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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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

A problem you run into when making buckles using stone moulds

I have had the ambition for years now to make good pewter buckles, of the sort in use in the 14-15th centuries for holding shoes shut. Numerous examples can be found in the Museum of London book “Dress accessories” and other books of finds.

The mould I am using is is made of soapstone, which I am using purely because I don’t have access to easily carved limestone or siltstone.

Note that I’m cramming quite a few items into one mould, which was done in the past. Good stone isn’t so cheap and easy to find, and it makes sense to use it for several objects, or use two sides for different objects, as seen in some of the Coventry moulds.

The slight problem is, how do I make sure that both sides have the correct half on them? Dan Towse (http://pbsn3.pbworks.com/w/browse/#view=ViewAllObjects), an experienced SCA caster, reckons they squared the mould sides off and measured. That would work well enough I think; being a cheapskate I don’t always use squared off moulds in order to maximise the amount of stone I can use, yet most moulds I have seen are squared off which certainly strengthens his hypothesis.

So, given the situation I was in, how should I mark either side? I tried using damp red clay in the already carved side, but it did not stick very well and rather marked one side of the stone as you can see at the bottom left:

soapstone buckle moulds in the making

Then I tried pouring wax in, hoping it would stick to the appropriate area. It did, but also spread a little beyond, so it was not as accurate as I would have liked. Still, it was a start.

Next I tried casting metal into it, hoping the white stone would be marked by it. No luck, the white soapstone remained unmarked.

When test casting into the mould though, I found that a partly complete cast could indicate the other side of the mould easily enough:

soapstone mould half done with test casts soapstone mould half done without test casts

So perhaps they did it that way? It would be nice to find some objects that are clearly half made, and that show both sides of the mould. Once you have carved the edge of the area to be carved out, you can carry on carving with the occasional test casting in order to see how you are doing:

I would expect real half-done moulds to show some evidence of this or another way of carving, although if the carver is sufficiently good and artistic they could do it virtually freehand I am sure.

Bronze casting at the Experimental Archaeology Conference

Part of the conference (https://storify.com/artefactual_KW/experimental-archaeology-conference-9) which I attended last week was a couple of hours at the UCD experimental archaeology site, which is a small grassed area beside the campus. Whilst there we watched some bronze casting done using bronze age methods, carried out by Umha Aois: (https://www.facebook.com/umha.aois).

That meant they used bag bellows like this:

And old fashioned crucibles like this, which are handled by means of a wood stick stuck into the handy socket at the back:

which is being used in a furnace in which the air came in from the top through a right angled tuyere.

Sinking the fireplace into the ground has some advantages, such as fairly good insulation and it can be made and prepared quite quickly. On the other hand I do wonder about the limits of the size of melt that can be made, since the crucibles had to be small enough to be easily handled by sticks.

Anyway, here’s the two videos:

It was a nice little axe that they made. The man who broke it out the mould was much more gentle than I would be, I prefer to smash the mould material off it in a couple of goes, rather than gently break it by tapping. Note too that the tongs used don’t quite fit the crucible, which can cause some difficulty in working, but they managed well enough.

Observe too the colours of the charcoal before they remove the crucible from the fire, and the colour of the liquid metal. They had no thermometers, only their eyes and skin, and so experience tells you when the melt is ready.

So, a nice part of an interesting couple of days. I reccomend the conference to anyone, it should be on again next year, although I think the location is still to be decided. Meanwhile I am jealous of UCD for having their own area for experimental archaeology.