I recently learned a new thing (to me) in low-temperature casting: hollow forms cast in tin. It’s a medieval practice, easily documented, and used for an exciting variety of forms, including ampullae, whistles, bells, wearable vases, and even tiny reliquaries, as ten minutes of happy searching through the Kunera database showed me. (Go look for yourself! It’s awesome!)
The pewter I ordinarily cast with is 98% tin, but that 2% makes a difference. Pure tin stays liquid longer than pewter, not going through a ‘slush’ stage as it cools. This means that it can be poured into a mold, then dumped out, leaving a thin layer of hollow metal in the mold.Tin tends to be a bit softer than the pewter I use, but not prohibitively so.
Why would you want a hollow form instead of a solid one? With some things (bells and whistles), it makes them work as instruments. Ampullae and reliquaries are intended to hold material from a sacred site or person, so a hollow is essential. Of course, a hollow item is also cheaper to produce, as it uses less material.
A mold for a hollow form isn’t much different from one for a solid piece. If it will be two-sided (having carving on both faces) or even more complex, the mold will need to be fitted with registry pins so that the pieces
always fit precisely. These are simple enough: drill through one side and into the other a bit with the pieces fitted exactly as you intend them to be. Widen the openings at each end of the drilled holes. Heat the mold, fit it together, and pour molten tin into the holes. Wipe off excess with a gloved hand and allow to cool. Why use tin? Well, although in period, pure lead was used, if there’s a problem with your mold, you may need to put the whole thing in the pot to melt out a locked casting. The pins will melt out, too, so unless you want another metal in your tin pot, you may as well use the same thing for every part.
Because the tin needs to be poured back out, the opening for the pour must be wider than usual, and the body of the hollow also is wider than a flat form would be, to ensure some liquid tin will be able to pour out. You’ll need to watch the temperature of your mold carefully: a hot
mold won’t cool the tin quickly enough, and if you don’t wait, it may pour out half of your design with it. Of course, if you have really thin parts (like filigree, pin backs, etc), you’ll want your mold to be warmed up, or they won’t cast. It will take practice, but as with pewter, if it doesn’t come out, put it back in the pot and recycle it right away.
The open edge will vary depending on the type of item you are making. No matter where it is, you will want to clip off the excess once it cools. You can finish by filing or tumbling if desired. You don’t have tremendous control over thickness with this method; if you want the interior to be very smooth, you may need to do finishing work on it. If you want an exact thickness, you may as well use a wooden plug or other means to ensure that and cast with pewter. However, for odd-shaped pieces where a plug will not work, and for pieces that don’t need to be precise on the inside, this is an easy, medieval period method that works well. Happy casting!