the properties of copper noticeably except in the case of bismuth, which even in small amounts makes copper brittle.
The first major and deliberate addition to copper seemed to have been arsenic (at least in the Middle East). Copper–arsenic ores were widely available in this region, but alloying also was achieved by mixing arsenic-containing ores with copper ores during smelt-ing (see Plate 4.1). Artifacts from 3000 B.C. found in the Middle East contained up to 7% arsenic as a second constituent to cop-per. In this context, an archaeological find needs to be mentioned which was made in 1961. In the almost inaccessible “Cave of the Treasure,” close to the cave where the Dead Sea Scrolls were un-earthed, 429 objects were discovered, of which all but 13 were made of a copper–arsenic alloy. They must have been brought there around 3000 B.C. by refugees and may have belonged to a temple or shrine. The cache harbored 10 crowns, 240 elaborately deco-rated mace heads, chisels, and axes of different sizes and shapes.
Copper–arsenic alloys, however, were used only for a limited time. Eventually, it must have been realized that the fumes which were emitted by the arsenic during smelting killed some metal-smiths. Eventually, tin was found to be the ideal addition to cop-per which was alloyed in an optimal proportion of 10 mass–%.
This copper-tin alloy is generally referred to as bronze. The melt-ing temperature of Cu–10% Sn is about 950°C (compared to 1084°C for pure Cu). The melt flows freely into molds and no problems with gas bubbles, that is, porosity, are encountered.
Most importantly, however, the alloy is hard immediately after casting and subsequent cooling but can be hardened further by hammering. Finally, copper–tin is harder and less brittle than copper–arsenic. See in this context Plate 4.2.
FIGURE4.1.A portion of a mural from the tomb of the Vizier Rekh-Mi-Re at Thebes (Egypt) depicting workers carrying a piece of copper in the shape of an ox hide (see Figure 1.3) and baskets containing probably tin and lead for bronze production.
An overseer “supervises” the porters. Second millenium B.C. (Reprinted by permission from B.
Scheel, Egyptian Metalworking and Tools, Shire Publications, Aylesbury, UK)
There are several intriguing questions that demand answers.
One of them is concerned with the query about whether or not bronze was “invented” in only one region of the world (namely the Middle East, as many scholars used to believe) or indepen-dently at several places. The final word on this has not been spo-ken yet. However, recent archaeological evidence indicates that besides the Mediterranean area (considered by many westerners to be the “cradle of civilization”), independent bronze-producing centers existed in northern Thailand (Ban Chiang) during the third or fourth millennium B.C., and additionally in the isolation of China during the Shang dynasty starting at about 1400 B.C.
The mode of transition into the bronze age in the just-mentioned areas, which were considerably separated from each other, seemed not to have been identical. For example, Indo-China (which is said to have given to mankind a number of es-sential food plants such as rice, bananas, coconuts, yams, taro, and sugarcane) had a remarkable bronze production. The in-habitants of this area lived in light bamboo houses, made pot-tery, and domesticated pigs, chickens, and cows. In this region, bronze axes, spearheads, socket tools, bronze bracelets, clay cru-cibles, and sandstone molds have been found dating back as far as 3000 to 2300 B.C.
The most interesting find is that the Ban Chiang people seemed to have skipped copper production and arsenical bronze alto-gether and jumped immediately into the tin–bronze age. The raw materials for bronze were certainly available at essentially one and the same place (in contrast to the Near East, as we shall elu-cidate below). Indeed, rich alluvial1deposits of tin as well as cop-per ores are found from southern China down to Thailand and Indonesia. Another interesting observation was made by archae-ologists who state that the Thai people seemed to have lived in a “peaceful bronze age” since no swords, battle axes, daggers, or mace heads have been found. Instead, bronze was mostly used for decoration and adornment. Its possession did not seem to ex-press a status symbol since many children were buried with bronze bracelets.
In contrast to this, early Chinese bronze, made during the Shang dynasty (1600–1122 B.C.), was mainly utilized for cere-monial vessels, that is, for offering of food and wine to ancestral spirits (Plate 4.3). The bronze contained from 5 to 30% tin and between 3 and 5% lead (which makes the melt flow easier).
1Soil deposited by flowing water.
Bronze pieces from the Shang period are richly decorated by re-lief patterns depicting animals such as elephants, water buffalos, tigers, mythical dragons, and others. The Chinese were masters of a cast technology (Plate 4.3). They utilized fired clay molds in which the patterns were carved. No subsequent metalworking such as hammering, etc., was used. An example of their skills is a large cauldron on four legs which weighs 875 kg and whose body was cast in one piece. (It was unearthed near Anyang in 1939 and then utilized by villagers for storing pig food.)
Another astounding recent find is a set of bronze bells which were discovered in a tomb for the Marqui Yi, dated 433 B.C. (Fig.
4.2). They were shaped in a manner that, when hit with a ham-mer at the center, produced a lower pitch than on the edge. Mod-ern scholars believe that the two sounds reflected the Chinese concept of a universe that is governed by two opposing, yet har-monious, forces called Yin and Yang (such as day and night, heaven and earth, sun and moon). The two-tone bells are said to have demonstrated how two forces can interact in harmony.
Mu-FIGURE 4.2. Chinese two-tone bell made of bronze. Eastern Zhou dy-nasty, 6th century B.C. Unearthed in 1978 at Sui Xian, Hubei Province. An entire set of those bells is called “Bian Zhong”.
Arthur M. Sackler Gallery, Smith-sonian Institution, Washington, DC. Gift of Arthur M. Sackler S1987.285.
sic, which was important to the Chinese of that time, was a means to communicate with their forefathers, who gave them life and wealth. Music from bells and drums was not a form of enter-tainment but a ceremonial feast for and with their ancestors. It is still a mystery how the Chinese of the fifth century B.C. were able to cast two-tone bells. The bronze bells of the Marqui Yi weigh about 2500 kg, they are nicely decorated, and must have represented a large portion, if not all, of the wealth of their owner.
It is believed today that China developed its bronze technology only slightly later than the West and independently from the out-side world. It grew, as in the West, out of the ceramic tradition.
Chinese potters achieved kiln temperatures as high as 1400°C which allowed them to produce their unique porcelain. In con-trast to Thailand, China briefly went through an initial copper smelting period (at around 2000 B.C.). But arsenic–bronzes seem to be absent in China. Furthermore, two types of bronzes were developed, one consisting of the usual copper–tin alloy and the other of copper with lead. Also interesting is a find of a piece of brass (copper–zinc), dating back to about 2200–2000 B.C., which was probably smelted from zinc-bearing copper ore.
As outlined above, both the Chinese and the Thais possessed ample copper as well as tin raw materials. This was definitely not true for the Chalcolithic man residing in the Near East. Ma-jor bronze-producing centers in 2000 B.C. were in Mesopotamia,1 Assyria, Anatolia, and Cyprus.2,3All of these centers were blessed with abundant deposits of copper ore, as described in Chapter 1.
However, no tin seemed to have been found in the vicinity of these places. As a matter of fact, the major tin deposits as known today are in China, Thailand, Malaysia, England, Germany, Nige-ria, Zaire, Australia, Bolivia, and Mexico. They consist of tin ox-ide, or cassiterite (see Plate 4.1), which is inserted into granite in the form of veins. Cassiterite4 is broken down by water and washed into rivers where it can be panned like gold. Minor de-posits were possibly in Italy, Spain, France (at the mouth of the river Loire) and Sardinia.
In short, reputed archaeological evidence seems to point to the fact that, during the Chalcolithic time, no major known tin sources were situated in the Near East except possibly for some
1Mesopotamia once lay between the lower Tigris and lower Euphrates rivers and is today part of Iraq. (From Greek: mesos middle and po-tomos river.)
2Kypros (Greek) copper.
3Aes cyprium (Latin) copper; aes (Latin) ore.
4Kassiteros (Greek) tin.
native tin in the Zagros mountains on the eastern edge of the Mesopotamian plain. If they existed, however, they were quickly exhausted. Instead, documents have been found which record immense tin transporting caravans. Further, coastal ships could have carried tin from the Far East into the Mediterranean area.
(An ancient shipwreck bearing tin has recently been found off the coast of Israel.) All taken, it is still a mystery today how the Bronze Age could have started already around 2000 B.C. in the Mediterranean area when no tin was readily available for exper-imentation or the accidental discovery of bronze. The transfer of bronze technology from the Far East or another area hitherto unknown to the West therefore should be considered to be a pos-sibility.
One of the theories postulates that tin may have come from the southern slopes of the Caucasus (now Armenia) where both malachite (copper ore) and cassiterite (tin ore) are found. These minerals may well have been accidentally smelted together, thus yielding tin–bronze.
Other scholars believe that trade connections between the Mid-dle East and Eastern Europe (where tin occurs in Bohemia, Sax-onia, etc.) existed as early as 2500 B.C. This brings us to focus our attention on the Europeans, in particular on the Uneticians (named after a village near Prague) who by 1500 B.C. had become the dominant people in Europe. Their influence extended over a large territory, that is, from the Ukraine to the Rhine valley. The Uneticians were skillful bronzesmiths who manufactured in large quantities items such as pins (to hold garments together), tools (such as axes and plowshares), weapons, and jewelry. Actually, one item, a neck ring, was produced in such large quantities that it served as a kind of a currency, that is, it was exchanged for gold, furs, amber, and glass beads. These neck rings were, inci-dentally, quite similar in appearance to those found in Syria. The Uneticians traded not only with the south, but also with the British Islands, Scandinavia, and Ireland, where the fruits of their work have been found in many graves. They were ingenious in-ventors of new applications or copied items which they liked. In-deed, the safety pin was a product of the Uneticians. Archaeolo-gists have found at Unetician sites such items as knitting needles, the remnants of an elaborate loom, and a strainer for the pro-duction of cheese. A large find in bronze-metal artifacts (axes, chisels, spears, knives, bracelets, pins, and a chain) was uncov-ered in a peat swamp located at the Federsee (a lake in southern Germany) which remarkably preserved a Bronze Age settlement.
Another area where bronze technology was practiced was lo-cated near the Indus river in ancient northwest India. There, a
highly developed civilization, called the Harappan people, had settled between about 7000 and 1500 B.C. (until the Aryans in-vaded the land). Excavations at Mehrgarh (today’s Pakistan) have demonstrated that the Harappans must have been skilled bronze workers as early as 2300 B.C., applying the lost wax casting tech-nique, annealing, and riveting. They produced human figurines, vessels, arrowheads, spearheads, knives, and axes. The sickles which were unearthed suggest that bronze articles were utilized to support agriculture. Copper ores for these activities probably came from plains near the south end of the Indus river (Mohenjo-daro) and from areas northwest of the Indus valley, which is to-day’s Afghanistan, as evidenced by large heaps of copper slag.
The copper ingots of this area had the form of a semicircle. As in the Near East case, the origin of tin is, however, not quite clear at present. Some scholars believe that it came from the Deccan Plateau in central and western India. Copper smelting was not confined to the Harappan urban area. Instead, many regional In-dian cultures likewise smelted copper and bronze even though not always with the same sophistication.
A written document concerning bronze can be found in Greek mythology. Homer, in his Iliad (which is assumed to have been created between 800 and 700 B.C.), reports of Hephaestus, the Greek god of fire, who throws copper and tin along with silver and gold into his furnace, thus creating a superior shield for Achilles.
Finally, the early inhabitants of the central highlands of Peru before and during the Inca period engaged in some bronze tech-nology which is believed to have started around A.D. 1450 or pos-sibly even somewhat earlier. In contrast to the European and Asian customs, however, the maximal arsenic content of the goods (pins, chisels, axes) was only 1.5% and the tin concentra-tion was 3% or less. Many artifacts found in this area did not contain alloy constituents in amounts that would significantly al-ter the mechanical properties. It is therefore questionable that the alloying was done intentionally. In any event, potential tin sources would have been close by, i.e., in northern Bolivia.
In summary, elaborate bronze technologies existed in various areas of the ancient world, not only in the Middle East, as some-times assumed.
Now that we know from the above presentation that bronze is harder than copper and that bronze has a lower melting point than copper, we certainly should be eager to know which mech-anisms govern these properties. We shall explain this in the chap-ters to come.
W.T. Chase, Ancient Chinese Bronze Art, China House Gallery, China Institute in America, New York (1991).
B. Cunliffe (Editor), The Oxford Illustrated Prehistory of Europe, Oxford University Press, Oxford (1994).
O. Dickinson, The Aegean Bronze Age, Cambridge University Press, Cambridge, UK (1994).
W. Fong (Editor), The Great Bronze Age of China, Knopf, New York (1980).
C. Higham, The Bronze Age of Southeast Asia, Cambridge Uni-versity Press, Cambridge, UK (1996).
N.G. Langmaid, Bronze Age Metalwork in England and Wales, Shire Archaeology Series, Shire Publications, Aylesbury, UK (1976).
J. Mellaart, The Chalcolithic and Early Bronze Ages in the Near East and Anatolia, Khayats, Beirut (1966).
Suggestions for Further Study
5
Pure materials have a number of inherent mechanical properties, as discussed in Chapter 3. These features, such as strength or duc-tility, can be altered only to a limited degree, for example, by work hardening. In contrast to this, the properties of materials can be varied significantly if one combines several elements, that is, by alloying. In this chapter, we shall unfold the multiplicity of the mechanical properties of alloys and compounds with particular emphasis on the mechanisms which are involved. Specifically, we shall discuss a number of techniques which increase the strength of materials. Among them are solid solution strengthening, pre-cipitation hardening (age hardening), dispersion strengthening, and grain size strengthening. In order to understand these mech-anisms, we need to study the fundamentals of phase diagrams.
When certain second constituents such as tin or nickel are added to copper, the resulting alloy has a noticeably larger yield strength than pure copper, as depicted in Figure 5.1. The added atoms (called solute atoms) may substitute up to a certain limit regular lattice atoms (called solvent or matrix). The resulting mixture is then said to be a substitutional solid solution. In many cases, the size of the solvent atoms is different from the size of the solute atoms. As a consequence, the lattice around the added atoms is distorted, as shown in Figure 5.2. The movement of dislocations upon application of a shear stress is then eventually obstructed and the alloy becomes stronger but less ductile. (Only in cop-per–zinc alloys do both strength and ductility increase simulta-neously.) This mechanism is called solid solution strengthening.
Solid solution strengthening is greater (up to a certain limit) the more solute atoms are added (see Figure 5.1). Specifically, the yield strength of an alloy increases parabolically with the solute