Hi, I no expert but I'm wondering if the thinset requires a curing time in addition to drying time. Maybe check the package directions to see if it does. Good luck.

Kati,

I'm a little confused, were you trying to make a steppiing stone?? Hopefully some one will step in, but I would think you want to use thinset to glue tesserae etc to a substrate, or to use as grout to hold them together, but I dont' know about putting int into a mold, I would think you would want to use concrete to put in a mold, and that does take curing time depending on the type of concrete, some of the special ones for stepping stones, cure in an hour or so..

Not sure if I misunderstood, or not.

Good luck