You should use silicone as the adhesive when you mosaic on glass.
Make sure the glass surface is clean and free from any oiliness before you start. Stick the mosaic tiles in place with silicone. Don’t apply too much – it must not ooze out between the tiles. Use your finger to apply an even coating on the back of each tile. When all the tiles are in position and the silicone is dry, you can make the grout according to the instructions on the pack and apply it to the tiles.
Decorate a drinking glass with mosaic tiles and use it as a vase. Or use lead glass tiles and make a candle holder – the light will shine through the lead glass.