ARSENICAL COPPER

Arsenical Copper

In a strict meaning, arsenical copper is a copper that contains a proportion of arsenic. The use of this combination it is believed to be the forerunner of the copper tin alloy, consequently -arsenical copper was the first copper alloy which was used in the early beginning of melting and casting of metal-^[1]. However, several authors have tried to investigate if this „mixture“ was an alloy (deliberated) or a bullion (result of accidental smelting of certain copper ores in which arsenic was present as a natural impurity).

History

It is widely thought that the most important alloy created during the Bronze Age was the copper tin. However, archaeometallurgical investigations have brought out a fancy combination of arsenic and copper, called, on the one hand, arsenical copper and, on the other, copper arsenic or arsenic bronze (depending on the amount of arsenic found into the alloy); it was widespread distributed in almost all the world (from Asia to America, without forgetting Europe).

The use of this alloy is dated for the Old World (Europe and Asia) from the fourth millennia (3500 BC in Western Asia ) until 1600 BC. Nonetheless the dates for the New World (given in this case by Lechtman (1991)) go from 850 to 1500 AC, when the Inca state imposed the general use of tin.

Meanwhile the end of the use of arsenical copper in the New world is clear, in the Old world it isn´t. With the available data we are only able to know that while arsenical copper was in use, tin appeared, and in an indeterminate moment tin turned into the -new big metal- of the Bronze Age, disappearing arsenical copper, why-, the reasons already are unclear, but then I will try to explain some of the possible causes.

Dirty copper or chosen alloy-

I have chosen this title from the Lechtman´s article to expose one of the central problems that the study of arsenical copper has brought: Was this material produced in a deliberated way or only was the consequence of mere chance when choosing the ore-.

Arsenic (As) appears in the nature both in native and combined forms. In the native state occurs as a dull-grey, brittle mass (fig. 1)often associated with native antimony or silver and in combined forms occurs with sulphides of copper , iron and lead, such as olivenite (Cu3As2O8Cu(OH)2) (fig.2), enargite (Cu3AsS4) (both are the main source of arsenical copper), lollingite (FeAs2) or with oxides ( arsenolite (As2O3) or white arsenic). Is a very toxic and volatile metal when is heated.

The optimal question once we have reached this point is: why was not the arsenical copper achieved at the same time that copper smelting beginning-, in view of the fact that some of the ores used to smelt copper were after employed to create the copper arsenic alloy. The logical answer is that at an early date (5000 BC) smiths didn-t have the technical skills or the knowledge to create the appropriate conditions required to prepare such as volatile product. As the time advance, the skills developed which implies changes, this changes contributes to control better the smelting conditions, which are basic to create arsenical copper in an intentional or unintentional way.

The production of arsenical copper is also a point of discussion among experts, several experiments (Lechtmann 1999, Pollard 1989, Zwicker 1991, Budd 1992) have been carried out to discover which smelting technique was employed by the ancients to get the amount of arsenic that generally contains the archaeological material found and as an attempt to clarify if the alloy was consciously made or not. The most common arsenical rates in copper are: arsenical copper (< 0,1% As) this is normally the unintentional one, copper arsenic alloy (0.1%-0,5% As) and arsenic bronze (0,5%-7%) which is believed to be the deliberated one: -thousands of copper objects have been analysed and good majority of them contain less than 1% arsenic. It is quite likely that these cases the arsenic was present in the copper ores as an impurity. But whenever the percent of arsenic rises above 1% or so, it is most likely that deliberate alloying was done^[2] -. Nonetheless this data can be confused because as Pollard has demonstrate, a high level of arsenic can be achieved with a reducing environment, which shows that is not needed to control the alloying in a conscious way .

Before star explaining the method utilized in the production of arsenical copper, we have to make clear why this alloy was complicated to reach: First, arsenic has a boiling temperature of 616 ºwhile the melting point of copper is 1084º (Fig.3). -So when copper and arsenic are smelted together the resulting melt would have a high vapour pressure over arsenic solution in copper and hence much of the arsenic would be lost in the process^[3]- . Another point that we have to bear in mind is that the maximum solid solubility of arsenic in copper is of the order of 8%, as a result is quite difficult to get a higher amount of arsenic in the solution.

One of the most simplest process of alloying arsenic and copper is from natural arsenic copper, such us algodonite (Cu5As) (fig.4), in this case, it is only needed to control the melted bulk at a lower temperature, to restrict the volatilization of arsenic.

Certain objects (needles, rings-(fig.5) ) found in India, Aegean region, South America- contain a high quantity of arsenic (3-6%); it is thought that this type of instruments with such us amount of arsenic were produced consciously by adding arsenic in native conditions.

However, the experts think that the two main ways to produce an arsenic copper alloy were either smelting a copper oxide ore containing arsenic, in this case it was only necessary to control the oxygen, -The smelting enclosure must contain a reducing environment, provided by the partial burning of charcoal to carbon monoxide (CO).The monoxide combines with oxygen in the ore mineral, reducing the ore to its metallic component.CuAsO4 + 4 CO - Cu,As + 4 CO2-^[4]-. Some experiments conducted by Pollard (1989) has shown that this technique (utilized from the early beginning of the metal age) could be the result of undelivered alloy, -resulting from inclusion of arsenic in the ore# POLLARD, A and Al (1989); Some experiments concerning the smelting of arsenical copper. Archaeological sciences, 169-174 -. Or cosmelting -Most of the world-s arsenic-bearing ores are sulphides so the only option available to early metalworkers for extracting metal from sulphide ores was to follow a two-step process: (1) roast the ore to drive off the sulphur as sulphur dioxide gas; (2) follow the roast by a direct smelt of the oxide ore produced during roasting. (1) roasting 8 CuAsS + 22 1/2 O2- 6 CuAsO4 + 2 CuO + As2O3 + 8 SO2 (2) direct smelt CuAsO4 + 4 CO - Cu,As + 4 CO2# LECHTMAN, H (1999); The production of copper arsenic alloys by cosmelting, Journal of archaeological science, V.26, 497. -. Suggesting a more intentional process, because it requires a high level of knowledge.

Another way to produce arsenical copper could had been from the iron or lead-sulphur arsinides speiss(Thornton 2008, Zwicker 1991) which is very rich in arsenic. -Dissolve up to more than 66% in copper so that arsenic bronze can be produce easily by addition of these types of speiss. The speiss in equilibrium with a copper alloy can contain 8.5% As which is the maximum solubility of arsenic in copper^[5]-. However, this system have not been widely studied, and we have a few reference research about this issue.

Properties

An important question has also been asked about why, in the case that the alloy was deliberated, did the ancients chose this alloy and not other-.

The answer is owing to the physical properties that the arsenical has in the copper such us: strength, malleability, ductility, hardness, brittleness, can be hot or cold worked, is a deoxidizing agent ( arsenic tends to counteract the embrittling tendency of abnormal amounts of oxygen and the tendency to develop cracks) and gives to the final product a silvery colour. Perfect to make flat axes and harder instruments like daggers- .

However, metallographic analysis on the real archaeological artefacts (Budd 1992, Delibes 1997, Northover 1987, Lechtman 1981) carried out to see until what extend were the properties of this alloy exploited, have demonstrated that in some cases like a group of axes and daggers from Palestine (Northover 1987) the properties were fully exploited (cutting edges, flat surfaces-) which means that the smith worked the instrument (forging and casting ) to achieve that all the qualities of the alloy come out; but in other cases such us flat axes (fig.7) and daggers in Spain(Delibes 1997) and Ireland (Nothover 1987), the advantages that the arsenic is supposed to transfer to the alloy were not enough worked ( were little hardening, without cutting edges-).

So, does it mean that Palestinian instruments were deliberated alloying and Spanish and Irish doesn´t-.

Tin vs. Arsenical copper

As I said before, tin-copper alloy replaced arsenical copper in order of importance, becoming one of the most important alloys of the history. Nobody knows exactly why these replacement took place and many hypothesis have been written:

	On the one hand experts (Charles 1967) have attempted to assess the mechanical superiority of tin copper alloy respect arsenical copper, but laboratory studies have demonstrated that exit very little differences between this two alloys; in other words, as metals they work in a similar way (hardness, resistance-), however, arsenical copper is much more ductile (better to produce flat axes) and less brittle (Fig. 8) ; on the other, several authors ( Charles 1967) have spoken about the toxicity of the arsenic fumes as one of the causes of the replacement, but it has been completely rejected. 

More credible are the theories that are about an increase of the metal demand that was supplied with tin, because was more widespread distributed and easier to smelt (it doesn-t need a high control of temperature, the melting point is slightly lower and is easier -to stick the desired composition of the alloy-^[6]); and others have postulated about „political“ (Lechtman) or aesthetic (different colours) topics.

Revisions

As we can see at this moment nothing is complete clear, we know the complexity of the process, the different techniques that could have been used, the advantages and disadvantages of the alloy, but it is fairly impossible to concrete what it is deliberate or unintentional or why it was.

It is quite a pessimist point of view, but I don-t event see better future in this subject, in part due to the archaeological material in itself as Craddock (1995)related there is a lack of studies in materials with high amount of arsenic (they are rarely encountered in the modern world), almost all the studies have concentrated in objects with low amounts of arsenic, the crucibles that we have from this kind of alloys have an amount of arsenic much higher than the artefacts, in part because -the arsenic being more volatile, will have preferentially left the alloy and penetrated into the refractory# CRADDOCK, P.T (1995); Early metal mining and production, Edinburgh. --, all kind of little things that complicate the investigation.

Another big problem of this mess is that the only approach that the experts have done , has been through analytical experiments : metallurgic analysis to check if the metal have been worked enough or not-, test to prove how the alloy was made-and this is a good procedure, and necessary, but we have to be aware of using it as the unique information, as Historians we must achieve historic conclusion, I mean, try to see the artefacts within the society and cultural sphere, without this we only have data, data and more data that can help us to comprehend how the artefact was done, but not why-.

We are supposed to think that the wonderful Palestinian artefacts were deliberately made because all the properties that arsenic give to copper were exploit, however, the Spanish or Irish tools were unpremeditated owing to that characteristic didn-t appear, as a consequence of the -poor smith skills-. But we have to think that perhaps, the smith didn-t want a harder cutting edge on the instrument because it had to be utilized in other way that wasn-t cut wood- or perhaps only he chose this alloy because of the finish silvery colour. It is also true that some articles (North over (1989), Budd (1991), Delibes(1997)) admit this possibility, but they don-t try further steps.

References

1. ^ # ZWICKER, U (1991), Natural copper-arsenic alloys and smelt arsenic bronzes in early metal production, Decouverte du metal, 331-337.
2. ^ # BHARDWAJ, H( 2000); Metallurgy in Indian archaeology, Varasi.
3. ^ # BHARDWAJ, H( 2000); Metallurgy in Indian archaeology, Varasi.
4. ^ # LECHTMAN, H (1999); The production of copper arsenic alloys by cosmelting, Journal of archaeological science, V.26, 497.
5. ^ # ZWICKER, U (1991), Natural copper-arsenic alloys and smelt arsenic bronzes in early metal production, Decouverte du metal, 331-337.
6. ^ # LECHTMAN,H (1996); Arsenic bronze: Dirty copper or chosen alloy-. A view from the Americas. Journal of field archaeology, V.23, 477-514.

More references

BHARDWAJ, H( 2000); Metallurgy in Indian archaeology, Varasasi.

BUDD,P and OTTAWAY, B (1989); The properties of arsenical copper alloys, Archaeological science 1989: proceedings of a conference on the application of scientific techniques to archaeology, edited by P.Budd, Bradford.

BUDD,P (1991); A metallographic investigation of Neolithic arsenical copper, Historical metallurgy, v.25, 99-111.

CHARLES, J (1967); Early arsenical bronzes. A metallurgy view. American Journal of archaeology, V.71, 21-26.

COGHLAN, H.H (1975); Notes on the prehistoric metallurgy of copper and bronze in the world, Oxford.

CRADDOCK, P.T (1995); Early metal mining and production, Edinburgh.

DELIBES, G (et al.) (1997), Las primeras etapas metallurgical en la Península Ibérica, V.2.

LECHTMAN, H (1981); Copper arsenic bronzes from the north coast of Peru, Annales of the New York Academy of sciences, V.376, 77-121.

LECHTMAN, H (1991); The production of copper arsenic in Central Andes, Journal of field archaeology, V.18, 43-76.

LECHTMAN,H (1996); Arsenic bronze: Dirty copper or chosen alloy-. A view from the Americas. Journal of field archaeology, V.23, 477-514.

LECHTMAN, H (1999); The production of copper arsenic alloys by cosmelting, Journal of archaeological science, V.26, 497.

MADDIN,R (1988); The beginning of the use of metals and alloys, Cambridge.

MCKENEL, H and TYLECOTE, R (1972); The working of copper-arsenic alloys in the Early Bronze, Proceedings of the prehistoric society, V.38, 209-218.

NORTHOVER,P (1989); Properties and use of Arsenic-Copper alloys, Old world Archaeolometallurgy, 1987, 111-118.

POLLARD, A and Al (1989); Some experiments concerning the smelting of arsenical copper. Archaeological sciences, 169-174.

THORNTON, C.P (et al.) (2008); The production of speiss 8iron arsenide) during the early bronze age in Iran, Journal of Archaeological science.

TYLECOTE, R.F (1980); Summary of results of experimental work on early copper smelting, Aspects of early metallurgy occasional paper, V. 17, 5-12.

TYLECOTE, R.F (1987); The early history of metallurgy in Europe, London. ZWICKER, U (1991), Natural copper-arsenic alloys and smelt arsenic bronzes in early metal production, Decouverte du metal, 331-337.

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