 Grolier Multimedia 1997: Iron “At carbon contents below that of steel is wrought iron, which is nearly pure iron. Because of its low carbon content (usually below 0.035 percent) it is forgeable and nonbrittle. Iron of high carbon content (3 percent to 4 percent), obtained when pig iron is remelted and cooled, is called cast iron. If cast iron is cooled quickly, hard but brittle white cast iron is formed. If it is cooled slowly, soft but tough gray cast iron is formed.”
9 Grolier Multimedia 1997: Iron
10 Grolier Multimedia 1997, Annealing “Annealing is a method of heating and cooling a metal, alloy, or glass under precise controls to remove internal stresses and make the material more ductile and less brittle. The method is applied after a metal has been shaped by forging, extruding, rolling, or drawing, at temperatures where softening does not occur (cold-working). At this point the metal tends to resist further working, and a condition known as work-hardening occurs. If the metal is worked after hardening, it will crack or break. Annealing returns the metal to its original state so that it can be worked further.”
11 Grolier Multimedia 1997: Tempering: “Tempering is the process of toughening glass and metals, particularly steel. During its manufacture, steel is heated to a high temperature and quenched or cooled quickly. This rapid cooling creates a buildup of internal stresses that cause the metal to become brittle. The tempering of steel involves the subsequent reheating of the metal to a temperature below the point to which it was first heated. It is then allowed to cool slowly. This reheating and cooling process softens the steel, relieving the internal stresses set up by the original heating and quenching operations. The softening is accompanied by an important increase in toughness that is a direct result of the alleviation of brittleness of the steel. Temperature and rate of cooling will vary depending on the type of steel used, the desired properties, and the intended use of the steel. Generally, steel is tempered at temperatures ranging from 200 deg to 600 deg C (400 deg to 1,100 deg F).”
12 Grolier Interactive 1997: Metallurgy: History
13 Grolier Multimedia 1997, Metallurgy: History
14 Grolier Multimedia 1997, Annealing
15 Grolier Multimedia 1997, Iron: Magnetism: “Iron at room temperature exhibits ferromagnetism, a strong magnetic behavior that the metal may retain even in the absence of an external, applied magnetic field. When iron is heated to 768 degrees C, it loses this property. ”
16 The exception to this is when the metal is pounded in the direction of the magnetic field.
17 Grolier Multimedia 1997, Annealing: “Heating affects cold-worked metal in two stages, recovery and recrystallization. Recovery occurs as the temperature of the cold-worked metal is gradually raised. Internal stresses are relieved as the atoms in the metal rearrange themselves into the positions that they occupied in the preworked state. Recrystallization occurs as the temperature of the metal is raised further and nuclei for the growth of new, stress-free crystals begin to form. The higher the temperature, the more nuclei are precipitated. As these nuclei form in cold-worked metal, the stress-free crystals exhibit most of the original physical properties of the metal.”
18 Grolier Multimedia 1997: Annealing “The degree of softening resulting from an annealing treatment depends on the temperature to which a metal is heated as well as the length of time for which it is heated. The time and temperature of heating are inversely related. Different metals require different annealing times and temperatures, which are carefully regulated to gain the optimum structure development.”
19 Grolier Multimedia 1997, Metal Fatigue