The workers are doing this detail work in what we would consider quite dim light. After the fretwork has dried, other workers fill the spaces between the frets with brightly colored sand, using an eye dropper and water to pick it up as a slurry, and deposit it into the right space. At this point, the piece is fired in a kiln to melt the sand, and fuse it into glass. That's probably why they keep the formula a secret. After the firing, the piece is inspected, and any voids or bubbles in the glass are again filled with sand, and then re-fired.
Most pieces take three rounds through the fill and fire.
Finally, the piece is polished by chucking it in a lathe, and workers hold grinding stones to the spinning piece, under a constant flow of water. Notice the tape to protect the workers fingers from the abrasion, as well.