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.
Reversed Volumes by Mischer’Traxler #ceramics #casting #formwork
sculpture murale, magnifique… wall sculpture, Jeannine-Marchand
Wood Wall Sculpture Minimalist Art Modern Art Abstract Sculpture. $525.00, via Etsy.