Arsenical bronze
Encyclopedia
Arsenical bronze is an alloy
in which arsenic
is added to copper
as opposed to, or in addition to other constituent metals. The use of arsenic with copper, either as the secondary constituent or with another component such as tin
, results in a stronger final product and better casting
behaviour.
Since copper ore is often naturally contaminated with arsenic, the term "arsenical bronze" when used in archaeology
is typically only applied to alloys which contain greater than 1% by weight of arsenic, in order to distinguish it from potentially accidental additions of arsenic.
Although arsenical bronze occurs in the archaeological record
across the globe, the earliest artefacts
so far known have been found on the Iranian plateau
in the 5th millennium BCE. Arsenic is present in a number of copper containing ores
(see table at right, adapted from Lechtman & Klein, 1999) and therefore some contamination of the copper with arsenic would be unavoidable. However it is still not entirely clear to what extent arsenic was deliberately added to copper and how much its use arose simply from its presence in copper ores that were then treated by smelting
to produce the metal.
A possible sequence of events in prehistory involves considering the structure of copper ore deposits, which are mostly sulphides. The surface minerals would contain some native copper
and oxidised minerals, but much of the copper and other minerals would have been washed further into the ore body forming a secondary enrichment zone. This includes many minerals such as tennantite
, with their arsenic, copper and iron. So the surface deposits would be used first, and with some work deeper sulphidic ores would have been uncovered and worked, and it would have been discovered that the material from this level had better properties.
Using these various ores, there are four possible methods that may have been used to produce arsenical bronze alloys. These are:
Furthermore, greater sophistication of metal workers is suggested by Thornton et al. They suggest that iron arsenide was deliberately produced as part of the copper smelting process, to be traded and used to make arsenical bronze elsewhere by addition to molten copper.
Artefacts made of arsenical bronze cover the complete spectrum of metal objects, from axes to ornaments. The method of manufacture involved heating the metal in crucibles and casting it into moulds made of stone or clay. After solidifying it would be polished or in the case of axes and other tools work hardened by beating the working edge with a hammer, thinning out the metal and increasing its strength. Finished objects could also be engraved or decorated as appropriate.
Secondly, it contributes to greater work hardening
of the object than copper alone, so that it performs better when used for cutting or chopping. There is an increase in work hardening capability with increasing percentage of arsenic, and it can be work hardened over a wide range of temperatures without fear of embrittlement. Its improved properties over pure copper can be seen with as little as 0.5 to 2 wt% As, giving a 10 to 30% improvement in hardness and tensile strength.
Thirdly, in the correct percentages, it can contribute a silvery sheen to the article being manufactured. There is evidence of arsenical bronze daggers from the Caucasus
and other artefacts from different locations having an arsenic rich surface layer which may well have been produced deliberately by ancient craftsmen, and Mexican bells were made of copper with sufficient arsenic to colour them silver.
, and the Amorites, all based around the Tigris
and Euphrates
rivers and centres of the trade networks which spread arsenical bronze across the middle east during the Bronze Age.
The use of arsenical bronze spread along trade routes into North western China, to the region Gansu
– Qinghai
, with the Siba, Qija and Tianshanbeilu cultures. However it is still unclear as to whether arsenical bronze artefacts were imported or made locally, although the latter is suspected as being more likely due to possible local exploitation of mineral resources. On the other hand the artefacts show typological connections to the Eurasian steppe.
The Eneolithic period in Northern Italy, with the Remedello and Rinaldone cultures in 2800 to 2200 BCE, saw the use of arsenical bronze. Indeed it seems that arsenical bronze was the most common alloy in use in the Mediterranean basin at this time.
In South America
, arsenical bronze was the predominant alloy in Ecuador and north and central Peru, because of the rich arsenic bearing ores present there. By contrast, the south and central Andes, southern Peru, Bolivia and parts of Argentina, were rich in the tin ore Cassiterite
and thus did not use arsenical bronze.
The Sican Culture
of north western coastal Peru is famous for its use of arsenical bronze during the period 900 to 1350 AD. Arsenical bronze co-existed with tin bronze for in the Andes, probably due to its greater ductility which meant it could be easily hammered into thin sheets which were valued in local society.
, Peru
and the Caucasus suggests that arsenical bronze was produced for a time alongside tin bronze. At Tepe Yahya its use continued into the Iron Age for the manufacture of trinkets and decorative objects, thus demonstrating that there was not a simple succession of alloys over time, with superior new alloys replacing older ones. There are no real advantages metallurgically for the superiority of tin bronze, and early authors suggested that arsenical bronze was phased out due to its health effects. It is more likely that it was phased out in general use because alloying with tin gave castings which had similar strength to arsenical bronze but did not require further work hardening to achieve useful strength. It is also probable that more certain results could be achieved with the use of tin, because it could be added directly to the copper in specific amounts, whereas the precise amount of arsenic being added was much harder to gauge due to the manufacturing process.
Chronic arsenic poisoning leads to peripheral neuropathy
, which can cause weakness in the legs and feet. It has been speculated that this lay behind the legend of lame smiths, such as the Greek god Hephaestus
.
, being defined as copper with under 0.5 wt% As, below the accepted percentage in archaeological artefacts. The presence of 0.5 wt% arsenic in copper lowers the electrical conductivity to 34% of that of pure copper, and even as little as 0.05 wt% decreases it by 15%. Therefore there is no demand for copper containing arsenic in one of the major modern uses of copper and steam engine boilers are no longer made from it, leading to no modern use.
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
in which arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
is added to copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
as opposed to, or in addition to other constituent metals. The use of arsenic with copper, either as the secondary constituent or with another component such as tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, results in a stronger final product and better casting
Casting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
behaviour.
Since copper ore is often naturally contaminated with arsenic, the term "arsenical bronze" when used in archaeology
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
is typically only applied to alloys which contain greater than 1% by weight of arsenic, in order to distinguish it from potentially accidental additions of arsenic.
Origins in pre-history
| Ore name | chemical formula |
|---|---|
| Arsenopyrite | FeAsS |
| Enargite | Cu3AsS4 |
| Olivenite | Cu2(AsO4)OH |
| Tennantite | Cu12As4S13 |
| Malachite | Cu2(OH)2CO3 |
| Azurite | Cu3(OH)2(CO3)2 |
Although arsenical bronze occurs in the archaeological record
Archaeological record
The archaeological record is the body of physical evidence about the past. It is one of the most basic concepts in archaeology, the academic discipline concerned with documenting and interpreting the archaeological record....
across the globe, the earliest artefacts
Artifact (archaeology)
An artifact or artefact is "something made or given shape by man, such as a tool or a work of art, esp an object of archaeological interest"...
so far known have been found on the Iranian plateau
Iranian plateau
The Iranian plateau, or Iranic plateau, is a geological formation in Southwest Asia. It is the part of the Eurasian Plate wedged between the Arabian and Indian plates, situated between the Zagros mountains to the west, the Caspian Sea and the Kopet Dag to the north, the Hormuz Strait and Persian...
in the 5th millennium BCE. Arsenic is present in a number of copper containing ores
Orés
Orés is a municipality in the Cinco Villas, in the province of Zaragoza, in the autonomous community of Aragon, Spain. It belongs to the comarca of Cinco Villas. It is placed 104 km to the northwest of the provincial capital city, Zaragoza. Its coordinates are: 42° 17' N, 1° 00' W, and is...
(see table at right, adapted from Lechtman & Klein, 1999) and therefore some contamination of the copper with arsenic would be unavoidable. However it is still not entirely clear to what extent arsenic was deliberately added to copper and how much its use arose simply from its presence in copper ores that were then treated by smelting
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
to produce the metal.
A possible sequence of events in prehistory involves considering the structure of copper ore deposits, which are mostly sulphides. The surface minerals would contain some native copper
Native copper
Copper, as native copper, is one of the few metallic elements to occur in uncombined form as a natural mineral, although most commonly occurs in oxidized states and mixed with other elements...
and oxidised minerals, but much of the copper and other minerals would have been washed further into the ore body forming a secondary enrichment zone. This includes many minerals such as tennantite
Tennantite
Tennantite is a copper arsenic sulfosalt mineral. Its chemical formula is Cu12As4S13. It is grey-black, steel-gray, iron-gray or black in color. A closely related mineral, tetrahedrite has antimony substituting for arsenic and the two form a solid solution series. The two have very similar...
, with their arsenic, copper and iron. So the surface deposits would be used first, and with some work deeper sulphidic ores would have been uncovered and worked, and it would have been discovered that the material from this level had better properties.
Using these various ores, there are four possible methods that may have been used to produce arsenical bronze alloys. These are:
- The direct addition of arsenic-bearing metals or ores such as realgarRealgarRealgar, α-As4S4, is an arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic". It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral, orpiment . It is orange-red in colour, melts...
to molten copper.
- This method, although possible, lacks evidence.
- The reductionRedoxRedox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of antimonyAntimonyAntimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
-bearing copper arsenates or fahlore to produce an alloy high in arsenic and antimony.
- This is entirely practicable.
- The reduction of roasted copper sulfarsenides such as tennantite and enargiteEnargiteEnargite is a copper arsenic sulfosalt mineral with formula: Cu3AsS4. It takes its name from the Greek word enarge, "distinct." Enargite is a steel gray, blackish gray, to violet black mineral with metallic luster. It forms slender orthorhombic prisms as well as massive aggregates...
.
- This method would result in the production of toxic fumes of arsenous oxide and the loss of much of the arsenic present in the ores.
- The co-smelting of oxidic and sulphidic ores such as malachiteMalachiteMalachite is a copper carbonate mineral, with the formula Cu2CO32. This green-colored mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses. Individual crystals are rare but do occur as slender to acicular prisms...
and arsenopyriteArsenopyriteArsenopyrite is an iron arsenic sulfide . It is a hard metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it becomes magnetic and gives off toxic fumes...
together.
- This method has been demonstrated to work well, with little in the way of dangerous fumes given off during it, because of the reactions together of the different minerals.
Furthermore, greater sophistication of metal workers is suggested by Thornton et al. They suggest that iron arsenide was deliberately produced as part of the copper smelting process, to be traded and used to make arsenical bronze elsewhere by addition to molten copper.
Artefacts made of arsenical bronze cover the complete spectrum of metal objects, from axes to ornaments. The method of manufacture involved heating the metal in crucibles and casting it into moulds made of stone or clay. After solidifying it would be polished or in the case of axes and other tools work hardened by beating the working edge with a hammer, thinning out the metal and increasing its strength. Finished objects could also be engraved or decorated as appropriate.
Advantages of arsenical bronze
Whilst arsenic was most likely originally added to copper as a result of the ores containing it, its use probably continued for a number of reasons. Firstly, it acts as a de-oxidiser, reacting with oxygen in the hot metal to form arsenous oxides which vaporise from the liquid metal. If a great deal of oxygen is dissolved in liquid copper, when the metal cools the copper oxide separates out at grain boundaries and greatly reduces the ductility of the resulting object. It can lead to a greater risk of porous castings due to the solution of hydrogen in the molten metal and its subsequent loss as a bubble. But any bubbles could be forge welded and still leave the mass of the metal ready to be work hardened.Secondly, it contributes to greater work hardening
Work hardening
Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements within the crystal structure of the material. Any material with a reasonably high melting point such as metals and...
of the object than copper alone, so that it performs better when used for cutting or chopping. There is an increase in work hardening capability with increasing percentage of arsenic, and it can be work hardened over a wide range of temperatures without fear of embrittlement. Its improved properties over pure copper can be seen with as little as 0.5 to 2 wt% As, giving a 10 to 30% improvement in hardness and tensile strength.
Thirdly, in the correct percentages, it can contribute a silvery sheen to the article being manufactured. There is evidence of arsenical bronze daggers from the Caucasus
Caucasus
The Caucasus, also Caucas or Caucasia , is a geopolitical region at the border of Europe and Asia, and situated between the Black and the Caspian sea...
and other artefacts from different locations having an arsenic rich surface layer which may well have been produced deliberately by ancient craftsmen, and Mexican bells were made of copper with sufficient arsenic to colour them silver.
Arsenical bronze, sites and civilisations
Arsenical bronze was used by many societies and cultures across the globe. Firstly, the Mesoptamian area, covering modern Iran, Iraq and Syria, has the earliest arsenical bronze metallurgy in the world, as previously mentioned. It was in use from the 4th millennium BC through to mid 2nd millennium, a period of nearly 2,000 years. There was a great deal of variation in arsenic content of artefacts throughout this period, making it impossible to say exactly how much was added deliberately and how much came about by accident. Societies using arsenical bronze include the Akkadians, those of UrUr
Ur was an important city-state in ancient Sumer located at the site of modern Tell el-Muqayyar in Iraq's Dhi Qar Governorate...
, and the Amorites, all based around the Tigris
Tigris
The Tigris River is the eastern member of the two great rivers that define Mesopotamia, the other being the Euphrates. The river flows south from the mountains of southeastern Turkey through Iraq.-Geography:...
and Euphrates
Euphrates
The Euphrates is the longest and one of the most historically important rivers of Western Asia. Together with the Tigris, it is one of the two defining rivers of Mesopotamia...
rivers and centres of the trade networks which spread arsenical bronze across the middle east during the Bronze Age.
The use of arsenical bronze spread along trade routes into North western China, to the region Gansu
Gansu
' is a province located in the northwest of the People's Republic of China.It lies between the Tibetan and Huangtu plateaus, and borders Mongolia, Inner Mongolia, and Ningxia to the north, Xinjiang and Qinghai to the west, Sichuan to the south, and Shaanxi to the east...
– Qinghai
Qinghai
Qinghai ; Oirat Mongolian: ; ; Salar:) is a province of the People's Republic of China, named after Qinghai Lake...
, with the Siba, Qija and Tianshanbeilu cultures. However it is still unclear as to whether arsenical bronze artefacts were imported or made locally, although the latter is suspected as being more likely due to possible local exploitation of mineral resources. On the other hand the artefacts show typological connections to the Eurasian steppe.
The Eneolithic period in Northern Italy, with the Remedello and Rinaldone cultures in 2800 to 2200 BCE, saw the use of arsenical bronze. Indeed it seems that arsenical bronze was the most common alloy in use in the Mediterranean basin at this time.
In South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, arsenical bronze was the predominant alloy in Ecuador and north and central Peru, because of the rich arsenic bearing ores present there. By contrast, the south and central Andes, southern Peru, Bolivia and parts of Argentina, were rich in the tin ore Cassiterite
Cassiterite
Cassiterite is a tin oxide mineral, SnO2. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem...
and thus did not use arsenical bronze.
The Sican Culture
Sican Culture
The Sican culture is the name that archaeologist Izumi Shimada gave to the culture that inhabited what is now the north coast of Peru between about AD 750 and 1375. According to Shimada, Sican means "temple of the moon". The Sican culture is also referred to as Lambayeque culture, after the name...
of north western coastal Peru is famous for its use of arsenical bronze during the period 900 to 1350 AD. Arsenical bronze co-existed with tin bronze for in the Andes, probably due to its greater ductility which meant it could be easily hammered into thin sheets which were valued in local society.
Arsenical bronze after the bronze age
The archaeological record in EgyptEgypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
, Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
and the Caucasus suggests that arsenical bronze was produced for a time alongside tin bronze. At Tepe Yahya its use continued into the Iron Age for the manufacture of trinkets and decorative objects, thus demonstrating that there was not a simple succession of alloys over time, with superior new alloys replacing older ones. There are no real advantages metallurgically for the superiority of tin bronze, and early authors suggested that arsenical bronze was phased out due to its health effects. It is more likely that it was phased out in general use because alloying with tin gave castings which had similar strength to arsenical bronze but did not require further work hardening to achieve useful strength. It is also probable that more certain results could be achieved with the use of tin, because it could be added directly to the copper in specific amounts, whereas the precise amount of arsenic being added was much harder to gauge due to the manufacturing process.
Health effects of arsenical bronze use
Arsenic is an element with a vaporisation point of 615°C, such that arsenical oxide will be lost from the melt before or during casting, and fumes from fire setting for mining and ore processing have long been known to attack the eyes, lungs and skin.Chronic arsenic poisoning leads to peripheral neuropathy
Peripheral neuropathy
Peripheral neuropathy is the term for damage to nerves of the peripheral nervous system, which may be caused either by diseases of or trauma to the nerve or the side-effects of systemic illness....
, which can cause weakness in the legs and feet. It has been speculated that this lay behind the legend of lame smiths, such as the Greek god Hephaestus
Hephaestus
Hephaestus was a Greek god whose Roman equivalent was Vulcan. He is the son of Zeus and Hera, the King and Queen of the Gods - or else, according to some accounts, of Hera alone. He was the god of technology, blacksmiths, craftsmen, artisans, sculptors, metals, metallurgy, fire and volcanoes...
.
Modern uses of arsenical bronze
Arsenical bronze has seen little use in the modern period. It appears that the closest equivalent goes by the name of arsenical copperArsenical copper
Arsenical copper contains up to 0.5% arsenic which, at elevated temperatures, imparts higher tensile strength and a reduced tendency to scaling. It is typically specified in boiler work, especially locomotive fireboxes. It also helps prevent embrittlement of oxygen free copper by bismuth, antimony...
, being defined as copper with under 0.5 wt% As, below the accepted percentage in archaeological artefacts. The presence of 0.5 wt% arsenic in copper lowers the electrical conductivity to 34% of that of pure copper, and even as little as 0.05 wt% decreases it by 15%. Therefore there is no demand for copper containing arsenic in one of the major modern uses of copper and steam engine boilers are no longer made from it, leading to no modern use.