THE CASTING OF METAL is a prehistoric technology, but one that appears relatively late in the archaeological record.
There were many earlier fire-using technologies, collectively called by Wertime pyrotechnology, which provided a basis
for the development of metal casting.
Among these were the heat treatment of stone to make it more workable, the
burning of lime to make plaster, and the firing of clay to produce ceramics. At first, it did not include smelting, for the
metal of the earliest castings appears to have been native.
The earliest objects now known to have been have of metal are more than 10,000 years old (see Table 1) and were
wrought, not cast. They are small, decorative pendants and beads, which were hammered to shape from nuggets of native
copper and required no joining. The copper was beaten flat into the shape of leaves or was rolled to form small tubular
beads. The archaeological period in which this metalworking took place was the Neolithic, beginning some time during
the Aceramic Neolithic, before the appearance of pottery in the archaeological record.
9000 B.C. Earliest metal objects of wrought native copper Near East
6500 B.C. Earliest life-size statues, of plaster Jordan
5000-3000 B.C. Chalcolithic period: melting of copper; experimentation with smelting Near East
3000-1500 B.C. Bronze Age: arsenical copper and tin bronze alloys Near East
3000-2500 B.C. Lost wax casting of small objects Near East
2500 B.C. Granulation of gold and silver and their alloys Near East
2400-2200 B.C. Copper statue of Pharoah Pepi I Egypt
2000 B.C. Bronze Age Far East
1500 B.C. Iron Age (wrought iron) Near East
700-600 B.C. Etruscan dust granulation Italy
600 B.C. Cast iron China
224 B.C. Colossus of Rhodes destroyed Greece
200-300 A.D. Use of mercury in gilding (amalgam gilding) Roman world
1200-1450 A.D. Introduction of cast iron (exact date and place unknown) Europe
Circa 1122 A.D. Theophilus's On Divers Arts, the first monograph on metalworking written by a craftsman Germany
1252 A.D. Diabutsu (Great Buddha) cast at Kamakura Japan
Circa 1400 A.D. Great Bell of Beijing cast China
16th century Sand introduced as mold material France
1709 Cast iron produced with coke as fuel, Coalbrookdale England
1715 Boring mill or cannon developed Switzerland
1735 Great Bell of the Kremlin cast Russia
1740 Cast steel developed by Benjamin Huntsman England
1779 Cast iron used as architectural material, Ironbridge Gorge England
1826 Zinc statuary France
1838 Electrodeposition of copper Russia, England
1884 Electrolytic refining of aluminum United States, France
Native metals were then perhaps considered simply another kind of stone, and the methods that had been found useful in
shaping stone were attempted with metal nuggets. It seems likely that the copper being worked was also being annealed,
because this was a treatment that already being given store. Proof of annealing could be obtained from the microstructures
of these early copper artifacts were it not for their generally corroded condition (some are totally mineralized) and the
natural reluctance to use destructive methods in studying very rare objects.
The appearance of plasters and ceramics in the Neolithic period is evidence that the use of fire was being extended to
materials other than stone. Exactly when the casting of metals began is not known. Archaeologists give the name
Chalcolithic to the period in which metals were first being mastered and the date this period, which immediately preceded
the Bronze Age, very approximately to between 5000 and 3000 B.C. Analyses of early cast axes and other objects give
chemical compositions consistent with their having been cast from native copper and are the basis for the conclusion that
the melting of metals had been mastered before smelting was developed. The furnaces were rudimentary. It has been
shown by experiment that it was possible to smelt copper, for example, in a crucible. Nevertheless, the evidence for
casting demonstrates an increasing ability to manage and direct fire in order to achieve the required melting temperatures.
The fuel employed was charcoal, which tended to supply a reducing atmosphere where the fire was enclosed in an effort
to reduce the loss of heat. Smelting followed.
The molds were of stone The tradition of stone carving was longer than any of the pyrotechnologies, and the
level of skill allowed very finely detailed work. The stone carved was usually of a smooth texture such as steatite or
andesite, and the molds produced are themselves often very fine objects, which can be viewed in museums and
archaeological exhibitions. Many are open molds, although they were not necessarily intended for flat objects. Elaborate
filigree for jewelry was cast in open molds and then shaped by bending into bracelets and headpieces, or cast in parts and
then assembled. Certain molds, described by the archaeologist as multifaceted, have cavities carved in each side of a
rectangular block of stone. Such multifaceted molds would have been more portable than separate ones and suggest
itinerant founding, but they may simply represent economy in the use of a suitable piece of stone.
The Bronze Age
The Bronze Age began in the Near East before 3000 B.C. The first bronze that could be called a standard alloy was
arsenical copper, usually containing up to 4% As, although a few objects contain 12% or more. This alloy was in
widespread use and occurs in objects from Europe and the British Isles (Fig. 2) as well as the Near East. The metal can
sometimes be recognized as arsenical copper by the silvery appearance of the surface, which occurred as a result of
inverse segregation of the arsenic-rich low-melting phase to the surface. This is the same phenomenon that produces tin
sweat on tin bronzes, and it led earlier excavators to describe these artifacts as silver plated. A few examples of arsenic
plating on tin bronze can be seen on objects from Anatolia and Egypt, but the plating method is not known.
The use of 5 to 10% Sn as an alloying element for copper has the obvious advantages of lowering the melting point,
deoxidizing the melt, improving strength, and producing a beautiful, easily polished cast surface that reproduces the
features of the mold with exceptional fidelity--vitality important properties for art castings (Fig. 3). There are several
hypotheses to explain the development of tin bronze. One is that of the so-called natural alloy, that is, metal smelted from
a mixed ore of copper and tin. Another suggests the stream tin (tin ore in the form of cassiterite) may have been added
directly to molten copper. The more vexing question has been the sources of the tin, copper, and silver that have been
excavated from sites in such areas as Mesopotamia, which lack local metal resources. Cornwall or Afghanistan was long
thought to have been the source of this early tin, but more recent investigations have located stream tin in the Eastern
Desert of Egypt and sources of copper and silver as well as tin in the Taurus mountains of south central Anatolia in
modern Turkey.
Recent experiments have shown that metal cast into an open mold is sounder if the open face is covered after the mold
has been filled. This observation may have led to the use of bivalve (permanent two-part) molds. They were in common
use for objects having bilateral symmetry, such as axes of various designs and swords. The molds were made such that
the flash occurred at the edge, which required finishing to sharpen (Fig. 4). These edges are often harder than the body of
the object, evidence of deliberate work hardening. There is also evidence in the third millennium B.C. for the lost wax
casting of small objects of bronze and silver, such as the stag from Alaça Hoyük, now in Ankara. This small object is also
of interest because the casting sprues were left in place attached to the feet, clearly showing how the object was cast.
Although there is abundant evidence from such objects that lost wax casting was employed early in the Bronze Age, the
remnants of the process, such as broken investment and master molds, have eluded researchers. Wax may well have been
the material of the model; other material may have been used, but no surviving evidence of any of these materials has
been recognized. Similarly, the mold dressings used then and later remain unknown. Nevertheless, discoveries are
occasionally made that greatly enlarge the geographical area in which lost wax casting in thought to have taken place.
One of these discoveries occurred in 1972 at a site in England called Gussage All Saints.
At Gussage, an Iron Age (first century B.C.) factory was excavated. The lost wax process was used in this factory for the
mass production of bronze bridle bits and other metal fittings for harnesses and chariots. More than 7000 fragments of
clay investment molds were recovered (Fig. 5), along with crucible fragments, charcoal slag, and other debris thought to
represent the output of single season. The bronze was leaded and in one case had been used to bronze plate a ring of
carbon steel by dipping. This is the first site in Great Britain where direct evidence of lost wax casting has been found, yet
the maturity of the industry suggests that earlier sites remain to be located.
The Far East
The Bronze Age in the Far East began in about 2000 B.C. more than a millennium after its origin in the Near East. It is
not yet clear whether this occurred in China or elsewhere in southeast Asia, and there are vigorous efforts underway to
discover and interpret early metallurgical sites in Thailand. The later date for the development of metallurgy in the Far
East let to an obvious assumption that the knowledge of metal smelting and working had entered the area by diffusion
from the West. This assumption was countered by mapping the geographical distribution of dated metallurgical sites in
China, which indicates development in a generally east-to-west direction. The question of independent origin for the
metallurgy of southeast Asia remains open.
Casting was the predominant forming method in the Far East. There is little evidence of other methods of metalworking
in China before about 500 B.C. Antique Chinese cast bronze ritual vessels were of such complexity that it was the opinion
until recently that these must have been cast by the lost wax method. This had also been the opinion of Chinese scholars
in recent centuries. In the 1920s, however, a number of mold fragments were unearthed at Anyang, prompting
reevaluation of the lost wax hypothesis. The molds were ceramic, and they were piece molds.
Very early Bronze Age sites, approximately 2000 B.C., in Thailand present similar evidence. At one of these sites a burial
was unearthed that contained the broken pieces of an apparently unused ceramic bivalve mold. The bronze founder had
been buried with a piece of the mold in each hand.
The Chinese mold was a ceramic piece mold, typically of many separate parts. The wall sections of the vessels cast in
these molds are quite thin and testify to very fine control over the design of the molds and pouring of the metal. The
metal, usually a leaded tin bronze, was used to great effect but also in an economical manner. Parts, such as legs, which
could have been cast solid, were instead cast around a ceramic core held in position in the mold by chaplets. The chaplets
took several forms; some were cross shaped, others square. They were of the same alloy as the vessel but can clearly be
seen in radiographs. They have occasionally become visible on the surface because their patina appears slightly different
from that of the rest of the vessel.
Metal parts that in the Western tradition would have been made separately and then joined by soldering or welding were
incorporated into Chinese vessels by a sequence of casting on. Handles and legs might be cast first, the finished parts set
in the mold, and the body of the vessel then poured Elaborate designs demanded several such steps. An unusual
feature of this way of thinking about mold making and casting metal is the deliberate incorporation of flash into the
design elements.
The surface decoration of the vessels sometimes employed inlay or gilding, but even in these examples much of the
decoration is cast in. Various decorative elements may have been molded from a master model, impressed into the mold
with loose pieces, or incorporated by casting on metal elements. By using a leaded tin bronze, the founder increased the
fluidity of the melt and consequently the soundness of the casting even in the usual thin sections. However, such a fluid
melt also has a greater tendency to penetrate the joints between the pieces of the mold so as to produce flash. If the
surface of the bronze is meant to be smooth, the flash must be trimmed away. The Chinese founders eventually took this
casting flaw and made it a deliberate element of their design. The joints of the mold were placed in relation to the rest of
the surface decoration such that the flash needed only to be trimmed to an even height to be accepted as part of the cast-in
decoration.
Cast Iron
Cast iron appeared in China in about 600 B.C. Its use was not limited to strictly practical applications, and there are many
examples of Chinese cast iron statuary. Most Chinese cast irons were unusually high in phosphorus, and, because coal
was often used in smelting, high in sulfur as well. These irons, therefore, have melting points that are similar to those of
bronze and when molten are unusually fluid. The iron castings, like the Chinese cast bronzes, are often remarked upon for
the thinness of their wall sections.
There is some dispute concerning the date of the introduction of cast iron into Europe and the route by which it came.
There is less disagreement about the assumption that it was brought from the East. The generally agreed upon date for the
introduction of cast iron smelting into Europe is the 15th century A.D.; it may have been earlier. At this time, cast iron
was less appreciated as a casting alloy than as the raw material needed for "fining" to wrought iron, the form in which
iron could be used by the local blacksmith.
The mass production of cast iron in the West, as well as its subsequent use as an important structural material, began in
the 18th century at Coalbrookdale in England. Here Abraham Darby devised a method of smelting iron with coal by first
coking the coal. He was successful because the local ores fortuitously contained enough manganese to scavenge the sulfur
that the coke contributed to the iron. The vastly greater amounts of cast iron that could be produced by using coke rather
than charcoal from dwindling supplies of timber were eventually put to use nearby in erecting the famous Iron Bridge
The dome of the United States Capitol Building is an example, as is the staircase designed by Louis Sullivan for the
Chicago Stock Exchange now at the Metropolitan Museum in New York City. Cast iron architectural elements were
usually painted; the Capitol dome is painted to resemble the masonry of the rest of the building. Finishes other than paint
were also used. The Sullivan staircase was copper plated and then patinated to give it the appearance of having been cast
in bronze. Another method suitable for interior iron work was the treatment of the surface by deliberate light rusting,
followed by hydrogen reduction of the rust. This produced a velvety black adherent layer of magnetite (Fe3O4) that was
both attractive and durable.
Granulation
Not all casting requires a shaped mold. The exploitation of surface tension led to granulation. The tiny spheres produced
when small amounts of molten metal solidified without restraint were being used as decoration in gold jewelry by 2500
B.C. Granulation was primarily done in gold, silver, or the native alloy of gold and silver called electrum. Some granules
were attached to copper or gilt-silver substrates. The finest work in granulation was done by the Etruscans in about the
seventh century B.C. Its fineness has given it the name "dust granulation," the granules being less than 0.2 mm (0.008 in.)
in diameter. Many thousands of granules were used to create the design on a single object. The Etruscan alloy was gold
with about 30% Ag and a few percent of copper. The method of joining the granules varied. Sweating or soldering have
both been observed, but the exact method used is often still a matter of dispute.
Tumbaga
New World metallurgy is a metallurgy almost without iron. The exception was the use of meteoric iron, which was most
important among the Eskimos, who traded it all across the North. Copper-using cultures flourished further south until the
sources of native copper were exhausted. There is no evidence of smelting among the native population of what is now
the United States until the arrival of the Europeans.
In South America, however, the story is quite different. Early European explorers were overwhelmed by the amount of
gold and silver objects they found. Many of these objects were of sheet gold or its alloys, and it has been suggested that
sheet metal was viewed then as a kind of textile, as textiles in these cultures were not limited to clothing and were used
for weapons and armor. The most interesting castings are of an alloy called tumbaga, which contained gold, silver, and
copper in various proportions. Molds have been found (some never used) that were made by the lost wax process. After
an object had been cast in tumbaga, it was pickled in a corrosive solution that attacked the silver and especially the copper
and, when rinsed off, left a surface layer enriched in gold. This method of gilding is called mise-en-couleur, or "depletion
gilding."
Africa
Africa, where sculpture is often the province of the blacksmith, presents several interesting traditions of casting. Among
them are the famous Benin bronzes of Nigeria and the gold weights of Ghana, formerly the Gold Coast. Both of these
traditions produced castings in brass, with the brass having a high enough zinc content to appear golden. The source of
the brass, or at least that of the zinc, may well be indicated by the portrait of a Portuguese trader in a Benin bronze (Fig.
8). Recent discoveries of zinc furnaces and distillation retorts at Zawar, near Udaipur in India, as well as the very long
trade routes that were opened in the 17th century, suggest the possibility that the metal may have been traded from India.
The Benin bronzes were cast by the lost wax process, and the traditional method has been recorded on film.
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