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.
DIY Concrete Leaf Bird Bath DIY Concrete Leaf Bird Bath - love this idea! Those leaves look like rhubarb leaves to me.. Hmmm may have to do something like this.. Especially if I do a cob house some day, this would be a great wall feature/shelf.
43 DIY concrete crafts - Metallic Plated Concrete Bowls- Cheap and creative projects and tutorials for countertops and ideas for floors, patio and porch decor, tables, planters, vases, frames, jewelry holder, home decor and DIY gifts. http://diyjoy.com/diy-concrete-crafts-projects