For most of us, 3D printing means printing in plastic of some sort — either filament or photo resin. However, we have all wanted to print in other materials — especially more substantial materials. Metal printers exist but they aren’t cheap. However, it is possible to print molds and cast metal parts using them. [Amos Dudley] prints molds. But instead of metal, he casts parts out of glass.
[Amos] covers several techniques. The first is creating a relief (that is a 3D shape that grows out of a base). According to the post, this prevents difficult undercuts. He then casts a mold from silica and uses a kiln to melt glass into the mold. You might expect to do that with a full-size kiln, but you can actually get an inexpensive small kiln that fits in your microwave oven.
The second technique is a lost-wax style process in which the plastic burns away in a high-temperature kiln. The final technique creates an internal void to make it appear an object is inside another object.
For small objects, this looks like it would be very accessible. Art suppliers have inexpensive kits for glass casting for which you only need a kiln or a microwave. Small kilns are reasonably cheap, or you may be able to find someone to give you or rent you kiln time.