What I did on my holiday this year – bronze casting

Once again, I was on the foundry at Kentwell hall in Suffolk.

This time, I was the master of the foundry. Unfortunately there was a slight lack of assistants, so there were 2 of us most days. Still, I managed to get a few things done and refine my understanding of the processes and equipment.

Last year, which I meant to write up and publish but never really did, involved a group of us trying to make cannon. We failed, due to lack of time and lack of appreciation of the difficulties. Replicating the techniques of 16th and 17th century foundry work, even with at least 3 of us having a great deal of expertise, was something that would have taken more than the 8 days that we had, and so we failed. Just two more days would have been enough though. Which isn’t bad, all things considered.

For those who don’t know, this is the foundry at Kentwell, first the bellows end and then the furnace end:

bellows end of kentwell foundry 2016 furnace of kentwell foundry 2016

It is based on one shown in Biringuccio’s De pyrotechnia, i.e. early 16th century. I don’t think there’s any evidence for one like it in England, but the possibility is always there. It is on it’s second set of bellows and this is the 3rd (or maybe 4th) revival of foundry work after a break, since it was first built around 1994 or 1995.
So, this year, our aims were a lot simpler- the others were new to the foundry, and willing to listen to me. One had been at Kentwell before, so knew the patter and just had to learn some foundry specific stuff. Another was new to everything, so didn’t do or say much the first couple of days, but was helpful. Both ended up casting in pewter in carving they had done themselves and talking to the public about it all, which is what we want. And had a good time, which is also one of the aims of Kentwell.

As for me, I took along some old moulds and fiddled about with newer ones. I also took copper and tin and made brass and bronze for future use.

As for the purpose of the foundry, it was as much to wow children and visitors with the size of our flames and sparks that came from them. I managed to show red and yellow hot crucibles and liquid metal to quite a few too. My 2 year old pewter casting scars came in handy as well, since I was able to show them to the children as proof of the dangers of metal casting.

In this post, the failures.

I did several sand, clay and horse dung moulds that failed for various reasons, mostly due to hurrying the mould manufacture. Nevertheless they were useful practise.

failed buckle pour kentwell 2016

See how the bronze escaped from the proper ingate because I didn’t squish the mould material together properly.  The moult itself worked fine.

I also took along the tin oxide and tried to use it, but made it too damp so it was very sticky and difficult to work so never got around to using it. I might need to temper it with larger bits of tin oxide and other stuff.

Moreover, with just 2 of us, it was really hard to run the bronze casting, carve moulds and do some pewter to show people. Nevertheless, we managed. When operating the furnace, it is a two person job to pump the bellows and monitor the furnace itself, the problem being to talk to the public at the same time. I think it best to have at least 3 people at any active station at Kentwell, for precisely this reason. It also would allow a sensible division of labour, insofar as whoever is good at talking and not so much at doing can concentrate on that when the public are around. Some of us don’t like talking to the public much and so are better off doing practical things.

I was going to put the successes in here, but found that I hadn’t photographed them, so will put them into another post along with one of the failures.

Aside from that, my mastery of the furnace itself improved. Despite not having done much casting for a while, somehow all the weeks I’ve spent there on the foundry in the last 7 years came together and I was able to act and sound experience, and moreover do what I wanted to do. I recall the first time I did a big pour, around half a kilo of bronze to make a pestle (it didn’t quite work, the pestle you can see at the top of this blog, but it is misshapen due to the mould not being quite right), and I was scared and worried about the heat and the weight of the liquid metal. This year, I carried out similar pours, and had no problem at all.

cooling poured bronze kentwell 2016

This picture shows the poured metal not long after I had poured into a mould.  Note the red glow as it is still quite hot.

With my greater experience, I was running the furnace in a better way than in many previous occasions, and so everything happened much more easily. For starters, we had a lid on it somewhat, but not too much so as not to cause backpressure of air within the furnace, so the bellows blast could reach all parts of it easily.  Note the flame blowing out of the furnace in this photo:

furnace in operation kentwell 2016

Then, I paid a lot more attention to the colour of the charcoal within, wanting it to reach yellow. At orange it is perhaps just hot enough to melt bronze, but I knew from previous times and use of a thermocouple, that yellow hot is the right colour. And that you have to keep it at that for a few minutes to let the heat soak through the crucible. And if you overblow the bellows you will blow too much air through the furnace, removing heat from it and causing CO to burn to CO2 outside it, which makes lots of flames, but isn’t efficient.

Then, I paid a lot more attention to the colour of the charcoal within, wanting it to reach yellow. At orange it is perhaps just hot enough to melt bronze, but I knew from previous times and use of a thermocouple, that yellow hot is the right colour. And that you have to keep it at that for a few minutes to let the heat soak through the crucible. And if you overblow the bellows you will blow too much air through the furnace, removing heat from it and causing CO to burn to CO2 outside it, which makes lots of flames, but isn’t efficient. This photo shows it about the right temperature around the crucible, less so away from it:

yellow heat furnace kentwell 2016

The yellow hot is around 1100C, according to Rehder, which matches my thermocouple readings from previous bronze castings.

Then there was the matter of the size of charcoal within the furnace. From “The mastery and uses of fire in antiquity” by Rehder, as well as experience in 2015, I knew that the pressure needed to blow air through the furnace varied depending on the size of charcoal lumps and therefore also the surface area of charcoal available for combustion. Thus blowing gently when the lumps are large is a good idea, and then harder when they have shrunk. There comes a time when the lumps are too small and the air really has trouble getting through the mass of charcoal, as can be seen in this photo, with many of them in the centre being a cm across or less and all sunk together in the most efficient packing possible which makes the passage of air very difficult:

charcoal in furnace kentwell 2016

Experience and skill indicate how long it takes for the charcoal to reduce in size, and this is why I didn’t actually do any bronze casting the first time we ran the furnace properly, in order that I could judge it’s behaviour properly. It depends on the starting size of the lumps and the shape of the furnace itself. In this particular case the lumps were easily half fist sized, so we started off with many big ones, which slowly burnt down, and judicious additions of more once or twice with each melt (and we could do two before the furnace became choked and no use) were the best way forwards. I also reduced the volume of the firebox using firebricks so we didn’t have to use so much of it. The extra charcoal used acts as an insulator over the crucible too, so as to retain heat within the fire.

Experience and skill indicate how long it takes for the charcoal to reduce in size, and this is why I didn’t actually do any bronze casting the first time we ran the furnace properly, in order that I could judge it’s behaviour properly. It depends on the starting size of the lumps and the shape of the furnace itself. In this particular case the lumps were easily half fist sized, so we started off with many big ones, which slowly burnt down, and judicious additions of more once or twice with each melt (and we could do two before the furnace became choked and no use) were the best way forwards. I also reduced the volume of the firebox using firebricks so we didn’t have to use so much of it. The extra charcoal used acts as an insulator over the crucible too, so as to retain heat within the fire. Of course doing a second melt is a little fiddly, because it is hard to push the crucible down within the charcoal. The importance of the lid should also not be forgotten, in retaining heat within the furnace, and I am sure that we would have had to use a lot more charcoal to keep the heat in if we didn’t have the lid. It was a two man lift on and off the furnace, with our welding gloves sorely tested by the heat.

The first day of using the furnace, we just heated it through, and attempted no casting or melting. The following days we carried out one or two castings, and also made a lump of bronze that weighed about a pound, which I handed round the public:

Bronze lump kentwell 2016

The first day of using the furnace, we just heated it through, and attempted no casting or melting. The following days we carried out one or two castings, and also made a lump of bronze that weighed about a pound, which I handed round the public:

Now, compared to last year when we were melting in the furnace itself, melting in a crucible is easier for several reasons. Firstly, you don’t have so much trouble sealing the furnace bottom since the metal is all kept safe in the crucible. Secondly, you don’t need to consider how much heat you are putting into the structure of the furnace itself, which you have to do to keep the bronze molten if melting in the furnace, and nor do you have a narrow window in which it is of the proper liquidity, not to mention it being contaminated with stuff from the charcoal that it is within, and if you look closely at the lump of bronze above you can see how charcoal within the crucible is stuck in it.

But one of the drawbacks of the crucible method is that you can’t melt more than you can lift out of the furnace yourself. So I think I can’t safely manage more than about a kilogram of metal. The crucible can hold 2 or 3, but I’m not sure about that without getting bigger, better tongs.

The next post shall cover the two items I made that might be useful. What I’d like to do some time, given the capabilities of the foundry, is cast a cannon and also cauldrons and other stuff like mortars and pestles. The difficult bit is in making the moulds, and that is actually quite skilled work and requires a lathe too, which I need to source. So, more work to do. If I can arrange a lot of that over the winter, there’s a chance we could actually make more useful things at Kentwell next year if I’m on the foundry again.

Anyway, here’s a posed picture of me cutting some pewter bits from their sprue:

me at kentwell foundry 2016

 

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