I found this on BB for you -
Tips: Have you ever had your bath bombs crack? I’m raising my hand. Most cracking is caused by too much or too little moisture so try to find that happy medium, where it sticks together nicely in the palm of your hand. Check out my Bath Bomb Cupcake tutorial to see an example of a too dry and a just right mixture. Other times, cracking can mean that the bath bomb wasn’t packed tightly enough so use those muscles and pack it as tight as you can! Or try adding clays to your bath fizzy to improve the structural integrity of your bath fizzy and make it stronger.