New article available through EXARC. See the link below for the full article
It is accepted knowledge that when re-melting alloys, some of the metal with a lower melting temperature is lost through oxidation, and more metal must be added in order to maintain the desired alloy proportions. In order to understand the changes in alloy content when recycling using Bronze Age technology, experiments were undertaken by the author and others, using a charcoal furnace. These experiments included recycling bronze to quantify the loss of tin, and how alloys were affected by co-melting metals. The results were then compared to modern metallurgical practices using electric and gas furnaces. The initial results were presented at the Historical Metallurgy Society’s Research in Progress Conference in November of 2009. However, this paper includes further experiments that build on the earlier work. The conclusions indicate that knowledge of earlier practices was lost with the advance of technology, and that broad assumptions cannot be made about earlier technological practice based on work done with modern equipment.
In October 2015 I had the great adventure of doing some bronze casting with Il Tre de Spade (The Three of Swords) at the Archaeological Park and Open-Air Museum of the Terramare in Montale. The museum is located just south of Modena and recreates Bronze Age houses surrounded by a palisade and a marsh, appearing as it would have in the central and later phases of the Bronze Age there (1600-1250 BC). The museum hosts demonstrations and activities, along with a recreation of the original excavation.
The houses are nicely furnished, with well laid out areas for cooking, sleeping, food storage, and workshops. One house has a workbench for wood and antler working, and another area set aside for weaving and textile crafts. The other has a metalworking workbench with stone anvils, moulds, bellows, and all the needed kit stored neatly on shelves and a work bench. Unlike the recreated roundhouses that are often seen in the UK, you can get a good idea about how the ancient people lived here and where they put their all the things they used in every day life.
I had been invited to join the casting demonstrations by Claude Cavazzuti, who is also a member of EXARC. He is one of Il Tre di Spade (along with Pelle and Scacco) , who were some of the first metalworkers at the museum after it opened several years ago. They do regular demonstrations (sometimes with hundreds of visitors per day) and also run workshops that introduce people to Bronze Age casting and metalworking with a focus on the Middle and Recent Bronze Age (1700-1150 BC) of Northern Italy.
The furnaces there are small clay-lined trenches about 25 cm deep and 50 cm long, something that would be nearly invisible archaeologically and easily interpreted as a cooking hearth (and really, there’s no reason why they couldn’t be both). The furnaces heat up quickly and work efficiently. The charcoal is concentrated around the crucible with more warming beside it so that the fuel is hot before it gets raked around to cover the crucible.
My first new introduction was the bag bellows. They were larger than I have used in the past, and made of much heavier leather. They put out an enormous volume of air, and their size also allows the person pumping the bellows to sit on a low stool.
The real challenge for me was the valve and the way the bellows open at the top. The bag bellows I’d used before have straight sticks in the handles that either open parallel to each other, or are hinged at the back to open like a “V”. These bellows have two sticks on both sides, so that they open into a diamond shape making another hinge where your hands hold the bellows. It took a little bit to get used to them and to figure out where best to position my fingers. I never did get it quite tight enough and could feel a bit of air blasting on the back of my arms, but they still delivered a powerful amount of air. The tuyere was a large clay tube that curved downwards at a 90° angle, and was positioned so that it was directly above the crucible. This meant that the charcoal had to be moved frequently to keep the crucible covered. Without the layer of charcoal above, the air coming from the bellows would cool the metal in the crucible.
One nice innovation is that they put a large stone in the bottom of each bag. That prevents people who are new to bellowing from lifting them too high and causing the bags to collapse.
Casting in Stone and Sand
The crucibles are a flattened dish-shape, with some that were larger and a bit more of a bowl shape to hold more metal. They have a grooved tab on one side that is used as a handle, and have a small lip for pouring. The shallow design and lip cause the metal to pour more quickly and flow out in an arc, rather than almost straight down like the triangular bag-shaped crucibles used in Britain. It took a couple tries to get used to the trajectory of the metal in order to get all the metal in the pouring cups.
The shallow crucibles also mean that the pouring has to be done more quickly than with the deeper crucibles that I was used to.Because they are wide open, the metal cools quickly, so there is little time for dragging out charcoal and skimming. As soon as the metal was molten, we gave it a quick stir with a stick, pulled the crucible out with the wet tongs, and poured the bronze into the mould while holding the stick across the top to keep the charcoal back.
The tongs they use are beautifully crafted from wood and cord. They are kept in a bucket of water to keep them from burning when holding the heated crucible. The entire organisation of tools and materials is efficient and elegant, and there is nothing that could not have been made from materials available during the Early Bronze Age.
For this session we used the stone moulds that were on display in the house. The moulds were made of a very fine-grained local stone, the same that had been used by the Terramare circa 3500 years ago. The moulds were warmed next to the furnace and then strapped together with leather strips. We took turns bellowing and pouring.
We cast sickles, knives, and daggers using the stone moulds. Later we moved on to using sand moulds. I borrowed one of the antler spindle whorls from the woodcarving house to see if I could try casting that as an experiment. It worked sort-of. It will take a bit of work to get finished up, but I think with a couple more tries we could have got it spot-on.
I was wearing my bronze torc bracelet that day and one of the new people wanted to try casting that. We used it as a model for a sand mould and after a couple tries, we got a cast that made a perfect duplicate.
Casting a bronze sword
The museum was also performing public demonstrations that day. The main event was casting a sword using a sand mould. The sand they use is local, and perfect for casting. Commercial casting sand consists of fine sand mixed with dry bentonite clay. The sand from the Po River delta is exactly that, sand that has been reduced to almost a powder by erosion, combined with clay and silt that has been washed into the river (I encountered the same natural mixture in Albuquerque in the dry bed of the Rio Grande River Valley). The Po River sand leaves a much finer texture on the finished objects than the coarser commercial sand that I have used in the US and the UK. Looking at the quality of the sand, I was able to understand how different sands might affect the regional quality of casting, and might even have had value as a commodity.
The mould for the sword was made using the standard cope and drag method, with the pouring cup on the flat side of the sword, rather than pouring from the top. A large vent was also placed at the tip of the sword. Before casting, the mould was propped up at an angle. As the crowd gathered and settled onto benches under a marquee, Claude explained Bronze Age metalworking techniques, and then the sword was cast. The angle and the pour are calculated so that the metal cools just before it exits the vent hole at the bottom. Everyone was impressed and even more so when the perfectly cast sword was taken from the mould.
Often after the casting is done for demonstrations the bronzes are taken home and finished using drills and angle grinders, but not here. We had a relaxing time using wet sand on the hard, fine-grained anvil stones to grind off the flashing and excess metal. Some smaller stones that had a coarser texture could be held in the hand and used for working the inside curves and corners. I’d brought my small socketed hammer along and we used that to break off some of the excess metal. Between light hammering and patient grinding, the daggers were smoothed rather more quickly than I would have expected.
While we were casting, more tour and school groups came through to watch more casting demonstrations. School kids also had the opportunity to make their own copper bracelets. There was a square of tables set up with small stone anvils and hammerstones. Punches and chisels were available so they could decorate the strips of copper to make bracelets in patterns that would not have looked out of place in a Terramare village.
It was a great day, and one that was full of new experiences. It was particularly interesting to see how much variation there is in doing the same tasks and getting successful results. The different types of furnaces, moulds, bellows, and crucibles make for differences in how tasks are performed. Other small acts show variation between different metalworking groups. Tasks such as turning the crucible over to remove dross and leftover metal after casting was done into the furnace here, where in Ireland we have always done that on the ground next to the furnace. These are small things that could be seen archaeologically if excavation is done carefully, but also shows how customs and metalworking traditions develop with regional differences.
A special thanks to Il Tre di Spade, Claude, Pelle, and Scacco and the staff at the Parco archaologico e Muse all’aperto della Terramare for inviting me to participate in casting. I would also like to acknowledge Markus Binggeli & Markus Binggeli who are masters of bronze casting and replicating ancient metalworking techniques. They are mentors of Il Tre di Spade, and provide both inspiration and technical expertise for experimental archaeologists.
If you’d like to learn more about the Early Bronze Age in the Modena area, the work of Il Tre di Spade, and the Terramare Open Air Museum in Montale you can find links below.
I missed the first day of the conference. Instead I was at my PhD graduation ceremony. It was a wild trip. I graduated with the full regalia of cap and gown, had a quick couple glasses of wine at the archaeology department’s reception, and then we hopped on a fast flight to Dublin for the EAC9 conference at University College, Dublin. The conference was a collaborative effort brought together by EXARC, UCD, and the Irish National Heritage Park. It was a large conference with over 200 delegates, 20 papers, and 31 posters.
The Dublin University campus is huge and spread out, so we had a time trying to find the right building. I arrived just in time to deliver my paper and see the rest of the session. There were some interesting papers and posters given that explored the range of pyrotechnology in archaeology from cremation to glassworking and metalwork. In addition to the usual poster session, individual posters were given a ten minute presentation while being projected in the main hall. These included Jiří Hošek, Ryszard Kaźmierczak, Paweł Kucypera & Maciej Tomaszczyk (Nicolaus Copernicus University) with a presentation on steel carburising in a small shaft furnace, and Yuri Godino & Lorenzo Teppati Losè (University of Florence) presented a poster on their experiments on cupellating galena to produce refined silver.
I was also interested in the presentations on glassworking. There were two very different approaches to the subject with Marta Krzyżanowska & Mateusz Frankiewicz from Poland who spoke about producing Early Medieval lampwork type beads in an open hearth based on excavations in Ribe. Jonathan Thornton from Buffalo, New York spoke about replicating trade bead production based on evidence from Africa using glass frit in a clay mould .
The presentations that discussed metal began with my presentation on inverse segregation and its influence on chemical analysis of objects cast in the Bronze Age. Padraig McGoran of Umha Aois presented a poster on his experiments that included problems and solutions in casting into open one piece moulds.
After that I was off to the university’s experimental grounds to help set up furnaces and get ready for casting. The centre boasts a Mesolithic house, along with metalworking furnaces in varying states of decay. There are separate areas set aside for flint knapping, firing pottery, and active metalworking projects. The members of Umha Aois had already started building a variety of furnaces that included ones heated from below, from the side, and another with a tuyere that had a 90 degree bend that blew the air directly onto charcoal covering a flat, pan-shaped crucible. I worked at a portable ceramic furnace that was brought to the site by Fiona Coffey. It was set up inside Billy Mag Floinn’s newly constructed traveller’s tent. Despite it being wind and waterproof, the flaps ventilated it well and we kept warmer than the others who were set up under a tarp outside.
At lunch I was presented with a birthday cake. Surprisingly no one had anything bigger to cut it with than a pocket knife. The only solution was to get one of Billy’s bronze swords and carefully slice it. It was a most memorable birthday.
Bronze objects that had been created by the members of Umha Aois were on display, including swords, horns, tools, spears, and stone moulds. We spent the day casting axes, jewellery, tools, and more. There was a constant flood of visitors and regular announcements were made when one of us was ready to pour. For most of the day it was standing room only. The casting events continued all afternoon and into the evening.
Rather than head straight back to Sheffield the next day, I had arranged to see the Bishopsland Hoard and a hammer from the Garden Hill Hoard at the National Museum. I’d hoped that I could see some moulds, and to have some colleagues also examine the objects. Unfortunately emails were crossed and I just got to see the hoard and hammer. However, that was fascinating in itself, and I spent hours measuring, weighing, drawing, and photographing every detail of the artefacts.
Events like this are exhilarating and exhausting. We all learn more every time we meet, and we come away with new ideas as well as newly cast objects to finish up. This week I’ve been filing and polishing some of the bronze fibulae I cast and I still need to get to work on the replica I cast of the hammer from the Lusmagh Hoard. Meanwhile, there are more waxes and moulds to make to get ready for casting again.