An Encyclopedia of the History of Technology part 8 ppt

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52
relatively pure native metal. The rate of this transformation increased rapidly
soon after the establishment of the Tigris and Euphrates civilizations. The
wealth and specialized demand provided by these urban societies must have
stimulated early copper workers to prospect in the northern mountainous
regions where weathered outcrops of copper were most likely to be
encountered.
The earliest copper workers appear to have extracted their metal from oxide
or carbonate ores which, although not always rich or plentiful, could generally
be smelted successfully in the primitive furnaces then available. The early
smelters all appeared to understand instinctively that charcoal fires could be
adjusted to provide atmospheric conditions which simultaneously reduced
copper and oxidized iron. Methods were thus evolved which allowed relatively
pure copper to be separated in the molten state from iron and other unwanted
materials in the ore. These, when suitably oxidized, could be induced to
dissolve in the slag. The primary ores of copper are invariably complex
sulphides of copper and iron, and are generally disseminated in a porous rock
such as sandstone which rarely contains more than 2 per cent by weight of
copper. Such deposits were too lean to be exploited by primitive man, who
sought for the richer if more limited deposits produced by the weathering and
oxidation of primary ores. Thus, at Rudna Glava in Yugoslavia, a copper mine
worked in the 6th millennia, did not exploit the main chalcopyrite ore body,
but worked instead a thin, rich carbonate vein produced by leaching and
weathering. This concentrated ore contained 32 per cent of copper and 26 per
cent of iron. Quartz sand would have been added to such a smelting charge to
ensure that most of the iron separated into the slag.
At Timna, in the southern Negev, copper has been mined and smelted since
the dawn of history. Extensive workings, slag heaps and furnaces have
remained with little disturbance since Chalcolithic, Iron Age and Roman times.
These mines, traditionally associated with King Solomon, were in fact worked

by the Egyptian Pharaohs during much of the Iron Age until 1156 BC.
The primary ore deposit at Timna is based on the mineral chrysocolla and
is currently being exploited on a large scale. Since this ore contains only about
2 per cent of copper, it could not have been effectively smelted in ancient
times. During the Chalcolithic or historical periods copper was extracted at
Timna from sandstone nodules in the Middle White sandstone beds overlying
the chrysocolla deposits. The nodules contain between 6 and 37 per cent of
copper, which exists as the minerals malachite, azurite and cuprite. The
remainder is largely silica, and the nodules contain little iron. In the fourth
millennium BC copper was extracted from them in furnaces: a rough hole in
stony ground, approximately 30cm (1ft) in diameter, was surrounded by a
rudimentary stone wall to contain the charge, which consisted of crushed ore
mixed with charcoal from the desert acacia. Controlled quantities of the
crushed iron oxide haematite, was added to the charge as a flux, to reduce the
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Figure 1.3: Reconstructions based on the remains of smelting furnaces used (a) in
the twelfth century BC, and (b) in the eleventh century BC, at the ancient
copper smelting site of Timna in the Southern Negev region of Israel.
Courtesy of the Institute of Metals.
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54
melting point of the silicious material and improve the separation between the
copper and the slag.
The Chalcolithic smelting furnaces at Timna appeared to have had no tap
hole and no copper ingots were found. High concentrations of prills and blebs
of metallic copper were found in the slag, however, and it seems possible that
the metal never, in fact, separated from the slag in massive form: after smelting
the slag would have been broken up to remove the prills which were then
remelted together in a crucible furnace.

A more highly developed smelting furnace used by the Egyptians at Timna
around 1200 BC is shown in Figure 1.3 (a). The slags from such furnaces
contained up to 14 per cent of lime which was added to the charge as crushed
calcareous shells from the Red Sea. This addition would have improved slag
metal separation and allowed the reduced copper to settle to the bottom of the
furnace and to solidify below the slag as plano-convex ingots.
Smelting techniques appeared to have reached their zenith at Timna around
1100 BC. After the smelting operation the slag and metal appear to have been
tapped simultaneously from the furnace into a bed of sand where, as recent
simulation experiments by Bamberger have shown, they would have remained
liquid for about fifteen minutes, providing ample time for the molten copper to
sink beneath the slag to form well-shaped ingots about 9cm (3.5in) in diameter.
For Bamberger’s reconstruction see Figure 1.3 (b).
The rings and other small artefacts of iron found at Timna are now thought
to have been by-products of the main copper refining operation. Lead isotope
‘finger-printing’ has shown that the source of the iron was the haematite used
to flux the copper ore during refining. It would appear that when the ‘as
smelted’ copper was remelted in a crucible furnace in preparation for the
casting of axes and other artefacts, any surplus iron it contained separated at
the surface of the melt to form a sponge-like mass permeated by molten
copper. This layer would, in all probability, have been skimmed from the
surface of the melt before it was poured. At a later stage it must have been
found that the iron/copper residue could be consolidated by hot forging and
worked to the shape required. The presence of copper is known to improve the
consolidation of iron powder, and it would seem, therefore, that a sophisticated
powder metallurgical process, utilizing liquid phase bonding, was being
operated at Timna in Iron Age times.
Smelting of sulphide copper ores
From the presence of arsenic and other impurities in many of the early Copper
Age artefacts it must be concluded that much of the copper used was extracted

from sulphide rather than oxide or carbonate ores. In prehistory, as in modern
times, the bulk of the world’s supply of copper appears to have been obtained
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from ores based on chalcopyrite, a mixed sulphide containing equi-atomic
proportions of iron and copper. Chalcopyrite must be roasted in air to convert
it to a mixture of iron and copper oxides before it can be smelted. Moreover,
because chalcopyrite ores contain in general less than 2 per cent of copper, and
because of the presence of large quantities of unwanted earthy material, they
do not respond to simple smelting processes.
The sulphide copper ores exploited at the beginning of the Copper Age
appear to have been thin, localized and very rich deposits which lay some
distance below the surface of a weathered and oxidized primary outcrop.
Although the presence of such enrichment zones has been recognized by
mining engineers and geologists for many years, their significance as ancient
sources of copper has only recently been fully appreciated.
Due to the atmospheric oxidation which occurs at the surface of
chalcopyrite outcrops, the sulphides are partially converted to more soluble
compounds which are slowly leached away. The exposed surface of such an
outcrop is therefore slowly robbed of most of its heavy metals with the
exception of iron which concentrates at the surface as iron oxide, generally in
the form of limonite. The iron-oxide regions above rich copper deposits are
known as gossans, and the German term eisener Hüt to describe a gossan led
to the saying, ‘For a lode nothing is better than it should have a good iron
hat,’ and extensive gossans are noteworthy features of most of the sulphide
copper ore deposits which were worked in antiquity. At Rio Tinto in south-
west Spain, where copper has been extracted from the earliest times, iron is
so extensively exposed at the surface that the terrain resembles that of an
open-cast iron ore mine. Streams such as the Rio Tinto and Aguar Tenidas
which leave this gossan owe their names to the red contaminant iron oxide.

The ancient mines at Oman, which provided the Sumerians with copper, are
characterized by huge ferruginous gossans, and similar terrain exists at
Ergani Maden in Turkey.
From these gossans the copper content has been completely leached away,
and transferred in aqueous solutions to lower horizons, where, as the dissolved
oxygen becomes depleted, it is precipitated in sulphide form in the surrounding
strata. In this way zones of secondary enrichment are formed which contain
most of the metallic content which was originally uniformly distributed
throughout considerable depths of rock. The average copper content of the
thin secondary enrichment zones at Rio Tinto sometimes approaches 15 per
cent and values as high as 25 per cent of copper in the enrichment zones at
Ergani Maden have been reported.
The arsenical and antimonial minerals are associated with the copper in the
cementation zone, and the preponderance in the Copper Age of artefacts
containing substantial quantities of arsenic is therefore a further indication of
the fact that much copper of this period came from these thin zones of
secondary enrichment.
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Arsenical copper
When an awareness that many early copper artefacts were in fact dilute alloys
of arsenic in copper began to develop after 1890, it was thought that arsenic
had been deliberately added to improve the mechanical properties. Bertholet,
in 1906, was the first to demonstrate that the concept of alloying would have
been unknown when these artefacts came into general use, and that arsenical
copper, which was extensively used by the ancient Egyptians, was a natural
alloy obtained by smelting arsenical copper ore.
In the cast condition arsenical copper is only marginally harder than pure
copper, although its hardness increases very rapidly as a result of cold working.
This must have been a factor of great importance in the Copper Age when

edge tools were invariably hardened and sharpened by hammering. Arsenic
deoxidizes copper very effectively and the alloys are noted for their excellent
casting characteristics, a factor which early copper workers would much have
appreciated. Cast billets of arsenical copper would, however, have been more
difficult to work than pure copper, and it seems evident that when not cast
directly to the required shape weapons and cutting tools were fashioned by a
judicious mixture of hot and cold working. Such implements would certainly
have retained their cutting edges for longer periods than their pure copper
counterparts.
When arsenical copper ores are smelted with charcoal, the arsenic has little
tendency to escape because it is held in solution by the molten copper. Similarly,
when copper alloys containing up to 7 per cent of arsenic are melted in a crucible
under reasonably reducing conditions, little is lost. Arsenious oxide, however, is
very much more volatile than elementary arsenic, and toxic fumes must certainly
have been emitted in copious quantities during the roasting of copper sulphide
ores containing arsenic. Since sulphur dioxide fumes would also have been given
off in vast quantities, the additional health hazards caused by arsenic would not,
however, have been separately identifiable. Certainly nothing appears to have
inhibited the use of these accidental arsenical copper alloys, which were
extensively produced for a very long period over most regions of the ancient
world. Primitive man, when he stumbled upon the thin layers of concentrated
copper ores immediately below the gossan must soon have appreciated that they
produced artefacts having properties vastly superior to those of the copper
hitherto obtained from oxide or carbonate ores. On the basis of gradually
accumulated experience he would have sought for similar or even richer ores in
other localities, but in view of the absence of any concept of alloying it seems
unlikely that attempts were made to control the hardness of the copper obtained
by adjusting the mixture of ores fed into the smelting furnace.
The beginning of the arsenical copper era is difficult to date with any
certainty. The earliest Egyptian artefacts of arsenical copper were produced

around 4000 BC in predynastic times. In addition to arsenic these early
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weapons contained around 1.3 per cent of nickel. All the copper or bronze
artefacts found by Sir Leonard Woolley during his excavations of the Royal
Graves at Ur contained similar quantities of nickel which at the time these
discoveries were made was considered to be a most unusual constituent of
ancient copper.
It has been suggested that the Sumerians who made these artefacts came
from the Caucasus and were instrumental in transferring the arts of metallurgy
from the land of Elam (which now forms part of Iran) into Babylonia. The first
Sumerian kingdom was destroyed by the Semitic King Sargon, and one of his
inscriptions, dating from 2700 BC, indicated that the Sumerians obtained their
copper from the copper mountain of Magan. Shortly after the Royal Graves
excavation at Ur, the remains of extensive ancient copper workings were
discovered in the vicinity of the small village Margana in Oman. The impurity
spectrum of this copper ore deposit, including the nickel content, corresponded
exactly with that of the artefacts of Ur.
Oman, however, is a long way from Babylonia, and since nickel bearing
copper ores are also found in India and in the Sinai desert, the true origins of
Sumerian copper are still uncertain. Indirect evident for direct links between
Ur and Oman is provided, however, by evident similarities between the
smelting techniques used to produce plano-convex copper ingots at Suza in
Iran and at Tawi-Aaya in Oman, during the third millennium BC. The
archaeological evidence also indicates strong cultural links between Southern
Iran and Oman at this time.
Some of the slags found at Oman contained copper sulphide matte. Well-
roasted sulphide ores can be effectively reduced to metal by techniques similar
to those used to treat oxide or carbonate ores. Separation of the copper from
the slag is facilitated by small flux additions such as bone ash, and this echoes

the sea-shell additions made at Timna (see p.54). The essential difference
between the treatment of oxide and roasted sulphide ores, however, is that
small quantities of copper sulphide matte appear to be produced in that
furnace when the roasted sulphide ore is being treated under atmospheric
conditions which reduce the copper, but retain the iron in an oxidized state.
This matte, being insoluble in the slag, separates out as a thin silvery crust on
the surface of the copper ingot. The presence of copper matte in ancient slag
deposits is not, therefore, conclusive evidence of matte smelting.
TIN AND BRONZE
The Early Bronze Age
For well over 1500 years arsenical copper artefacts were extensively produced
in the ancient world, which had come to expect from its metallic implements
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standards of hardness and durability which were unattainable from pure
copper. After about 3000 BC, however, the arsenical content of Middle Eastern
copper artefacts begins to decline, perhaps because of the progressive
exhaustion of rich, accessible arsenical copper ore deposits. Artefacts
containing small quantities of tin made their first appearance during the early
stages of this decline although the quantities involved rarely exceeded about
2.5 per cent. Such alloys, which first appeared in Iran around 3000 BC, did
not reach regions such as England and Brittany, on the outer fringes of
civilization, until about 2200 BC.
These low tin bronzes must be regarded as the precursors of the later true
bronzes containing 8–10 per cent of tin. They presumably emerged
accidentally, either by the smelting of copper ores which contained tin
minerals, or by the use of tin-bearing fluxes.
If the possibilities of alloying had been grasped at this stage it seems logical
to assume that they would have been progressively exploited. No evidence for
such a gradual evolutionary process has been found, and virtually no artefacts

containing more than 2.5 per cent of tin have been identified which antedate
the sudden emergence of the 8–10 per cent tin bronzes in Sumeria between
3000 and 2500 BC. It would appear that the copper workers of Ur suddenly
discovered or acquired the concept of alloying, and rapidly developed and
optimized the composition of bronze. This sudden leap forward cannot be
separated from the rapid expansion of trade in the Middle East around 3000
BC, since it seems improbable that the significance of tin as an additive to
copper would have first been identified in a region where its ore was not
indigenous.
Sources of ancient tin
The most abundant and significant tin mineral is cassiterite, the oxide Sn O
2
,
which varies in colour from brown to black. Cassiterite is noteworthy in that
its high specific gravity of 7.1 is comparable to that of metallic iron, and also
because its hardness is comparable to that of quartz, so that it is highly
resistant to abrasion and tends to concentrate in gravels and alluvial deposits.
The name tin appears to be derived from the Chaldaean word for mud or
slime, which implies that the tin originally used at Ur came from alluvial
deposits. The Greek word kassiteros was taken from a Celtic term which has
been literally translated as ‘the almost separated islands’, presumably the
mythological tin islands. In classical times, kassiteros was loosely used to denote
tin, pewter and sometimes even lead. The Greek word for Celtic, kentikov, was
also used by Aristotle to describe metallic tin.
Four cassiterite mines have been located in the eastern Egyptian desert, one
of which was accurately dated from inscriptions associated with the Pharaoh
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Pepi II to around 2300 BC, approximately 500 years after the appearance of
the first Sumerian bronze. Copper artefacts containing significant quantities of

tin were not produced in Egypt before the Fourth Dynasty, around 2600 BC,
and the true Bronze Age in Egypt, when artefacts containing 8–10 per cent of
tin were produced, did not begin until 2000 BC.
It seems logical to assume that at some time around 3000 BC, the concept
of bronze manufacture was acquired by the Sumerians, either by trade or
conquest, from a region outside Mesopotamia where tin was a readily
available commodity. Many historians have claimed that by 3000 BC
commerce in the Middle East had developed so extensively that supplies of
tin might well have reached Sumeria by sea, up the Persian Gulf, from
regions as remote as Malaysia or Nigeria. Support for this general idea is, of
course, provided by the known importation by Sumeria of copper from
Oman. From the Gulf, the tin would have been transported by overland
caravan. Between 2600 and 2500 BC supplies of tin to Mesopotamia appear
to have been interrupted, many of the copper artefacts dating from the
Second Sumerian Revival being simple arsenical coppers containing little or
no tin. This occurred, of course, in biblical times, when the Land of Sumer
was devastated by floods. The shortage of tin, whether it was caused by
natural disasters or by political upheavals, was not prolonged, however, and
after about 2500 BC the use of 8–10 per cent tin bronzes expanded rapidly
throughout the Middle Eastern world.
Early Bronze Age developments have also been found in southern China,
Thailand and Indonesia, where alluvial tin and copper ore are sometimes
found in close association. It was not until 2000 BC that bronze, or even
copper, was manufactured in northern Thailand. The bronze artefacts,
including spears, axeheads and bracelets, recently found at the village of Ban
Chiang were obviously made locally, since stone axehead moulds were also
discovered. The alloys, which contained 10 per cent of tin, must have been
produced by craftsmen well-versed in bronzeworking technology; and since no
evidence of earlier or more primitive metal working has been found, it would
seem that this modest agricultural community suddenly acquired a fully

developed facility for bronze manufacture. It probably arrived with the
peaceful integration of a group of skilled foreign workers, perhaps displaced by
military disturbances in China. Chinese bronze containing 8 per cent of tin
was being produced in Gansu province as early as 2800 BC, apparently
independently of the emergence of high tin bronzes in Sumer.
The earliest tin bronzes so far identified appear to date from the fourth
millennium BC and were found in the 1930s at the site of Tepe Giyan, near
Nahavand in Western Iran. This mountainous region, situated midway between
the Persian Gulf and the Caspian Sea, was in the Land of Elam mentioned in the
Bible, from which the Sumerians were assumed to have obtained the arts of
metallurgy. Although Tepe Giyan does not appear to have a local source of tin, it
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is possible that there were alluvial tin deposits, now exhausted. The sudden
appearance of true bronzes at Ur around 2800 BC, and in Amuq, near Antioch
in Turkey, about 3000 BC would be consistent with the view that the concept of
bronze manufacture originated in a community such as Tepe Giyan, in the
Persian highlands, during the 4th millennium, and subsequently moved
southwards to Sumeria and the Persian Gulf, and westwards to the
Mediterranean seaboard, during the third millennium.
Bronze was first made in Italy between 1900 and 1800 BC, using tin from
deposits at Campiglia Marittima in Tuscany, although it seems possible that
cassiterite was also being obtained from mines in Saxony and Bohemia. Copper
extraction started in Spain in Neolithic times and during the third millennium its
copper and precious metal deposits were extensively worked (see p. 55)
Exploitation of the Spanish tin deposits, however, did not begin until 1700 BC,
when bronze artefacts were first produced at El Argar and other sites in the
south-east. Most of the bronze artefacts of this period, which contain around 8
per cent tin, appear to have been cast roughly to shape and finished by forging.
Evidence that the early Mediterranean cultures obtained significant quantities of

tin from Cornwall, Brittany or Saxony-Bohemia is singularly lacking, although
some of the tin used for Central European bronzes made between 1800 and
1500 BC appears to have come from the Saxony deposits.
Apart from a tin bracelet dating from 3000 BC found at Thermi in Lesbos,
very few Bronze Age artefacts made of metallic tin have been discovered. This
paucity is difficult to reconcile with the vast quantities of tin which must have
been used for bronze manufacture over the period. It has been suggested that
bronze was originally produced by co-smelting copper ore with minerals of tin,
and that at a later stage, when better compositional control was required,
weighed quantities of cassiterite were reduced directly with charcoal on the
surface of a bath of molten copper. This implies that cassiterite, rather than
metallic tin, was the standard commodity of trade. Metallic tin, however, far
from behaving at all times as a highly inert material can sometimes, when buried
for example in certain types of soil, disintegrate very rapidly into amorphous
masses of oxide and carbonate sludge, destroying the identity of tin artefacts.
The most enduring evidence of Early Bronze Age technology is provided by
a few accidentally vitrified clay tuyeres from between 2000 and 1800 BC.
Many early tuyeres were ‘D’ shaped in cross-section and appear to have been
used in primitive crucible furnaces. Here the crucible was heated from above
by radiation from a glowing bed of charcoal, much of which must inevitably
have fallen upon the surface of the melt.
The crucible in the furnace used at Abu Matar for remelting impure smelted
copper between 3300 and 3000 BC (see p. 51) would presumably have been
extracted via a hole in the side of the vertical shaft. Many crucibles used in the
Mediterranean region between 3000 and 2500 BC had, to facilitate handling, a
socketed boss moulded on one side. Clay-covered wooden or metal rods were
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inserted into these sockets so that the crucible could be removed. During the
Late Bronze Age, the crucibles used were thicker and more robust. The casting

arrangement shown diagrammatically in Figure 1.4 was used between 1550
and 1200 BC in the Greek Islands and also in Sinai. Pouring was
accomplished by rocking the crucible on its base, either by pulling with a hook
from the front, or by pushing from the rear of the furnace.
Bronze Age casting techniques
Few plano-convex ingots of arsenical copper have been found, although the
material appears to have been available at any early stage in the form of ingot
torcs. These Ösenhalsringe, or neck-rings with recoiled ends, resemble in shape
the later iron currency bars. Worked from cast ingots, they were traded as an
intermediate product suitable for the manufacture of pins, jewellery and other
small objects. No unworked Early Bronze Age ingots from which large
artefacts such as axeheads could have been cast have yet been found.
Figure 1.4: Rocking crucibles of this type were extensively used in the Greek
Islands and also in Sinai between about 1600 and 1200 BC to ease the problems of
lifting a heavy crucible of molten metal from a primitive furnace and subsequently
pouring from it in a controllable manner. The rocking crucibles had thick
hemispherical bases and were provided as shown with a pouring hole. When tilted
they rolled away from their charcoal bed and discharged their content of molten
metal simply and safely. One crucible of this type, found by Flinders Petrie at
Serabit in Sinai in 1906 was large enough to contain nearly 8kg of bronze.