How many times has this happened to you?
You get out of the shower to find the bathroom floor flooded because the water has seeped through the glass of your shower door.
How many times has this happened? That’s right, NEVER!
Glass is used in shower doors, and windows of all kinds, because it is IMPERMEABLE*.
Water, wind, gases, and debris won’t go through glass. Even when it’s clear.
So when you are using glass of any sort in mosaic art the glass itself doesn’t need to be sealed because it is already IMPERMEABLE. Sealants will only sit on the surface of the glass and, because it cannot soak into the glass (because glass is impermeable) it will eventually flake off if it has dried there.
That’s why you only want to use penetrating grout sealant when sealing mosaic art pieces. In glass mosaic art the grout is the only porous surface that could be sealed against staining. Cement-based grout will never be waterproof.
*An impermeable surface is a solid, non-porous boundary that completely blocks the passage of liquids, gases, and moisture.