Fill the balloon with water . This image illustrates why you don't completely fill the balloon you should always leave some room in the neck of the balloon, then as the form dries the pressure has somewhere to go and your clay wont split like that.
I am delicate,fragile, yet hardy and sturdy when the wind passes me by. I can hold up to anything. Except when you pull me from my life force, than i will wither and die.