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

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Interesting article on Viking/ Scandinavian casting methods, compared to what I know and have done

http://web.comhem.se/vikingbronze/casting.htm

It shows a number of things. One is that you can do casting with a comparatively small and simple hearth, with bellows feeding a tuyere. This is not of course news to many people, but it does also indicate the sort of improvements in technology that took place over the following 500 years as furnaces got bigger and bigger. Yet they could make some sophisticated and high quality items on such a small fire.

The hearth is clay lined, on stones, very simple to make, and only about 6 inches long.

He mentions some of his experiences, 15 minutes to melt a crucible of bronze is about right, I’ve done it in that time on my small setup. There is a photo of a double hearth, with one suggested for the melting, and one for heating moulds. I usually do both in the same hearth, which if it is large can mean losing a mould in the charcoal. Otherwise I just put a firebrick at the bottom at the far end of the oblong hearth, and thus less charcoal builds up there with less air getting at it, which ensures a lower temperature there, ideal for moulds.

He spends some time on the moulds, going over the advantages of lost wax casting.

It seems the crucibles were made from sand tempered clay, which is always good, but you could only get one or two meltings from each one, which I say is because it is hard to get the right sort of clay, he points out that the crucible turns glassy when used. If you use a whiter high alumina clay then it doesn’t get so glassy, but a high iron one glasses up very quickly. He then says that he uses sand tempered stoneware clay, as far as I can see stoneware clays are for higher temperature firing, and thus are more heat resistant, so of course that is what to use.

Interestingly potters have evolved their own separate nomenclature that is not directly based on scientific analysis, so they say stoneware or earthenware or describe a clay by it’s type of firing and colour.

Now, one of the interesting things is that he says the mould (made from clay, sand and horse dung) should be completely burnt through, with no organic matter left. This is partly to ensure that there is no unreacted lime within the mould, from the clay. Yet he says that this is in contrast to the historic moulds, which are often poorly burnt, oxidised red on the outside and black through the middle, which can make the casting hard to do. The reasons he suggests for successful casting are that they used zinc copper alloys which were better at getting into the moulds through being less viscous and having a lower temp.

Importantly he notes that the clay mould conduct heat slowly, so you have time to take it out the fire and pour into it without rushing, which is a good point. This also permits the manufacture of finely details pieces, because the metal stays liquid enough to get into all the corners.

Clearly I have a lot of practise left to do because my castings don’t turn out the way his do. The secret is in the mould.

Maybe a sighting of double action bellows?

In Lazarus Erckers book about the analysis and production of various minerals and salts, there is great use made of furnaces. But most are natural draft powered. However there is one to be found later in the book, I stumbled upon it by accident. The accompanying text merely calls them bellows, for the purpose of assaying copper ore.

And the picture is:

POssible double action bellows

From page 219 of the hardback of Sisco and Smith’s translation of the Treatise on Ores and Minerals.

The key point here is that it shows a man reaching into the furnace with tongs whilst manipulating the bellows with his other hand. Moreover the end of the pole seems to be attached to a rod which pulls up the bottom of the bellows and there is a valve on top. Of course the small issue is the presence of the valve, which would suggest that air goes in the top, when the bottom is falling down.

Modern double action bellows work by having a valve inside, splitting the top from the bottom, so that when you pull the bottom part up it pushes air into the top part, which then expells it out the front due to the force of gravity on the top piece of wood, whilst the bottom section is opening out again.

So in this case I don’t think I can say it definitely is double action bellows because of the apparent valve in the top. This image has pretty much everything you would expect for double bellows such assingle handed operation and only one set of bellows, but I can’t really be sure. I shall keep my eyes open anyhow.