Oh No!!! Never had this happen! I use GE Marine silicone adhesive for my bowling balls(don't use bowling balls anymore, use globe shaped light fixtures filled with concrete(too dicey that the bowling ball will crack with our weather), but can't figure out why this happened to you!!! WEIRD!!! I feel for ya, how frustrating!!! My friend Klinger, had a bowling ball wrapped in duct tape and all the tesserae came off, those balls are finicky!!!
(I will be using mortar on my globe balls tho as the globe is concrete)

I've always used GE II on all of mine. And I don't sand them or clean them or nuttin'. My oldest have been outside in the Texas weather for 3yrs. Can't imagine what happened. Don't give up!
Ann

How long did you let the thinset "cure" before applying the grout? Thinset is similar to concrete in that it needs to cure for at least a few days before grouting. I prefer to wait a week myself. If that isn't a factor, I can't imagine what could have gone wrong. I would think thinset would have been a good choice unless you were using stained glass or other transperant glass.

I am also for GEII for my outdoor projects. I use it on bbs and my stepping stones. Have not had any probs yet with these. Other glues I used turned out a disaster.

Kat

Thanks y'all. I guess next time I will use silicone. Makes sense...right now the other of two balls is hanging in there and I am not going to TOUCH it. It is, after all, about learning, so I have learned. Are all the pieces "lost", or do you think I can get the thinset off them? Guess all I can do is try. I sure appreciate your help and support.