Oh my! A Whole week has gone by and I haven’t added anything… I’m such a slack bastard.
OK, so the November Crown Ampullae haven’t gone so well… I have been busy with social engagements and haven’t really had the time to experiment as much as I would have liked. However if I add up the many hours of work I have already put into them and then document that, I should have a reasonable “Work in Progress” document to send in just for the fun of it. As the whole idea with attempting the Ampullae was the recreation of the Slush casting process so little reference to the chosen design is made. The design was chosen intially for it’s simplicity and ease of carving as well as it’s look and possible appeal, however little reference to period and significance was researched.
So what has been the process with this whole thing? It started early October when after recieving a book I had on order, I discovered that a fair number of period Pilgrim souvenirs were actually Ampullae that were hollow and crimped shut to hold usually liquid contents. Not much was said about how these were actually made, however thinking about it for a bit brought me to the conclusion that the molten metal was poured from the center of the mould. I created a rough mould to attempt this process so I could see if it was viable. I made several attempts using this mould and they were all rather thick-walled but the idea seemed to be right. Further research online confirmed that the method for hollow casting was called slush casting, and that it was exactly as I had theorised.
Discussions with Tanw one evening brought into light the possibility that the modern pewter (commonly called ‘Britannia’ as it contains no lead and is designed to mimic pewter) is nothing like what was used in period examples. So I began researching what was a typical pewter alloy for what I was attempting. As it turns out, period examples of pewter had a reasonably high lead content which changes the pouring consistancy of the metal. Period mixes of of pewer were generally around the 60/40 mix of tin/lead however were widely varied and there are examples varying from mostly tin through to mostly lead. As it stands the more common mixes were around the 60/40 mix, which happens to be what we call solder today. The next session of casting I had planned to attempt to cast with a variety of mixes. It is worth noting here that the alloy mixes varied depending on the type of casting required.
After purchasing some plumbers solder of a 40/60 mix of tin/lead, I roughly mixed this in equal parts with modern pewter to give me a guestimated 66% tin, 30% lead, 1% copper and 3% antimony. This mix however tended to shorten the time between pouring the metal into the mould and when it sets, causing more failures. A higher lead content was also tried by using the plumbers solder without adding any ‘Brittania’, however this also seemed to have the same issue (where the molten metal turns from a liquid into a solid very quickly), the higher the lead content the quicker this seemed to happen. Much frustration was had at this point, causing me to put the project aside for a few days.
Whilst researching more slush casting techniques on the internet, I came across a more modern referance to using a chilled bronze mould. This reference once again inspired me to tinker, first back to trying with the period style pewters and later, when it was confirmed that the lead content actually made the chilled slush casting process harder because it shortens the window from liquid to solid, the more modern high tin content pewter was used. Whilst the actual pouring technique still requires some practice, the examples I have created show that this method is indeed valid. Future attempts will start proving more fruitful, however a method of increasing the cooling rate of the moulds would ease the whole casting process.
So, in summary… Using an extremely high tin content pewter and chilling the mould to a sub-zero temperature are the two main keys to creating nice thin walled period ampullae. Of course these techniques are probably fully documented elsewhere, however I have thoroughly enjoyed rediscovering them through research and experience. Now that I have worked out the technique… finishing the mould and creating a number of finished samples is the next step. Stay tuned for photo’s of this later. :)