My new dilemma – where to put the ingate and vents

So I want to carve some soapstone for casting bronze havettes. I have a couple of largish pieces of stone that I just need to flatten properly on one side, to make a two piece mould. The question is where to put the ingate. Should I run the bronze down a channel and fill the mould from the bottom? That way the air is forced out the top, but I’ll need to carve an extra channel for the pointy bits. Overall this way has produced good results when I’ve done it using lost wax.

havette wilt-e0afbc one

Should I put it in the side and tilt the havette slightly so that it flows up? That isn’t too bad an approach, although it does rather hasten the filling and I think increases the chances of air bubbles.

havette wilt-e0afbc two

Should I just pour in the top and hope for the best? This is usually the worst way, unless the mould is open enough to be able to let air out all around, since the air coming out will have to mix with the liquid metal coming in.  It also leaves more messy sprue around the outside to clean off.

havette wilt-e0afbc three

I know, lets have a look at some real ones from the portable antiquities scheme.

Unfortunately, despite them being pretty good photographs, it isn’t at all clear. If I handled one myself I could have a better idea, but at the moment it isn’t obvious. They may even have been made with lost wax method, but what makes it difficult to say is that they will have been finished off with the sprue cut off and the area sanded down.

https://finds.org.uk/database/artefacts/record/id/897784

This one for instance, in order to make it work properly the hooky bits need to be filled. But it is pretty smooth all over, with perhaps a small bubble at the bottom where the hook joins the main body. The decoration is probably made using a file.

https://finds.org.uk/database/artefacts/record/id/882498

Is slightly bent, again, hard to judge what it was cast into or how it was cast into.

I like this one, it’s a bit pitted but clearly shows how pointy the spikes are.

https://finds.org.uk/database/artefacts/record/id/810783

I suspect that they were made with much larger spikes to be ground down into proper spikes. Which means that if the bronze came into the mould that way, there would be less evidence for it since it was ground away.

In the end all I can do is try a method and see how it turns out.  I am minded to go for the first and second ones, the stone is big enough to do that.

My next thing is working out how to hold the two halves of the mould together when the bronze is being poured into it.

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What I actually did on the foundry at Kentwell in 2017

This year, we had four of us, all having been to Kentwell before, and two with well over a decade each under their belt.

I was head of station, and often referred to the others on the foundry as Master Alexander. Which was especially nice when I had just corrected something or made a useful comment about things. My many years experience has given me a good foundation of knowledge and confidence about my own capabilities and knowledge.

The great thing about having experienced people there was that all I had to do was mention a few facts and answer their questions and they immediately incorporated it into their spiel to the public. I was initially a bit stressed on Sunday because it was the first day and I didn’t quite know what I had to work with, but the three of them swung into action and by Monday were talking convincingly to school children about various things.

The week did start a bit badly, because I found on Saturday night when dropping my stuff off (After driving 430 miles, so a bit tired) that someone had adapted the furnace to look like this:

Kentwell 17 furnace front top

 

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Making a forge the historic way

Several people I know are making or remaking blacksmiths forges from the medieval period. Now obviously there are some changes in precise equipment used and various other things, but the designs have ended up fairly similar, due to the need to have portable forges.

The base is usually, but not always, a wooden box, because what is more authentic than wood, and can be easily worked to your desired shape. The interior can be filled with fire bricks, insulation or sand, topped off with something clay like, which can resist heat and yet not look wrong. This photo is of an iron box, although the interior of it and other bit look authentic enough, I’m not so sure about using a box.

portable medieval firebox

So, now to the sources.

Firstly, we have Theophilus, circa 1122.

He seems to suggest using two wooden boards, upright, at right angles to each other, with the space inbetween and up them filled with clay:
“Then take some freshly dug clay, neither kneaded nor mixed with water, and start by putting a little of it into this space and compact it well with a round piece of wood; then put in some more and ram it down again. Continue in this way until two thirds of the space is filled, leaving a third empty. Then take away the board in front, and with a long knife trim the front [edge} and the top of the clay flat and smooth. Then strike the clay hard with a slender piece of wood. After this take some clay that has been kneaded and mixed with horse-dung, and build up the first and a hearth for it. Coat the wall [of the building] also so that it will not be burnt by the fire. Pierce the clay through the hole at the back of the board with a slender piece of wood. Build all smith’s forges in this way.”

When we turn to the 16th century, Biringuccio has a few pages on the art of the smith who works in iron, it is not his personal speciality or area of knowledge, so whilst he describes some secrets, and artistic descriptions of the work carried out, there isn’t anything directly related to forges.

The nearest we can get is with the descriptions of how to make various sorts of hearths for melting bronze in. These are functionally equivalent to forges.

For instance, “Then it is filled with clay, very well pressed and beaten in, and a hollow is dug out in the middle as deep and wide as you think will contain the material that you wish to melt. Having prepared the bottom and made a hole as an exist for the bronze, and having put in the iron pole, cover it all very well with ashes that are moistened with water in which salt has been dissolved. Then back it, and putting the bellows in their places where you have set the tuyeres, proceed a you did when you melted with a hearth.” (Page 289 of the Dover paperback edition)

Biringuccio does love his salt water moistened ashes as a final outer layer over any clay lining, and it does work well. Well burnt ashes are by their nature very refractory and survive a fire well.

Unfortunately I haven’t found much about any surviving medieval forge hearths. There are a few mentions in archaeology journal articles but they are hidden behind paywalls.

So it would be interesting to compare the performance of hearths built out of clay and coated with ash, or clay with an outer layer of horse dung mixed in. The horse dung would hold it together better as it dried, and give some porosity after being fired, which might help it be more insulating. On the other hand that might make it a little more fragile and prone to damage when moving things about in the fire.

 

Something different – iron smelting

A re-enactment group I know are good friends with a venue in the Lakes District, near where iron used to be mined. They are interested in carrying out a medieval iron smelt at some point in time.

This blog post therefore is a wee summary of what I know about it at the moment, based on one particular interesting paper I found recently, along with some pictures.

The paper is this one:

http://openarchaeology.info/bibliography/twenty-five-years-bloomery-experiments-perspectives-and-prospects

https://www.academia.edu/4263490/Twenty-five_years_of_bloomery_experiments_perspectives_and_prospects_2013_

There are many topics that the paper brings up, all of which are important if you wish to have as easy a re-creation of a medieval bloomery as possible. The simple fact is that our ancestors spent generations refining their metallurgical techniques, and ignoring what the archaeology and practical experiments tells us would condemn us to spend many years in re-creating the same things as have been done before. I see no need to do that, because, as with Tudor bronze casting, it will be quite difficult enough as it is even if we do everything right, so it is important to learn from previous attempts.

In the abstract is a list of topics that are examined that seem to me to be very important and I will first discuss them below based on my knowledge without having properly read the paper:

the nature of the furnaces, the bellows and blowing rates; the ore, charcoal and clay

types, quality and treatment; and the operating conditions, the products and the losses of material through the refining process.

To start with, the nature of the furnaces. I have been unable to find out much about lakes district furnaces in the late medieval period, but it seems obvious to me that since blast furnaces had not reached that part of Europe, they were the typical chimney shaped bloomery furnace. This will have to be fully confirmed by reading more widely in the archaeological literature.

They are called bloomery furnaces because they produce a ‘bloom’ of iron, like you can see being hit with hammers in the darkness in this photo:

Striking iron bloom

The furnaces would have looked something like this, with the second photo showing slag flowing out of the furnace:Iron smelt bloomery at work

Iron smelting tapping slag

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What I actually did on the foundry at Kentwell

This year, we had four of us, all having been to Kentwell before, and two with well over a decade each under their belt.

I was head of station, and often referred to the others on the foundry as Master Alexander. Which was especially nice when I had just corrected something or made a useful comment about things. My many years experience has given me a good foundation of knowledge.

The great thing about having experienced people there was that all I had to do was mention a few facts and answer their questions and they immediately incorporated it into their spiel. I was initially a bit stressed on Sunday because it was the first day and I didn’t quite know what I had, but the three of them swung into action and by Monday were talking convincingly to school children about various things.

The week did start a bit badly, because I found on Saturday night when dropping my stuff off (After driving 430 miles, so a bit tired) that someone had adapted the furnace to look like this:

Kentwell 17 furnace front top

It’s the oddest bodge job I’ve seen for a while. After fuming for a bit I worked out that we would be better to reduce the volume of it using old bricks and bits of tile and such, and scavenged them from the foundry and the pottery.

Which turned it into something like this:

Kentwell 17 crucible in fire2

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What I wanted to try and do at Kentwell this year

I was going to put this up two weeks ago, but forgot amongst all the hassle. So here it is as a taster for my next post.

It is nearly time for my annual pilgrimage to be a Tudor at Kentwell hall in Suffolk. This year, like last year, I am running the foundry. This year we have 4 of us, which is a good start, and three of us have prior experience.

So, what I want to try and cast are:

Lead spindle whorls, medieval and tudor date, with 6mm and 10mm internal holes.

Pewterbuckles and suchlike.

A bronze mortar and pestle.

Bronze seal matrices and purse bars, using the lost wax technique.

More complex but interesting things I might try include casting into tin oxide to make buckles, but also to try and make bronze moulds for making a seal matrix or bullets or suchlike. Which involves embedding the patron or sphere in the tin oxide on one half of the mould, and then casting the bronze into the other half of it in such a way that I have a lump of bronze with a shape like a seal matrix in it. Then I can bang out dozens of clean, good seal matrices.
This idea courtesy of the Historical Metallurgy society AGM and conference, where Kevin Leahy mentioned a bronze mould for casting seal matrices.

It is really nice to have actual confirmation of bronze moulds existing in the medieval period, and likely having been used for casting bronze or lead objects. I’ve been investigating casting for years but moulds really do not survive much at all.

We will also try to make a bell, since one of my co-workers is a campanologist, using sand casting and maybe using clay, it all depends on what we use as an original. I don’t have a bell, but there are some still on the foundry.

It should be quite a busy week, it all depends on the weather and how much charcoal we have.

Tinning copper and bronze, part 1

This is something I’ve been meaning to do for years, it happened often enough in the medieval period, to cooking pots and also some buckles and the like, basically anything which would look nicer with a shiny corrosion resistant surface. I tinned iron nails many years ago, but somehow never got around to trying it on copper or bronze. It is a sensible thing to do to a cooking vessel, to ensure it doesn’t taint the food you are cooking.
So, this is how you do it, according to Biringuccio (Page 369 of the paperback Dover edition):

“To do this, a little salt and vinegar is boiled and the vessels are cleaned well inside with this. Then some tin mixed with a fourth part of lead and with some powdered Grecian pitch is melted. The tin is applied as you wish by rubbing it all over the outside and inside with a brush of tow tied to the point of a tool or held with a pair of tongs.”

I’ll skip using the lead for obvious reasons. And I don’t have any tow, which presumably would be made from flax or hemp rope. The pitch acts as a flux, helping prevent oxidation of the liquid tin and lead. I have other substances which might do, such as lard or beeswax.

In order to tin the group cauldron therefore, I set it up like this on a fire:#

cauldron on fire

It took a while to heat through due to the mass of bronze. Continue reading