DTA and DTA-TGA results

CuAs-11, after 20K/min DTA

Arsenical bronze with 11 wt.% arsenic after cooling down with 20 K/min. Note the inverse segregation at the edge, and the massive appearance of (α+γ) eutectic.

The DTA analyses started in February 2016 are finished, 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!

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Visit to Ross Island, Ireland

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

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.

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Ross Island, relicts of prehistoric copper mining activities.

…the minerals collected were good, as I realised the next day at the costums at Dublin airport: security stopped me to see which kind of strange metals I have in my bag. Should be at least 40% copper, they said. I am going to figure that out soon!

first DTA analyses

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.

Project presentation at the IRAMAT-CRP2A, Bordeaux

IRAMAT-CRP2A : les séminaire mensuels ont repris!

L’Institut de recherche en archéomatériaux – Centre de recherche en physique appliquée à l’Archéologie (IRAMAT-CRP2A) propose un séminaire autour du thème “Chemical and metallurgical aspects of arsenical bronze: the case of arsenic-loss in prehistoric metal production“, présenté par Marianne Mödlinger, titulaire d’une bourse Marie Curie. Il aura lieu le 23 octobre 2015, 12:30 à Pessac (salle Archéométrie du CRP2A).

Update: casting

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terracotta moulds for casting the CuAs ingots

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…breakage during casting

Over the last weekend, I burnt the clay moulds and tested them with CuPb and CuSn last Monday. Unfortunately they broke during casting. So this means back to the start, and redo the moulds – this time with tempered clay and some hay, which makes the mould also much closer to the ones we know from the Bronze Age!

The sandstone moulds are in preparation as well; I also started with the first sand casts – more soon!

Preparing the moulds

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!

 

Terracotta moulds in preparation (in different condition).

Terracotta moulds in preparation (in different condition).

 

 

First casts started

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Right after adding the As to the molten copper, blue flames and arsenic trioxide in form of white fume are visible.

In order to study the loss of arsenic during prehistoric recycling activities, I started with the first casting experiments. 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!