Re: Silicone glass pre foam.
I have not done this yet, although I have read of quite a lot of hobbyists doing this.
I did sort of a similar thing when I was taking product photos of clay sculptures I was going to put up for sale. I used a matte black spray paint on one side of a sheet of plexiglass, then had the sculpture stand on the other side, so that the black surface was only visible from underneath the plexiglass surface, not above it. The result was an attractive black, glassy surface that sort of resembled a flat piece of obsidian --- if such a thing can be imagined.
I have not done the silicone method yet (although I plan to do so soon, I hope), but I bet going through 1/4" or so glass, the effect turns out rather nice-looking.