We often get questions about adding polymer clay to glass and I can say I have literally made hundreds if not thousands of pieces this way. So I know the ins and outs of covering glass with polymer clay. One factor on getting the clay to stay on the glass is to cover the entire piece with clay. When you add pieces of polymer clay here and there it has nothing to grab onto to keep it affixed to the glass. So covering the entire piece will help ensure the clay stays put. However there are times you don't want to cover the whole piece with clay and there are two ways to handle that. One you can use a clear drying glue that is bakeable like Sobo glue and paint that onto the back of your clay before you fire. Second you can bake the clay directly on the glass and pull it off after you bake it and glue it on.
Another thing to consider is glass moves, no it doesn't get up and run around (he he) but it expands and contracts with the temperature. I experienced this first hand when I was doing the art shows and left some glass covered pieces in my tent over night and it got cold out. I woke to find that many of my pieces were cracked from the expansion of the glass. Not a happy camper that day! So that is something to consider. Don't let this stop you from using glass as an armature I have made some of the coolest pieces by covering glass. You can see a few of them in a video I filmed 7 years ago. Boy has the quality of video improved from then. LOL Have fun!
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