The Mavis Furnace: Casting in Albuquerque or how to build a furnace for almost no cost!

This autumn I was visiting my sister who lives just outside Albuquerque, New Mexico. She has a bit of land, so I asked her if I could build a furnace and do some casting there. I’ve built plenty of pit furnaces, so this time I wanted to build one that was above ground and a bit more visible. It would also mean that no one would trip and fall if they forgot where I put the thing.

First off was to get the basic materials: Clay, Sand, and Horse Manure.

I thought that the clay would be the biggest hurdle. However my sister’s neighbour happened to be a sculptor who works in both iron and ceramics. She went to her regular clay supplier and got a couple feed sacks full of dried clay for free! Potters who work at wheels, or work with kids end up with a lot of clay that is scraped off of workbenches and wheels that is hard to re-use. It’s often mixed consistency, lumpy or part dried. If they want to use the clay again, it has to be reconditioned. So, they take their sacks of scrap back to the place where they buy the clay and pay to have it put through the mill. Since it’s not really usable, some potters are willing to just give it away. So thanks to Liz Fritzsche, who does amazing and beautiful work in porcelain, I was able to get started.

dscf4218
Dry clay

I threw some of the dried clay into a five gallon bucket and poured in enough water to cover the clay and let it sit. It was squishy and workable in a few hours. Next up was horse manure. A friend of a friend has horses, and they were grateful to have someone who would haul away a couple of barn buckets full. The final bit was the sand. My sister did have some beach sand that she’d bought for the garden, but I was more fascinated by the soil there. Her land is in the old Rio Grande River Valley, and the soil is a mixture of silty sand and a little clay that had been pounded to a powder. The soil was almost the consistency of dust. It mixed in perfectly and later I found the mixture was highly resistant to cracking.

The ingredients were mixed with some water, stomped, and adjusted. New Mexico is a lot drier than England, so I ended up adding more water as I worked. It was also nice knowing that it was unlikely for any rain to fall while I was working.

The silt/clay/dung mixture made a durable clay, and after I flew back to Minneapolis, I was please that the crucibles and moulds I packed in my check-in luggage survived airport handling.

dscf4219
The Mavis Furnace: note the small tabs inside. These support the Teapot Stand.

The furnace walls are about 8 cm wide (about the width of my palm) and the inside diameter is about 25 cm in diameter. I let it dry for a couple days and then moved it over to an unused area behind a greenhouse where I would have more room to work. I set a small fire with cottonwood branches, fed it for a couple hours, and then let it die out overnight.

dscf4220
Mavis the dog immortalised in a furnace!
dscf3520
Mavis in a calm moment. Her nickname is the Dog-nado.

 

 

 

 

 

We all like to put some decoration on our furnaces. My students have done everything from dragons and demons to cats. I had been thinking about doing some imitation petroglyphs, but as I was working it just seemed natural to do a portrait of Mavis, my sister’s exuberant German Shepherd.

Finally I adjusted the mix to have a bit more of the silty sand and horse dung to make the teapot stand and crucibles. For more information about teapot stands and how they work in furnaces, check out my article on the Umha Aois website here.

Later I added more water to make the slip for dipping waxes in for the first stages of mould making. The silt made a wonderfully fine mould that picked up all the details.

dscf4253
The Mavis furnace, teapot stand, and some crucibles

I had some cracking on the upper part of the furnace, and noted that the part that cracked was where I used the commercial sand. The silty New Mexico soil held up much better. If it weren’t so heavy (and probably not allowed) I would have hauled bags of it on the plane to use for more projects.

Both Mavises performed very well (Mavis the Dog is into barrel racing and advanced obedience classes).  I’ll be interested to see how it holds up over the winter. Winters in New Mexico tend to be mild, so I’m hoping that the Mavis furnace will be available for friends in Albuquerque to use for some time to come.

 

Here’s a very short video of the furnace in action.

 

Like this post? Please share it!

20 February 2015 – Fibulae

I finally got the fibulae I cast in Dublin last month finished and fitted with pins. They polished up nicely and look a lot more shiny than their counterparts in museums. I still have a hammer and a couple other castings to clean up. Meanwhile a new batch of waxes are in moulds and if the weather behaves they could get cast up in the next couple weeks.

This is a large one, 8.5 cm long. It's the type you'd need for holding a heavy woolen cloak as you patrolled along The Wall.
This is a large one, 8.5 cm long. It’s the type you’d need for holding a heavy woolen cloak as you patrolled along The Wall.

 

This one is a bit smaller at 4.5 cm, but still a sturdy brooch.
This one is a bit smaller at 4.5 cm, but still a sturdy brooch.

 

This one was snapped up by Cordula before it was even out of the mould! Cordula was steadily pumping the bellows while I had the cushy job of pouring the metal.
This one was snapped up by Cordula before it was even out of the mould! Cordula earned it after steadily pumping the bellows most of the day while I had the cushy job of pouring the metal.

 

There’s even a slow motion film of one of these guys being cast thanks to the folks at UCD.

Like this post? Please share it!

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 this link  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.

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 needs to be 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 from the charcoal so the temperature gets hot enough to melt bronze.

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 plasticine, 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. 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.

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

 

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. 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. 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 come 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, not to mention trying to get it heated up for a wintertime demonstration when the ground was iced over. 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 described it in their reports as a cooking hearth.

Like this post? Please share it!

Traces of Empire – Video!

 

In the summer of 2014 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 put them into moulds. The process is 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 process.

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.

Like this post? Please share it!