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Chemical and metallurgical aspects of arsenical bronze: the case of arsenic-loss in prehistoric metal production

Periodic Reporting for period 1 - ARSENICLOSS (Chemical and metallurgical aspects of arsenical bronze: the case of arsenic-loss in prehistoric metal production)

Reporting period: 2015-07-06 to 2017-07-05

Arsenical bronze is known since the 5th mill. BC and is considered the first alloy ever produced on a voluntary base. In Europe, arsenical bronze was typically used in a particular short transition period from the Chalcolithic to the beginning of the Bronze Age. Arsenical bronzes represent the bridge between the utilization of unalloyed copper and the production of tin-bronzes. The time period and the duration of usage varies according to the geographical regions: while arsenical bronze was typical for Central Europe around 2000 BC, it was already known and widely used on the Iranian Plateau during the 5th mill. BC.
While from prehistoric copper base alloys tin bronzes are studied in all details, material characteristics of arsenical bronzes are rarely studied, even though their understanding is of high importance for research on the beginnings of metallurgy and the understanding why tin-bronze was considered to be superior at the beginning of the Bronze Age. This is even more surprising when the number of known and analysed prehistoric arsenical bronzes is taken into account: nearly 35,500 objects from the largest database of chemical analyses of Copper and Bronze Age metal finds (SAM - Studien zu den Anfängen der Metallurgie). Among these, almost a third are arsenical bronze and contain more than 1 wt.% of As; 10% more than 2 wt.% of As. C. 1100 objects were made out of arsenical bronze with 3 wt.% or higher content of arsenic. It is worth to mention that among the objects poorer in arsenic lay those produced with recycled arsenical bronzes, which previously contained higher amounts of arsenic.

Overall objectives
1) investigate and contribute to the construction of out-of-equilibrium phase diagrams for arsenic bronzes up to 10 wt.% arsenic;
2) evaluate mechanical properties and characteristics of arsenical bronze such as hardness or colour;
3) quantify and evaluate the loss of arsenic as it occurred during prehistoric manufacturing processes through re-melting, casting and annealing activities.
Cu-As alloys with different amounts of As were prepared (1, 2, 3, 4, 5, 6, 7, 11, 15 wt.% As) and cast in iron chill cast moulds. The produced CuAs-ingots (10 x 4 x 0.5 cm) are the base of all further analyses. Since it was not possible to cast ingots in moulds with the same geometry, but different mould material (sand, sandstone, clay), simulations of such castings were carried out with Matlab© and linked with the SDAS in order to calculate the cooling speed (see publications). The following analyses and works were carried out:
1. Establishing out-of-equilibrium phase diagrams of CuAs-alloys with up to 15 wt.% As at different cooling speed (2, 5, 10, 20 K/min) with DTA (Objective 1A)
2. Simulation of prehistoric casting technologies of arsenical bronze with up to 15 wt.% As in different mould materials (terracotta, terracotta at 600°C, tin-bronze, steatite, sand, iron) with Matlab© (Objective 1A)
3. Material characteristics: The color characteristics of different copper alloys, such as Cu-As, Cu-Sn, Cu-Sb, and Cu-Ni were evaluated photometrical by using the CIELAB color system to serve as a comparison database to evaluate the original metal colour of prehistoric metal objects via their alloy composition, and without destroying their surface through direct measurements (Objective 1B)
4. From different ingots with the composition relevant for archaeology (1, 2, 3, 4, and 5 wt.% arsenic), as well as the 10 wt.% As ingot, a number of specimens were taken and submitted to different thermomechanical histories in order to compare how these are affecting the microstructural features and, consequently, the mechanical properties (hardness; HV measurements) (Objective 1B).
5. Microstructural features of the experimentally produced ingots were compared with those typical of the original Chalcolithic and Bronze Age artefacts, and formation and occurrence of inverse segregation in prehistoric arsenical bronzes detected (Objective 1C).
6. The loss of arsenic was quantified and evaluated as it occurred during prehistoric manufacturing processes through re-melting, casting and annealing activities. Analyses were carried out with DTA, TGA, SEM-EDXS, Mass-spectrometry (Objective 2).

ADDITIONAL WORK:
1. Bronze Age daggers from the Caucasus (Georgia and Russia), made of arsenical bronze, were studied metallographically and chemically (SEM-EDXS, XRF). Also, the lead isotope signature was studied (HR-MC-ICP-MS).
2. In cooperation with the Marie Sklodowska-Curie Individual Fellowship “Breaking the Mould”, UCD School of Archaeology, Dublin (Barry Molloy), material evidence for the production of bronze objects in Bronze Age Europe was evaluated. A specific intention was to disaggregate the various steps in the lifecycle or functional biography of objects with the objective of assessing the potential ways that people could be involved in different varied aspects of the production cycle.
The creation of out-of-equilibrium phase diagrams might have an impact on industry, once arsenical bronze is re-discovered and re-used again. The interest in this alloy for the moment is limited to archaeometry and archaeometallurgical research.
Currently an intense debate on the question of metal recycling is ongoing in the field of archaeometry (see also the recently awarded ERC-Advanced grant FLAME; PI: Mark Pollard, Oxford university), on which the results of ArsenicLoss will have major impact. In recent years several scholars have used the compiled data from the Studien zu den Anfängen der Metallurgie (SAM) project, and reintroduced the idea that the chemical composition of ancient copper-based metal would have changed over time due to repetitive recycling and mixing of old metal and tried to explain the loss of arsenic and other elements commonly found in ancient alloys (see various studies from Pollard/Bray). For example, it has been suggested that arsenic and other element losses are gradual and easily predictable by categorizing the ‘presence’ of elements into groups. The idea being that these groups allow one to trace –assumed linear behaviour that is unaffected by other elements– through progressive recycling campaigns of metal over time and space. The problem, however, is that predetermined cut-off percentages are irrelevant to element loss kinetics, and the loss behaviour of elements is not linear. Also, these models do not consider primary and secondary processes that greatly influence the physical location of elements and substantially contribute to non-linear percentage losses and gains of elements (e.g. inverse segregation, reduced or oxidised atmosphere), as well as non-constant remelting conditions in prehistory. Moreover, changes in artefact chemistry that occur during remelting/recycling are dynamically influenced (and not linear!), additive and subtractive dependent upon total and varied composition, and based on the percentages of both major and minor elements present in a copper alloy. Starting with first experiments with CuAs alloys under a controlled set-up thus contributes significantly to understand the effect of ancient recycling on the chemical composition of the object, and whether or not arsenic can be used as an indicator of the amount of recycling activities in prehistory.
Inverse segregation in an experimentally chill-cast arsenical bronze
Arsenical bronze with 9 wt.% As
Polynomic lines of the CIELAB coordinates of different copper alloys
a) Summary DTA heating curves for the investigated Cu-As alloys. b) Summary of the TG curves.
Calculated out-of-equilibrium phase diagram for the As-Cu system
Cooling simulations for different mould materials with same geometry
SEM-images of selected microstructures of Bronze Age Georgian dagger blades