HOME

TheInfoList



OR:

Arsenical bronze is an
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...
in which
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
, as opposed to or in addition to
tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
or other constituent
metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typical ...
, is combined with
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
to make bronze. 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 Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
behavior. Copper ore is often naturally contaminated with arsenic; hence, the term "arsenical bronze" when used in
archaeology Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landsca ...
is typically only applied to alloys with an arsenic content higher than 1% by weight, in order to distinguish it from potentially accidental additions of arsenic.


Origins in pre-history

Although arsenical bronze occurs in the
archaeological record The archaeological record is the body of physical (not written) evidence about the past. It is one of the core concepts in archaeology, the academic discipline concerned with documenting and interpreting the archaeological record. Archaeological t ...
across the globe, the earliest artifacts so far known, dating from the
5th millennium BC The 5th millennium BC spanned the years 5000 BC to 4001 BC (c. 7 ka to c. 6 ka). It is impossible to precisely date events that happened around the time of this millennium and all dates mentioned here are estimates mostly based on geological an ...
, have been found on the
Iranian plateau The Iranian plateau or Persian plateau is a geological feature in Western Asia, Central Asia, and South Asia. It comprises part of the Eurasian Plate and is wedged between the Arabian Plate and the Indian Plate; situated between the Zagros ...
.
Arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
is present in a number of
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
-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 to what extent its use arose simply from its presence in copper ores that were then treated by
smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ...
to produce the metal. Reconstructing 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 is an uncombined form of copper that occurs as a natural mineral. Copper is one of the few metallic elements to occur in native form, although it most commonly occurs in oxidized states and mixed with other elements. Native coppe ...
and oxidized 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 is a copper arsenic sulfosalt mineral with an ideal formula . Due to variable substitution of the copper by iron and zinc the formula is . It is gray-black, steel-gray, iron-gray or black in color. A closely related mineral, tetrahedri ...
, with their arsenic, copper and
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
. Thus, the surface deposits would have been used first; 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
realgar Realgar ( ), also known as "ruby sulphur" or "ruby of arsenic", is an arsenic sulfide mineral with the chemical formula α-. It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in asso ...
to molten copper. : This method, although possible, lacks evidence. * The reduction of
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient t ...
-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 enargite. : This method would result in the production of toxic fumes of
arsenous oxide Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, it ...
and the loss of much of the arsenic present in the ores. * The co-smelting of oxidic and sulphidic ores such as
malachite Malachite is a copper carbonate hydroxide mineral, with the formula Cu2CO3(OH)2. This opaque, green-banded mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses, in fracture ...
and
arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elem ...
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 among 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. Artifacts 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

While arsenic was most likely originally mixed with copper as a result of the ores already containing it, its use probably continued for a number of reasons. First, it acts as a deoxidizer, reacting with
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
in the hot metal to form arsenous oxides which vaporize 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 Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...
of the resulting object. However, its use can lead to a greater risk of porous castings, owing to the solution of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
in the molten metal and its subsequent loss as a bubble (although any bubbles could be forge-welded and still leave the mass of the metal ready to be work-hardened). Second, the alloy is capable of greater work-hardening than is the case with pure copper, so that it performs better when used for cutting or chopping. An increase in work-hardening capability arises with an increasing percentage of arsenic, and the bronze 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. Third, 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 The Caucasus () or Caucasia (), is a region between the Black Sea and the Caspian Sea, mainly comprising Armenia, Azerbaijan, Georgia (country), Georgia, and parts of Southern Russia. The Caucasus Mountains, including the Greater Caucasus range ...
and other artifacts 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 color them silver.


Arsenical bronze, sites and civilisations

Arsenical bronze was used by many societies and cultures across the globe. Firstly, the
Iranian plateau The Iranian plateau or Persian plateau is a geological feature in Western Asia, Central Asia, and South Asia. It comprises part of the Eurasian Plate and is wedged between the Arabian Plate and the Indian Plate; situated between the Zagros ...
, followed by the adjacent Mesopotamian area, together 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 The 4th millennium BC spanned the years 4000 BC to 3001 BC. Some of the major changes in human culture during this time included the beginning of the Bronze Age and the invention of writing, which played a major role in starting recorded history. ...
through to mid
2nd millennium BC The 2nd millennium BC spanned the years 2000 BC to 1001 BC. In the Ancient Near East, it marks the transition from the Middle to the Late Bronze Age. The Ancient Near Eastern cultures are well within the historical era: The first half of the mil ...
, 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 The Akkadian Empire () was the first ancient empire of Mesopotamia after the long-lived civilization of Sumer. It was centered in the city of Akkad () and its surrounding region. The empire united Akkadian and Sumerian speakers under one rul ...
, those of Ur, and the
Amorites The Amorites (; sux, 𒈥𒌅, MAR.TU; Akkadian: 𒀀𒈬𒊒𒌝 or 𒋾𒀉𒉡𒌝/𒊎 ; he, אֱמוֹרִי, 'Ĕmōrī; grc, Ἀμορραῖοι) were an ancient Northwest Semitic-speaking people from the Levant who also occupied lar ...
, all based around the
Tigris The Tigris () is the easternmost of the two great rivers that define Mesopotamia, the other being the Euphrates. The river flows south from the mountains of the Armenian Highlands through the Syrian and Arabian Deserts, and empties into the ...
and
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 ( ''the land between the rivers''). Originating in Turkey, the Eup ...
rivers and centres of the trade networks which spread arsenical bronze across the Middle East during the Bronze Age. The
Chalcolithic The Copper Age, also called the Chalcolithic (; from grc-gre, χαλκός ''khalkós'', "copper" and  ''líthos'', "stone") or (A)eneolithic (from Latin '' aeneus'' "of copper"), is an archaeological period characterized by regular ...
-period
Nahal Mishmar hoard The Nahal Mishmar hoard is the hoard of archaeological artifacts found by a 1961 expedition led by Pessah Bar-Adon in a cave by Nahal Mishmar in the Judaean Desert near the Dead Sea, Israel. The collection wrapped in a straw mat found under d ...
in the Judean Desert west of the Dead Sea contains a number of arsenical bronze (4–12% arsenic) and perhaps
arsenical 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 embr ...
artifacts made using the
lost-wax process Lost-wax casting (also called " investment casting", "precision casting", or ''cire perdue'' which has been adopted into English from the French, ) is the process by which a duplicate metal sculpture (often silver, gold, brass, or bronze) ...
, the earliest known use of this complex technique. "Carbon-14 dating of the reed mat in which the objects were wrapped suggests that it dates to at least 3500 B.C. It was in this period that the use of copper became widespread throughout the Levant, attesting to considerable technological developments that parallel major social advances in the region." Sulfide deposits frequently are a mix of different metal sulfides, such as copper, zinc, silver, arsenic, mercury, iron and other metals. ( Sphalerite (ZnS with more or less iron), for example, is not uncommon in copper sulfide deposits, and the metal smelted would be brass, which is both harder and more durable than copper.) The metals could theoretically be separated out, but the alloys resulting were typically much stronger than the metals individually. The use of arsenical bronze spread along trade routes into northwestern China, to the Gansu
Qinghai Qinghai (; alternately romanized as Tsinghai, Ch'inghai), also known as Kokonor, is a landlocked province in the northwest of the People's Republic of China. It is the fourth largest province of China by area and has the third smallest po ...
region, with the
Siba Siba or SIBA may refer to: Places * Siba, Luocheng County, Guangxi, China, a town * Siba State, a princely state of India until 1947 * Siba subdistrict, Basra Governorate, Iraq * Šiba, a village in Bardejov District, Slovakia * Siba Castle, ...
, Qijia 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 BC, 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 is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
, 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 The Sican (also Sicán) culture is the name that archaeologist Izumi Shimada gave to the culture that inhabited what is now the north coast of Peru between about 750 and 1375. According to Shimada, ''Sican'' means "temple of the Moon". The Si ...
of northwestern coastal Peru is famous for its use of arsenical bronze during the period 900 to 1350 AD. Arsenical bronze co-existed with tin bronze 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
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...
,
Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = National seal , national_motto = "Firm and Happy f ...
and the Caucasus suggests that arsenical bronze was produced for a time alongside tin bronze. At
Tepe Yahya Tapeh Yahya () is an archaeological site in Kermān Province, Iran, some south of Kerman city, south of Baft city and 90 km south-west of Jiroft. History Habitation spans the 6th to 2nd millennia BCE and the 10th to 4th centuries BCE. ...
its use continued into the
Iron Age The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age ( Paleolithic, Mesolithic, Neolithic) and the Bronze Age ( Chalcolithic). The concept has been mostl ...
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 few 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
vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon ...
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 nervous system, eyes, lungs and skin. Chronic arsenic poisoning leads to
peripheral neuropathy Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or or ...
, which can cause weakness in the legs and feet. It has been speculated that this lay behind the legend of lame smiths in many cultures and myths, such as the Greek god
Hephaestus Hephaestus (; eight spellings; grc-gre, Ἥφαιστος, Hḗphaistos) is the Greek god of blacksmiths, metalworking, carpenters, craftsmen, artisans, sculptors, metallurgy, fire (compare, however, with Hestia), and volcanoes.Walter B ...
. A well-preserved
mummy A mummy is a dead human or an animal whose soft tissues and organs have been preserved by either intentional or accidental exposure to chemicals, extreme cold, very low humidity, or lack of air, so that the recovered body does not decay fu ...
of a man who lived around 3,200 BC found in the
Ötztal Alps The Ötztal Alps ( it, Alpi Venoste, german: Ötztaler Alpen) are a mountain range in the Central Eastern Alps, in the State of Tyrol in western Austria and the Province of South Tyrol in northern Italy. Geography The Ötztal Alps are arrayed ...
, popularly known as
Ötzi Ötzi, also called the Iceman, is the natural mummy of a man who lived some time between 3350 and 3105 BC, discovered in September 1991 in the Ötztal Alps (hence the nickname "Ötzi") on the border between Austria and Italy. Ötzi is believed to ...
, showed high levels of both copper particles and
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
in his hair. This, along with Ötzi's copper axe blade, which is 99.7% pure copper, has led scientists to speculate that he was involved in copper
smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ...
.


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 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 embr ...
, 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%.


See also

*
Arsenical 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 embr ...
*
Arsenical brass Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other with ...


References


External links

* * {{DEFAULTSORT:Arsenical Bronze Bronze Age Copper alloys History of metallurgy Coinage metals and alloys Arsenic