Your rediscovery of the way of casting pewter tokens, medallions and coins. Pewter is the preferred metal as it has about the same melting temp as most general solders. With out all the heavy metal/lead side effects. From the carving of the molds out of cuttle bone, to making the molds out of clay.
A lot on here are old enough to remember back in the day. When adults didn't care about their children, and let them have chemistry sets with real chemicals in them or even atomic material. and there was a whole side of the isle of chemistry sets of all sizes.
One of the standard experiments,was making of a sulfur mold of a nickle, Heating the sulfur it it was liquid and then pouring the material around the coin held in a small tin foil pan.
You also can use plaster of paris as a mold. You just have to make sure it is dry all the way through. Gently heating for a while depending on the size of the block.
Now for the safety notes. You do understand that handling, your lead coinage leaves trace on your hands along with any of the heavy metals that may be in the alloy of solder you are using. The fumes are also considered an inhalation hazard, so if you don't have a fume hood. You should be doing out side, and keep your nose out of the way. The heavy metals/lead in your body is a cumulative problem, It does flush out over time. And a hanky over your nose doesn't help.
Secondly it is law that any articles made or contain lead be clearly marked that they contain lead!