My pieces are made on either fine silver, which is unalloyed pure silver, or on copper. The fine silver does not get black when heated, so it is easier to work on. Thin wires are attached to the silver, making "cells" which are then filled with finely powdered glass. I mostly use transparent glass, because the metal shines through from beneath. I also make paint from really fine glass powder, and paint it on the metal. The piece is then fired many times (around 10-20) at high temperature (between 1325 F and 1450 F), using a kiln or a torch. Before each firing, I add another layer of color.
I just realized how similar this technique is to doing mosaics on mirror, where the light shines through. No wonder I love enameling!