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.[1]
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.[2]
Contents |
| 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 across the globe, the earliest artefacts so far known have been found on the Iranian plateau in the 5th millennium BCE.[3] Arsenic is present in a number of copper containing ores (see table at right, adapted from Lechtman & Klein, 1999[4]) 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 [5] 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.[6] 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.[3] These are:
Furthermore, greater sophistication of metal workers is suggested by Thornton et al.[8] 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.[6] Finished objects could also be engraved or decorated as appropriate.
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.[1]
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.[1] 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.[7]
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,[9] and Mexican bells were made of copper with sufficient arsenic to colour them silver.[7]
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.[5] Societies using arsenical bronze include the Akkadians, those of Ur, 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.[5]
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.[10]
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.[11]
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.[7]
The Sican Culture of north western coastal Peru is famous for its use of arsenical bronze during the period 900 to 1350 AD.[12] 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.[7]
The archaeological record in Egypt, 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,[3] 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,[1] 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.[6] 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.[7]
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.[13]
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.
Arsenical bronze has seen little use in the modern period. It appears that the closest equivalent goes by the name of arsenical copper, 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%.[7] 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.