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

Tin and building works – what is it for?

I was having a quick look through “The Welsh castles of Edward I”, by Arnold Taylor, when I found mention of the purchase of 160 pounds of tin for use in the building of Beaumaris castle in Anglesey in the late 13th century.

This naturally had me wondering what use tin was in building works. Tin is a weak metal, and not much use by itself. Almost every use requires it to be alloyed with something else. Lead and tin make pewter, tin and copper make bronze. Actually quite a few nice shiny pilgrim souvenirs and ampullae used nearly pure tin because of that shinyness. But you don’t make them for building a castle, gold and silver are much more popular with the workmen.

You certainly don’t put it in the lead for roofing, that would make it stronger but also less able to be beaten out into shape and around corners.

The last use I could think of would be tinning iron nails or other ironwork. Not only does this make it shiny it also helped inhibit rusting.

Theophilus, writing in the 12th century, recommended tinning iron nails and sheet iron used to bind wood together for making an organ. Biringuccio in the 16th century uses tinning on the inside of bronze or copper vessels to ensure that they do not taint food. He no doubt would have tinned iron as well had it been his job.

I also think that some small or large copper alloy castings were tinned to make them look like silver.

So I think the most likely use of the tin was for tinning nails. I have tinned nails before, using Theophilus’ recipe, but it is trickier than you would think, I really should have another go at it.  This photo shows the tinned nails.

tinnednailslanark08

You need a deep crucible for the liquid tin, nice clean iron nails, and to get the temperature of the tin and nails just right, as well as a steady hand when dipping them.

 

Since I wrote this wee post, I have found a mention of using tinned nails in buildings, but cannot recall where I read it.

Three original spindle whorls

I got these from ebay last year. Now, to have a closer look at them.

Firstly, lead spindle whorls are hard to date, and there is little agreement about when they are from, because in Britain at least, there aren’t any properly dated examples. Instead they are found in the countryside where they have fallen from someone’s spindle.

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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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Casting into powder, a method from Biringuccio

Something I’ve been wanting to try since last year is casting into powder to make small objects, which is an interesting thing to test for real experimental archaeological reasons. This is done by following the instructions of Biringuccio in his Pyrotechnia (published in the 1540’s). (page 324 for the powders, and 326 for the method for making up the boxes, of the Dover paperback edition)

The simplest way to do it for me was to make a wee casting box from sheet wood that I had lying around:

Wooden casting box April 2016

Note the dowelling rods to hold it together, and the fact that I carved the open parts out by drill and saw. This photo is of one side, with the outer piece of wood to hold the powder in, underneath the middle hollow bit.

It is big enough to make 1 large or 2 medium sized buckles, or some buttons or something. If I make it too big I’ll need lots of powder to fill it and it’ll be more of a pain to deal with. It is then bound together with string when ready to use, and you can see the pouring hole at the top, which will rapidly get burnt from the molten metal. Only I forgot the string, so held it together between two bricks when I was casting.

The archaeological aims are to see if it is possible to use the powders, and how well they do, and whether there are any obvious indications of such a method on the finished product, i.e. how much work would be required to finish it off, compared to my usual clay casting. Then there is the question as to whether the powder is damaged by the heat and thus forms a sort of ash or slag that can be identified or overlooked in a dig. It also seems to be an area of experiment that nobody else has tried, although I lack the language skills to tell if someone else abroad has done it before.

Now, the powders that Biringuccio says to use are quite varied, but you can immediately see that they are all heat resistant. Quote:

“Powders are also made of crushed brick, tripoli, vine ashes, tiles, and glazed drainpipes, or burned emery, calcined tin, straw, and of burned paper and horse dung as well as of young-ram’s-horn ashes and many other things. The goodness of all these depends on three things; namely on receiving the metal well, on being so fine as to be almost impalpable, and on their being made with a magistery that renders them hard and strong when dry.”

He also mentions another of two parts pumice and one of iron scale, pounded finely. The other powder he spends most words on is made from a burnt and pounded loam originally made from fine grained earth or gravel or river silt, mixed with wool cloth cuttings, spent wash ashes and horse dung.

This last one is a little time consuming to make, so will be made later.

This post is about the use of calcined tin, which was some purchased from a chemical supplier, and of a fine, floury consistency. Biringuccio above said to add a magistery to them, by which he meant the magistery of salt. Fortunately I tried burning salt in a silica rich environment last year, so added some salt solution made from that salt. (More on the magistery of salt in another post)

Now, onto the trial.

Making up the bronze to be cast was simple enough, so I will ignore that.

Instead, the interesting and complex bit was making the mould to cast into. Firstly, I made the tin oxide damp with the magistery of salt solution, but it ended up a bit lumpy. I tried pressing it into the mould, which was easily enough done, but it was never of a smooth consistency. Ultimately, when I was trying to press the buckle into it I came to the conclusion that I should have dampened it a lot more. Biringuccio wrote:

“…that has been slightly moistened for moulding, as I told you, so that by pressing with the ends of the finders and with the hand it holds together as well as possible.” (Page 326)

But there is a fair bit of room for error here, I think it holds together at a wider range of dampness than the actual wetness that would make the moulding turn out best.

Nevertheless I soldiered on. The first photo shows the buckle on the powder:

Casting powder buckle in frame April 2016 Continue reading