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:
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 The other powder he spends most words on is made from a burnt and pounded loam originally made from fine grained earth or gravel or river silt, mixed with wool cloth cuttings, spent wash ashes and horse dung.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:
This one shows it in the box, with the separation between the two layers of oxide ensured by sprinkling it with ashes, which worked well, and I left it for a while to get hot all the way through. Note that I still have to put an ingate at the right.
So I fired up the furnace, and got it going so well it turned out like this:
Not quite sure why, I think good dry charcoal of the right diameter, strong air pump and lack of ash in the furnace meant the thermocouple was reading over 1300C. Shortly after the thermocouple was this colour:
Which given that is stainless steel, is a little worrying and according to the usual charts of steel colour and temperature is over 1200C. The end of the thermocouple broke off after a little more heating and poking, which was annoying, but confirmed that 2 or 3 years of occasional use and frequent bending are not good for them. (A place I used to work would burn them out in months despite them being jammed in place, because of the high temperature variations and repeated exposure to oxidising and reducing atmospheres)
Anyway, I dried the moulds on the end of the furnace.
As you can see the powder was a little lumpy and you wouldn’t believe how hard it is to find a good straight edged metal object to smooth them flat.
The casting went well, but some metal fell through the mould because I hadn’t managed to secure it shut properly, which at least indicates that the metal was a good temperature to pour, and you can see it glowing in this photo, it took a while to cool:
Now, the moment of truth:
As you can see it sort of worked, but not quite, due probably to not smoothly casting all the metal in one go. Moreover the ingate seemed to collapse slightly, so not all of the metal got in. I don’t think air bubbles were a problem, because as you can see there were lots of gaps around the mould where metal got into the actual casting powder.
The interesting thing is that Biringuccio was correct about being able to re-use the moulds. This photo clearly shows the top lot of powder, which was intact after the casting.
As for the casting, the surface finish is very smooth:
Which is nice to see. I have trouble getting good smooth surface finishes with casting into clay moulds, but this method gives you a perfectly smooth object straight away. This would make a foundry-man’s life much easier, and not require lots of time to smooth out before selling.
Now, the slight irritation was that the tin oxide is clearly unaffected by the casting, as I rather expected. It is just not reactive enough at these temperatures, and the environment inside the mould is not reducing, so the SnO will not get broken down. So nothing would survive archaeologically except a layer of tin oxide or if we were lucky, some powder trapped in a hole in the surface of a casting.
So, next time I need to make the tin oxide a lot more liquid, and smooth it off better before putting the mould together, and it needs to be bound by string tightly. I think all that is manageable. I shall also ensure there is a better ingate and some air channels to let air out. Otherwise, it is a fairly simple method and shows great promise.