Casting Bronze in the Italian Bronze Age Way

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.

The furnace. Not much to see here, and even less after a few thousand years!

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 tuyere with the moulds set next to it to warm up. The small bowls are the crucibles. Note that they are used without lids.

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.

Beautifully constructed wooden tongs

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.

Some of the replica tools and ornaments cast at the Terramare Museum

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.

A nice flat anvil stone for grinding the flashing off of the dagger blade. There’s evidence from the originals from this area that the holes for rivets were punched rather than drilled, or cast in place. Other stones are used for coarser grinding. Note my hammer at the bottom. It came in handy for breaking off some of the flashing. A bit of time travel here since this is a replica of a Late Bronze Age hammer!

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.

Useful links

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.

A film about the Terramare and Bronze Age metalworking (in English)

Il Tre di Spade also have a Facebook page.

 

Sedgeford Archaeology: Metallurgy and more videos!

In the summer of 2014 I had the pleasure to assist Dr Eleanor Blakelock in running week-long seminars in archaeometallurgy and experimental archaeology at the  Sedgeford Historical and Archaeological Project (SHARP) in Norfolk. The current excavations are focused on two areas of an Anglo Saxon village, however their “primary objective is the investigation of the entire range of human settlement and land use in the Norfolk parish of Sedgeford”. The excavations have been going on since 1996, and the organisation provides comprehensive teaching in a wide area of archaeological subjects. You can read all about the project here.

The previous year SHARP began a new course in archaeometallurgy. Ellie wanted to expand the course, so I lent a hand with some of the hands-on and experimental work. We built furnaces, made moulds, crucibles, mixed alloys, and cast bronze. We even got some local ore to smelt iron. It was an intensive week and the hottest one I have ever experienced in England.

During the event, Ellie took some videos of us in action. The first one shows us building  a pit furnace for casting bronze and a pit for heating the moulds. The video then follows us through the casting process.

One interesting phenomenon of the week was how we became separated from the rest of the SHARP community. We were given our own space that wouldn’t interfere with the the trenches or the campground, and was not archaeologically sensitive. The first two days when we were building furnaces we kept to the same schedule as everyone else. However, once the casting began we couldn’t stop for meals or keep to the schedule that everyone else had. One of our group would go down to the mess tent and bring back food for the rest of us. We were effectively isolated, although the others always knew where we were from the rising smoke. Later when we started showing up with freshly cast  bronze jewellery there was a bit of envy and wonder. In the space of a couple days we had gone from being part of the community to the people who were “over there” with special knowledge, and who didn’t conform to the regular schedule or tasks that everyone else did. We kept odd hours, were continually covered in soot, but had become somewhat wizard-like in our knowledge of metalworking (not to mention regularly getting out of kitchen chores!).

On the final evening, everyone joined us for the iron smelt. The week had been intensely hot so we started stoking the bloomery furnace in the late afternoon, after the heat of the day. As the sun set and most folks had finished their supper, they came by to see the furnace in full swing with fire shooting from the top. Of course everyone wanted to take a turn at the bellows. It was a magical evening. Stories were told, and mysteries presented. The people who worked on the furnace and participated in the smelt had a new appreciation for the process of making iron. The archaeologists also had a more rounded knowledge of what a smelting site could have looked like, including the resources needed for smelting iron, and the physical space that people would have inhabited while at work. It was past 1 AM when we finally called it quits for the night and let the furnace cool.

In a way the course was an initiatory experience for the archaeologists who participated in the event. It was hard work, but they learned the secret knowledge of the smiths and have stories and their bronze castings to prove it. They also gained a well-rounded introduction to archaeometallurgy that included both theory and practice.

I have been back to help run the course and plan to be there again this summer. The course will run again this July. If you’re interested in learning more about the course or being part of the excavation please sign up on the SHARP website.

 

 

Traces of Empire – Video!

Some months ago Weston Park Museum here in Sheffield approached me about making a film about how metalwork would have been done in Roman Britain. Most of what I do is related to the Bronze Age, but I jumped at the chance to do something new. We set up a time to go take a look at the brooches they would have on display. After photographing them and taking measurements, I made some waxes and then made some moulds. The process is pretty well explained in the video.

I also realised that bag bellows would probably not be the way to go, so I built the bellows that are described in the tutorial on this website.

Alan Sylvester, the filmmaker for Museums Sheffield and Lucy Creighton, (now the acting curator of archaeology) both spent long hours at Heeley City Farm helping me build the furnace and pump the bellows. After a day of filming Alan felt he needed more shots of metal being poured, and so we went back for a second day of filming. This time I had some of the pieces I cast earlier, so we could show a bit of the clean-up.

It was a great experience. Later I gave a talk at Weston Park Museum about making the film and the importance of experimental archaeology. I also brought along the bellows and some of my tools. If you go see the exhibit, there’s a shorter version of the film on a loop near the display.

 

Making a portable furnace

While this technically isn’t experimental archaeology, this is a good way to facilitate archaeological experiments in metallurgy.  I regularly cast bronze and smelt metals using a small clay-lined pit furnace. However, there are occasions when I am asked to demonstrate Bronze Age metalwork, but am not allowed to dig a hole in the ground. Museums and schools do get fussy about grassy areas and holes that could be a tripping hazard.

I found the link below and I thought it would be a good solution to my portable casting problem. The tutorial was for a making a small iron working forge, but I decided to make one that would be a scaled up version that would enable me to do the same sort of casting that I do with a clay furnace. http://www.instructables.com/id/Make-a-Small-Blacksmith-s-Forge/?ALLSTEPS

When I say that I do Bronze Age casting, I have to be honest about it. So far, in Britain no intact Bronze Age furnaces have been excavated or properly identified. The furnaces I used are based on the work of others and best guesses as to how the technology was done[1]. I have used short shaft furnaces, similar to, but much smaller than bloomery iron smelting furnaces. I have also used clay bowl furnaces with the air supply coming across the top or from below. The most efficient type I’ve used is a two chamber furnace with the air introduced into a lower chamber with the charcoal and crucible supported above it. It fires up quickly and evenly, plus it has the advantage that the crucible has a stable support, so I don’t have to worry about spilling the metal because the charcoal is moved around.

Step one was to find a sturdy steel bucket and drill a hole in the side near the bottom for the tuyere, the tube through which air is blown into the furnace. The air is needed to increase the heat.

A bucket, a steel tube, and furnace cement. Let’s get going!

I had a length of steel tube and a flat drill bit (spade drill bit in the US) of the same diameter. The hole was drilled about an inch and a half above the bottom of the bucket to allow for at least one inch of space for the furnace lining.

furnace 2
The view from above. It all fits nicely.

The original website used furnace clay, but didn’t say much about it. I bought a tub of Cementone Fire Cement for £8 at B&Q. As you can see it did about half the job, with the clay packed about an inch thick. The directions said to use a trowel, however the cement has a texture like gritty plasticene, but not quite as rigid. Getting it smoothed in the bottom of the bucket was awkward with the trowel (I’m more used to removing material with a trowel than adding it) so I put on some plastic gloves and pushed it into place.

furnace 3
That’s how far one 5 kg bucket of furnace cement will go. Time to go out and get another…

Now the instructions didn’t say anything about getting it on your hands, it just had warnings about getting it into your eyes. However, knowing that many materials like this can be caustic I decided to err on the side of caution and wore nitril gloves. The container had a handy link to the MSDS (Material Safety Data Sheet) so I could check out all the possible material hazards. The MSDS is a great online resource that will let you know exactly what’s in a product and every possible statistic for it. Check it out here http://www.bostik.co.uk/diy/product/cementone/Fire-Cement/109. Note that it is caustic and there are precautions against getting it on your skin. It also tells you what to do in case of contact with skin or eyes.

Furnace cement doesn’t harden until it’s heated to 100 C, so that means that it won’t air dry until you heat it up slowly. In addition to lining the bottom and sides, I put in small knobs around the inside to hold up a small platform, known variously as a tea pot stand or a perforated clay slab. There’s more about those here: https://www.academia.edu/6564140/Of_Tea_Pot_Stands_and_Perforated_Slabs_A_report_on_the_comparison_of_experimental_bronze_casting_furnaces_with_artefacts_from_SE_England

I used to work with someone who was severely health and safety challenged. I survived, but in the process learned the effects of being too intimately acquainted with carbon monoxide. It should go without saying, but I’ll say it anyway. Do this outside with plenty of ventilation and don’t hover over the thing while it’s being heated. It’s not a barbeque (at least not yet, there’s plenty of time for that later).

I took out the tube that I was using for the tuyere and built a small wood fire in the furnace and faced it so the tuyere hole faced the breeze. The fire lit quickly and I kept it topped up with scrap wood and a little charcoal and let the whole thing burn down. Once it was cooled, the material was hard as a rock. The next step was to make the tea pot stand. It’s simply a flat plate that fits the diameter of the interior of the furnace with holes about an inch from the edge. The tea pot stand  allows the air to circulate freely below and then up through the holes to increase the heat of the charcoal. Once that was fired the furnace was ready to use.

Finished furnace after a few firings.
Finished furnace after a few firings.

I’ve used the furnace now several times and it holds up well doing high temperature work. My initial fear was that the seams in the bucket wouldn’t hold, but it remains intact after melting bronze and copper. I’ve used it both with bellows and electric pumps.

Furnace with tea pot stand (AKA perforated clay slab in place.
Furnace with tea pot stand (AKA perforated clay slab in place.

After I made this, there was some discussion about experimental archaeology and authenticity on the EXARC Facebook Group (a group well worth checking out if you have an interest in any aspect of experimental archaeology). I made the point that this isn’t designed to replicate a Bronze Age furnace, but it replicates the conditions of how we believe Bronze Age furnaces performed. Many early experiments were done using modern gas or electric furnaces, however those have oxygen enriched atmospheres. Charcoal fires have reduced atmospheres, meaning that the air immediately around the crucible is free of oxygen. This is good news because less dross and slag is produced since the environment won’t allow the surface of the molten metal to oxidise.

As for electric pumps, sometimes it’s valuable to have a controlled air-flow. Having an electric air pump means that I can control how much air goes into the furnace and replicate conditions from one pour to the next. This way I will be able to have multiple experiments conducted under conditions as close as possible to each other. It would be difficult to replicate the controlled airflow of an electric pump with bellows since there might be times when I get tired or there is some distraction and the air flow is slightly less than for the previous pour. Control in these situations is important for experiments where I would I want to compare the melting times of different alloys and need to control as many variables as possible.

Another advantage is that this furnace always starts out at the same temperature. While a bowl furnace dug into the ground is well insulated, there are often problems getting it dried out or warmed up after a night of rain. While it’s good to have the experience of getting a cold, damp furnace going, it’s also nice to have one where I don’t use up a couple kilos of charcoal getting it dry and heated.

By the way, it’s not only good for metal casting, but with a small grate, it does a good job as a barbeque.

 

[1] I should note that while there haven’t been any of these types of furnaces excavated in Britain, twice now when I have been demonstrating using a small bowl furnace, field archaeologists have told me that they excavated something that looks identical to what I was working with, but didn’t know what it was and wrote it up as a cooking hearth.