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

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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