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

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.

Modern information that helps us understand casting practises

Exhibit A, the phase diagram for a copper- tin alloy, usually known as bronze (Stolen from wikipedia or wherever):

 

Bronze phase diagram

Along the bottom is the weight % tin, i.e. if the object weighs 1kg, and is 50% tin, there is 500g of tin. Sure, that doesn’t seem as scientific as going by mols, but then a lot of metallurgy harks back to before it was properly integrated into modern chemistry. Continue reading