I am a member of various electronic messaging groups on topics of interest. So I got a message last week that should be of interest to people, about the production of pilgrims souvenirs. In this case, a form of tin ampulla, made by slush casting.
Slush casting is a simple but potentially messy way of making a hollow object; all you do is pour the pewter (or in this case tin) into the mould, wait a few seconds for the outer mm or two to solidify, and pour the liquid pewter out of the mould. That’s why I said messy, because unless you are careful you might spill it.
The pewterer is ‘Robert of Canterbury’, an experienced SCA pewterer. He has written up many of the items he has manufactured over the years, and he is way ahead of me in carving skills and moulds. The write up for the ampulla is here:
Here’s a video of it, note how quick it is to do:
Many hundreds of ampullae could be turned out in a day if you were practised at this. The video clearly shows 3 being produced in a minute, which when multiplied up, more in an hour and maybe 4 or 500 in a day, which is useful if you are selling them to lots of pilgrims.
It is important to note that this only really works with pure tin, as found by Trotman, working in the Museum of London in the 1960’s. I speculate that this would be for reasons of the different cooling properties of pewter and tin, or rather the way that crystals grow when it solidifies, and that lead/ tin pewter doesn’t freeze in the right way quickly enough in such a small space as an ampulla mould.
Robert has in his write up provided a clear set of instructions for the practical aspect of it (in italics below), indicating the sort of experiential knowledge which would be developed by a skilled pewtersmith. It is important to remember how much of this sort of knowledge is embodied in the craft worker of the period, who used touch and sight to monitor the progress of the process.
*Get the metal into the mould swiftly. Fill it in one quick pour.
*Fill the mould in a consistent fashion. I found I had to pour the metal in just so. Once I found the sweet spot, casting failure rates went down to almost zero.
*Emptying it after the correct waiting time. This takes experimentation to discover, and the time increases the hotter the mould gets. The closer to the perfect time you get the thinner the wall of your piece is, and the less metal you use. If you are getting small holes in the casting, you may just need to wait a little longer before emptying.
*Empty the mould swiftly by completely inverting it. you may occasionally need to give it a shake to break the meniscus at the top of the neck.
*slush casting is splashier than normal. Wear long cuffed tig welders gloves, if you don’t already. Work over a heatproof surface.
*keep your ladle full. There will be lots of metal going back and forth between the mould and your ladle, and you want to avoid the working temp. of the metal in your ladle going up & Down too much.
*Use pure tin. (for excellent technical reasons, see Pewter, but mostly because thats the medieval way (until the very late crude pieces))
These requirements also constrain the equipment to be used. It would be best to have a ladle that is not too large and heavy, otherwise it will get in the way of moving things around. I also prefer to use thin leather gloves because then you can feel the heat of the mould more easily and tell when it is getting too hot or too cold, or that you have successfully cast because the mould heats up from the metal within. It seems Robert uses some thin gloves too, and of course thick ones would inhibit movement and perhaps lead to spills.
The ladle should not be too small either, because that would cause untoward decreases in the heat of the metal. Each mould will have its own sweet spot, that can be found only with practise. Judging the waiting time is also a skill that can be learnt, with minimal teaching, but is not something you can automate until you have proper computer control, so historically was probably carried out well into the 20th century.
Pure tin is also guaranteed to look shiny and silver like for a long time, an important aspect of the ownership of such objects. The video does also show how not to do it, insofar as he is right handed, but the heat source is on the left and casting on the right, whereas it would be easier to cast on the left of the heat source, allowing him to pick up the ladle with his right hand without crossing his arms. On the other hand the fact that he can perform well enough with this handicap suggests that the process is easy enough that it does not matter greatly.
The impression I get from some historical documents I have read, such as wills, is that pewtersmith was basically a one man job. Unlike a foundry, which requires many men to operate it, pewter casting can be done by one man on an open fire, followed by some hours of work fettling the cast objects, that is, filing off the rough patches.