Did you use white grout ? The color of grout seems to determine whether your piece turns out or not. Did you wipe off the final haze on the glass ? That can give it that dull look. I use tile sealer & that gives the grout a brighter , richer tone. The only clear thing I've seen is silicone and that would be impossible to grout with. I hope you get some more good suggestions !

Thanks, toomuchglass! I used black grout which was probably a mistake but the problem was the light coming through the sides of the glass pieces was extinguished and I think any colour grout would have done that. I did wipe off the final haze and I agree with you that silicone would be terribly difficult.

Next time try using silver tape under your sg it helps brighten the glass.Hope you understand .

Yes,nanatricia, I have used britebak before but this was such a big piece and the tesserae were tiny so it would have been very tedious but thanks for the suggestion

You can cover the piece with epoxy resin instead of grout, I personally dont like that look, but some do.
I have had glass do that and it is disappointing. what nana says is good. also, I simply prefer to work with small grout spaces, very small. and some ppl butt the pieces right up to each other and dont grout.

I'm interested in the epoxy resin notion. Does anyone have advice on how to use it?