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Prehistoric arsenical bronze objects with silvery surfaces are known from the Chalcolithic to Middle Bronze Age periods in Europe and the Near East. The silvery surfaces of these objects are generally interpreted as the result of inversely segregated arsenic. Objects commonly reported with silvery surfaces include, apart from one awl, weapons such as swords, daggers or halberds, and other bronzes connected to high status or rank such as mirrors and statuettes (Table 1).

When a cast alloy begins to freeze the solute can be redistributed resulting in macro- and micro-segregation. Whether or not inverse segregation occurs, even in arsenical bronzes with sufficient percentages of As, depends on cooling rate, freezing range, and dendrite arm spacing. Fast cooling, wide freezing ranges, and small dendritic arm spacing favours the formation of inverse segregation. Interdendritic feeders force the arsenic rich (α+γ) eutectic to the surface, which results in significantly higher arsenic concentrations near the surface than the rest of the casting. The typical surface layer of (α+γ) eutectic transforms easily into an arsenic-rich α-solid solution once the object is annealed.

Different explanations for silvery surfaces on arsenical bronzes, such as inverse segregation, an arsenic-rich α-solid solution, cementation, or post-depositional precipitation, are discussed in this poster. The segregation of arsenic was studied in as-cast As-Cu ingots produced in chill cast molds at several compositions, which underwent surface treatment with an NaCl solution. The microstructure and surfaces of the As-Cu alloys were analysed using optical microscopy and SEM-EDXS.

Table 1 – Prehistoric arsenical bronze objects with 'silvery' surface (after Mödlinger - Sabatini 2016; updated). All objects were studiedmetallographically.

INTRODUCTION

EXPERIMENT AND ANALYSIS

CONCLUSIONThere are currently no known examples of prehistoric arsenical bronzes with inverse segregation. Instead, arsenical bronzes with silvery surface show recrystallized microstructures and γ-phase layers on their surfaces, as well as γ-phase precipitation at their grain boundaries. They do not show any (α+γ) eutectic and do not have a completely homogenized microstructure. The Horoztepe bull, which has an as-cast bulk metal of copper, is the only object with a surface layer of γ-phase, which was applied by cementation.

Arsenic has a natural tendency to precipitate or enrich at interfaces such as grain boundaries. If the object had been heavily worked and homogenized, and consequently supersaturated, as these boundaries are close to the object’s surface, γ-phase might precipitate even at room temperature. The presence of corrosion resistant γ-phase can be explained as post-depositional precipitation from an arsenic-rich α-solid solution. The thicker γ-phase layer on the surface of the objects can be explained a) by the previous presence of inverse segregation, or b) an arsenic-rich alpha solid solution with nearly as much as 8 wt.% arsenic located close to the surface. Apart from one awl, all arsenic bronze objects with silvery surfaces (such as daggers and swords) are commonly connected with high status or rank, which suggests arsenic-rich surfaces were produced intentionally. However, any inverse segregation present after casting would have been eliminated due to annealing.

REFERENCES• Mödlinger M. / Sabatini B. 2016. Segregation of Arsenic in Cu-As alloys and its influence on arsenic

losses during recasting and annealing. Journal of Archaeological Science 74, 60-74. DOI 10.1016/j.jas.2016.08.005

• Mödlinger, M. / Sabatini B. forthcoming. Caucasian daggers from the Late Bronze Age: masterpieces of arsenical bronze metal work.

• Smith, C. S. 1973. An examination of the arsenic-rich coating on a bronze bull from Horoz Tepe, in: Young, W. J. (ed). Application of science in the examination of works of art (Boston), 96-102.

ACKNOWLEDGEMENTSThe authors would like to acknowledge the financial support provided by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Actions, grant agreement no. 656244.

Inverse segregation was reproduced by casting arsenical bronze with different amount of arsenic (1, 2, 3, 4, 5, 6, 8, 10 wt.% As) into iron moulds, since fast cooling favours the formation of inverse segregation. Inverse segregation was noted in several of the ingots, and in particular in samples from the ingots made with 5, 6, 8, and 10 wt.% As. Of all the analysed samples only the surface of the 5 wt.% As ingot was mostly covered in (α+γ) eutectic (Figure 2). Annealing experiments at 720°C for 10, 20, 40, and 80min showed that the eutectic easily vanishes and an arsenic-rich α-solid solution is formed.

However, even the presence of small amounts of (α+γ) eutectic on the surface resulted in the appearance of a silvery surface (Figure 3). This colouring is mainly due to arsenic-rich zones with up to 8 wt.% As α-solid solution (Figure 4). The experimental microstructures were then compared to the microstructure of prehistoric arsenical bronzes with silvery surfaces (Figure 5). Apart from the Horoztepe bull, all of the prehistoric objects were reported to be annealed and cold worked, to not be fully homogenized, and to not show any (α+γ) eutectic in the matrix. Instead, these objects show γ-phase on the surface and along the grain boundaries.

1 IRAMAT-CRP2A – Institut de recherche sur les Archéomatériaux – Centre de recherche en physique appliquée à l’archéologie

UMR 5060 CNRS - Université Bordeaux Montaigne, Maison de l’archéologie, Esplanade des Antilles, 33607 Pessac, France. Email: marianne.modlinger@u-bordeaux-montaigne.fr2 Department of Materials Science & Engineering – Massachusetts Institute of Technology

13-5065, 77 Massachusetts Avenue, Cambridge, MA 02139-4307 (USA). Email: bsabatin@mit.edu

M. Mödlinger1; B. Sabatini2

Chemical and metallurgical aspects of arsenical bronze: inverse segregation in prehistoric Cu-As objects

Figure 5 – SEM-images of selected microstructures of some arsenical bronze dagger blades from the Koban Culture (Mödlinger / Sabatini, forthcoming). a) The light grey γ-phase precipitates on the surface are rather thick (partly covered by corrosion), and thinner at the grain boundaries. Corrosion is noted at the grain boundaries (small to big black zones). b) The darker α-phase contains about 4.5 wt.% As, while the light grey γ-phase contains about 30 wt.% As. Antimony is enriched in the corrosion. c) The light grey band between sound metal and corrosion (dark grey) consists of γ-phase. d) Vast amounts of grey γ-phase is showed precipitated all along the surface (thicker bands), and along the grain boundaries (thinner bands). The dark spots are corrosion, which start at grain boundaries, and especially at the crossings of grain boundaries.

no. Object Find spot wt.% AsAnalytical method used for chemical characterisation

1 dagger, two axes Mondsee, Austria 1.5-5.4 EPMA2 dagger Forêt de Carnoët, Finistére, France 2.6-5 SEM-EDXS (?)3 dagger Spain (Los Millares culture) 5 SEM-EDXS (?)4 4 swords Palestine (3rd mill. BC) 5-7 EPMA5 15 daggers Caucasia (Maikop-culture) 4-8.1 SEM-EDXS (?)6 3 daggers Caucasia (Koban culture) 6-10 SEM-EDXS, RFA6 6 daggers Novosvobodnaija, Caucasia (Maikop Culture) 4 to 5 'spectro-analytical methods'7 2 daggers Usatovo, Ukraine (late Tripolye culture; 2nd half 3rd mill. BC) 4-5 'spectro-analytical methods'8 rivet (dagger) Monte da Cabide 3, Portugal 26.5 ± 1.3 micro-EDXRF9 awl Pechina, Almería, Spain 1.3 XRF

10 dagger Porriño, Pontevedra, Spain 2.9 XRF11 dagger Rianxo, A Coruña, Spain 3.2 XRF12 dagger Santa Comba, A Coruña, Spain 6.5 XRF13 dagger Cuevas del Almanzora, Almería, Spain 2 XRF14 bull figurine Horoztepe, Turkey no As in bulk

metalXRF

Figure 1 – Late Bronze Age dagger from Koban, Republic of North Ossetia-Alania, Russia. The colour of the hilt was silverish while the blade had a warm, golden-like colour. The hilt contains about 9.8 wt.% arsenic and no tin, while the blade contains 10.1 wt.% tin and no arsenic. Because of heavy corrosion the original colours of the metals are no longer visible (Photograph: M. Mödlinger, © Naturhistorisches Museum Wien).

Table 2 – Arsenic-rich surfaces on prehistoric arsenical bronze objects (expanded after Meeks 1993, 269, tab. 21.2).

Type Microstructure Surface Process Archaeological examples

A as-cast γ-phase mainly in eutectic form; (α+γ) eutectic layer with different surface thickness; feeders clearly visible (see Figure 1)

inverse segregation none

B recrystallized γ-phase layer; γ-phase precipitations along the grain boundaries; no (α+γ) eutectic

precipitation of γ-phase from solid solution in the post-depositional environment in combination with intentional corrosion/cementation

Axes and daggers from Austria; daggers from France, Spain, Caucasia; swords from Palestine

C recrystallized γ-phase layer on surface; no or only small amounts of (α+γ) eutectic

cementation Egyptian mirrors were probably produced by cementation

D recrystallized or as-cast; no arsenic in bulk metal

γ-phase; no (α+γ) eutectic cementation The Horoztepe bull figurine

The microstructures of the (α+γ) eutectic and the observed precipitation of γ-phase on the surface, and around the grain boundaries, of prehistoric arsenical bronzes indicate that silvery surfaces can occur as a result of different processes (see Table 2). Arsenic rich surfaces can be achieved by:

• Intentional corrosion / copper depletion

Arsenical bronzes with <2 wt.% As can form a 50-500 µm thick silvery surface layer of arsenic-rich α-solid solution (4-8 wt.% As) (Figure 3). The silvery colour can be enhanced by placing the object in an NaCl solution for up to two days. This short post-casting treatment does not result in significant copper depletion, but of depletion of copper oxides on the surface. However, subsequent and repeated annealing results in more equally distributed As in the matrix, and consequently less As on the surface and a return to the colour of the original alloy.

• Post-depositional precipitation of γ-phase

The intermetallic phase Cu3As (γ-phase) precipitates both at the surface and at the grain boundaries (Figure 5). Precipitates may even form at room temperature when the alloy contains higher amounts of arsenic, is heavily worked, and homogenized (Northover 1998, 117; Meeks 1993, 270). The γ-phase precipitate most likely coalesces where the original feeders or As-rich zones had formed in the cast. All of the objects with silvery surfaces given in Table 1, apart from the Horoztepe bull, showed γ-phase precipitate along the grain boundaries as well as a at the γ-phase surface layer.

• Cementation with Cu3As

Apart from inverse segregation, deliberate corrosion, and precipitation of arsenic-rich compounds, a fourth method to produce a silvery surface on copper is by plating. Plating is achieved by applying an arsenic-rich compound to the surface and subjecting it to heat, which results in the formation of γ-phase. The only known example of this method is the Horoztepe bull, which has no As in the bulk (Smith 1973).

Figure 4 (right) – SEM-EDXS images of the silvery surface of the 2 wt.% As ingot shown in Figure 3. a) Light zones are rich in arsenic. Note the arsenic-rich bubbles that are always surrounded by arsenic-poor zones. b) Detail of bubbles embedded in an arsenic-poorer surrounding of up to 5 wt.% arsenic. c) Detail of the arsenic-rich surface with (α+γ) eutectic reaching the surface. The top of the dendrites have less arsenic (4-5 wt.%), while the base has more (5-7 wt.%). d) Detail of a freshly formed and complete bubble.

Figure 2 (left) – Microstructure of 5% wt.% As arsenical bronze with inverse segregation (presence of grey (α+γ) eutectoid on the surface).

Figure 3 (centre) – Ingot with 2 wt.% As. Note the silvery surface above and, more prominently, below the sampling location.