Marianne Mödlinger

Finally, the project’s first article A Re-evaluation of inverse segregation in prehistoric As-Cu objects is out (others will follow soon). It’s published in the current issue of The Journal of Archaeological Science. The study revaluates reported cases of prehistoric As-Cu objects with ‘silvery surfaces’, which are usually interpreted as the result of inverse segregation. Further possible explanations for such surfaces, such as an arsenic-rich alpha-solid solution, cementation, or post-depositional precipitation, are discussed. The segregation of arsenic was studied in As-Cu ingots produced in chill cast moulds 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. Special note of out-of-equilibrium As-Cu phases are discussed, as well as a comparison of inverse segregation to all other means of achieving surface silvering.

After November 1st, 2016, you can have access to the article via academia, or just send me an email.

The DTA analyses started in February 2016 are finished in May 2016, and an out of equilibrium phase diagram for the Cu-As system up to 15 wt.% arsenic was established. One of the characteristics of the diagram is also the appearance (even below 2 wt.% of arsenic) and temperature changes of the (α+γ) eutectic. I am currently preparing the article, which, once accepted, will be posted here. I noted also some losses in arsenic, which are mainly the result of adding the arsenic lump to the molten copper.

Recently started analyses with the DTA-TGA instead show significant losses of arsenic when melting the alloy, and less once it is cooling down. I am thrilled to see more results and curves, and to figure out how to connect them with the loss of arsenic during prehistoric recycling activities!

Following an invitation of the University of Cork, I was able to visit the Ross Island copper mines beginning of March 2016.

The prehistoric copper mines, which date back as far as 2400 BC, are located in the Killarney National Park. The mines are situated at the shore of the lake on an peninsula, less than half an hour walk from Ross Castle, a 15th century fortress. One can arrive to the mines by following a trail, passing through swamp and a wild forest, and the area of recent mining activity from the 19th century.

It is still possitble to see lots of traces from prehistoric mining on site, such as working ‘tables’, grinding stones, and remnants of fire setting. Unfortunately, the full extend of the underground mining is not anymore accessible today due to floods and roof collapses. Ceramic found on site connect the mining site with the Beaker material culture. Close to the mines, just a few meters higher above the lake, the miner’s work camp, which was only seasonally used, was found. Here, the minerals were crushed, hand-sorted, and smelted in shallow pit furnaces. No slags were found. The ingots produced were then transported to the nearby settlements and cast into other objects.

The arsenic content of the main copper mineral mined, tennantite, reaches up to 20%. Of course most of it was reduced during smelting, but the final objects still show significant amounts of arsenic (1-5 wt.%) and a distinctive impurity pattern (As>Sb>Ag). This, and the high amounts of arsenic, can be traced in the so-called ‘A’-copper, which circulated widely in Ireland in the period 2400-1900 BC, and also reached Britain. Ross Island is so far the only Irish copper mine with prehistoric mining known with such high amounts of arsenic, and is consequently most likely the source of this ‘A’-copper, which makes it the oldest known mine in Ireland.

After my presentation, we visited Ross Island with Prof. William O’Brien. Wind, rain, and temperatures below -10°C (at least that what it felt like for me) did not stop us to collect some tennantite, which I intend to use for several smelting and melting experiments.

In February 2016, first DTA tests were carried out with CuSn alloys to be certain that the protocol of investigation works and the alloy to be studied is not going to contaminate the DTA. For the analyses, a NETZSCH DSC 404C was used. The alloys (CuAs with 3, 4, and 5 wt.% As) were sealed in tantalum crucibles in Ar atmosphere. First cooling rates were fixed at 2, 5, 10, and 20 K/min. With the selected alloys we still remain in the range of the Cu based alpha solid solution. With the 2K/min cooling rate no eutectic transformation was noticed, which is coherent with the equilibrium phase diagram. 4 and 5 wt% As show already at 5K/min and 10K/min signs of eutectic transformation, which happens at lower temperature than in the phase diagram. The measurements at 20K/min are missing for CuAs-3 and will be re-measured. The measurements for the alloys with 1, 2, 6, and 7 wt.% As are currently carried out. Further analyses are planned with 8, 11, and 19 wt.% As, in order to achieve a complete set of data to draw the liquidus curve and the interception with the eutectic line.

To study the microstructure, segregation and material characteristics, four different mould materials will be used for casting: steel, terracotta, sandstone and sand. The steel mold is ready and was already tested for experiments on inverse segregation. The terracotta moulds are ready, but still need to be burnt… and the oven broke, so I will have to wait a bit to continue with these moulds. The sandstone moulds will be ready most likely by the end of the month – working sandstone is not that easy! …now I just need to prepare the wood box for the sand casting and I can start! (September 2015)

In order to study the loss of arsenic during prehistoric recycling activities, I started with the first casting experiments in July 2015. A 5% arsenical bronze ingot with 250g was produced (see image on the right) and re-cast already several times. Graphite crucibles were used both for casting and as mould. The bronze was melted in a small furnace in reduced atmosphere and cast at 1110°C. Thus, As is lost only during the casting process itself and the cooling down of the alloy. Once cooled down, the crucible containing the bronze was placed in the oven at 1110°C and recast in another graphite crucible. The loss of arsenic was noted by white fume for about 1 min after the cast. In a few days I will have a look at the samples with the SEM-EDXS… looking forward to it!

The annual conference of the European Association of Archaeologists (EAA) will take place in Glasgow, UK in September 2015. Together with P. Bray, A. Cuénod, and C.N. Duckworth, I organise a session on ‘Recycling things and ideas: linking scientific, archaeological and conceptual approaches to the reuse of materials in the past’, which will certainly result in valuable new ideas, discussions and approaches.

Session abstract:

Recycling touches upon all aspects of archaeology, but it often suffers from its perceived ‘invisibility’ in the material record. Reuse, reforming and mixing of material alters its composition and shape, and can therefore directly affect conceptions of source, provenance, technology and chronology. The malleability and reuse of material challenges simple ideas of identity, value, economy and exchange. Despite the profound effects of recycling it is often difficult to identify and quantify in the archaeological record, particularly for materials and artefact-based studies (by comparison with, for example, architecture). This means that the manufacture of objects from ‘prime’ raw materials is often implicitly assumed to be standard practice. We are also in danger on relying on modern conceptions of recycling that focus on ecological concerns and coping with scarcity. Improved communication and co-operation across a range of disciplines offers new ways to better understand the flow of materials in the past. This session aims to provide a forum for these discussions and will highlight new approaches and models. Papers will examine how lab-based science, experimental archaeology, excavation and artefact-led archaeology, and conceptual archaeology can come together to identify and quantify recycling and its effects. This will be in conjunction with considering the challenges involved in pursuing inter-disciplinary work.

Further activities at the EAA will be the organisation of a round table about the Sellout of our past: different strategies of how to deal with illicit trafficking of European Cultural Heritage and a presentation on metal analyses of Bronze Age defensive armour.

July 2015: Before work can begin on the arsenic bronze, all relevant safety aspects must be considered, i.e. I must ensure that the work will not harm myself or others in the lab. Reading the standard precautions for arsenic (e.g. from Sigma Aldrich) was informative if not a little scary.

Full respiratory equipment (e.g. full face particle respirator type N100 with appropriate cartridges and filters), a disposable complete suit protecting against dust and particles, and gloves will be worn as a precaution to ensure personal safety in the event of a toxic arsenic oxide vapor release during the experimental work.

The experiments will be performed under an extractor hood equipped with adequate filters in case any leakage occurs. The reactor consists of a crucible or a quartz vial sealed with a consumable glass / quartz pipe. The pipe is shaped to cool the vapours from the reactor and direct them into a container (e.g. a beaker) where a basic water solution will receive and fix the As-rich compounds in a stable arsenate salt. The glassware and the arsenate-rich solution will be stored as hazardous waste in secure gas and watertight containers and disposed of correctly by a licensed company, together with the laboratory gloves, coats and filters.

Read more about the physiologic effects of arsenic exposure here.

The core idea of this project has been in development for a very long time – by now, it must be over 10 years. While writing my PhD about European Bronze Age swords I came across the article of McKerrell – Tylecote 1972 and became curious about the amount of arsenic that actually remains in bronze after several (prehistoric) recycling processes. I tried to find further literature focusing on this but apart from the article of Lechtman 1996, the unpublished PhD-thesis and some hard-to-get articles of Paul Budd, as well as the article of Northover 1989, not much seemed to have been done until around 2005.

In the meantime I focused on other topics: I finished my PhD, documented Bronze Age weapons and tools in the Baltic States, then moved to Carinthia and started to work at the local Landesmuseum. Amongst other things, research projects of the Austrian FWF, CHARISMA and Marie Skłodowska-Curie actions subsequently permitted me to study and analyse e. g. European Bronze Age body armour at the Laboratorio di Metallurgia e Materiali in Genoa.

However, I remained intrigued by the loss of arsenic during prehistoric smelting and melting. Every time I saw diagrams in archaeological publications linking the amounts of As, Sb, Ni and Ag together in order to locate geographically a copper mine, I had the Ellingham diagram of the formation of common metal oxides in mind, noting that arsenic (as well as Sb) is far from being as stable as these publications sometimes would lead one to believe.

A literature study at the end of 2014 indicated that even up until that point, no further experiments had been published. However, recent studies carried out by M. Pollard, P. Bray and A. Cuenod demonstrate the importance of arsenic as a marker for the intensity and / or number of recycling activities in the Bronze Age, and how it may indicate direct or indirect connections between the owner of the final object and the mine(s) where the copper was originally produced. In addition, thermodynamic studies on CuAs are now close to being published (B. Sabatini).

Starting with a project to investigate the construction of out-of-equilibrium phase diagrams of Cu-As, to evaluate mechanical properties and characteristics of arsenical bronzes and finally, to quantify and evaluate the loss of arsenic as it occurred during prehistoric manufacturing processes, will certainly be a challenge – scientifically as well as for health reasons, since arsenic is not really famous for its positive effects on one’s health.

Ultimately though, I am thrilled that from July 2015 on I can finally start to work with arsenical bronze after all these years – and especially to have the opportunity to carry out this project at the IRAMAT-CRP2A at Bordeaux in cooperation with the LMM at Genova!

In April 2015, prior to the project’s official start, I was already able to present my ArsenicLoss project ideas and work plan at the ‘Glass and Metal Recycling in Archaeology and Archaeometry’ workshop at the University of Leicester, UK. The workshop was kindly organised by A. Cuénod and C.N. Duckworth (a warm thank you to both for everything!) in the frame of the ERC-funded Trans-Sahara project. This workshop and the associated discussion possibilities provided valuable hints for the experimental set-up and laid the groundwork for cooperation with other colleagues.