Waaaah. I've been casting cement garden mushrooms. I mosaic the top and paint the stem. My most recent one failed miserably. I covered the mushroom cap with small tumbled stones using thinset as my adhesive. All was well until I went to grout the thing a couple weeks later. One by one, the stones started falling off. At first I was going to just glue them on after the fact, but it seemed once one fell, others followed. I ended up throwing the whole thing in the trash!
Any speculation on what went wrong? Not enough thinset? Should I have burnished the adhesive side of the stones somehow? Maybe I was too vigorous in my "scrubbing" the grout between the joints? This is only my second 3D mosaic. The first mushroom cap went fine but I used marble and granite tiles instead of these shiny river rocks.
I'd like to think it was my material and not my technique because I'm working on another one right now using vitreous tiles and hope I don't end up with the same problem! Definitely, I'm going to be more gentle with my grouting as a precaution.