I have never heard of that happening. Did it happen as soon as the glue dried or after a time? Is it outside or in? Normally for outside only silicone or thinset are acceptable. Some people swear by Mac glue outdoors as well but I personally wouldnt do it. and thinset isnt transparent. Some more information about what happened would be helpful.

It cracked once it dried and the drier it got the more it cracked

It sounds as if the glue may have dried too fast. I have used Mac glue and NEVER had any problem.

becky_ia

I am totally stumped by this. Even if it dried fast I cant see that breaking the glass. The only thing I can think of is if the glass wasnt totally flat, like it had a wavy texture to it and as the glue dried it stressed the stained glass, kind of pulling on it, and cracked it.
Again, I know lots of people say they use mac outside. Im not one of em. So theres the choices-silicone, thinset, possibly Mac. for outside thats about all the options are.