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.
love the back wall built up and light
Hand Blown Concrete Bowls, Cool DIY Concrete Project Ideas, http://hative.com/cool-diy-concrete-project-ideas/,
Shanty2Chic Bottle Vase
Beautiful Concrete Garden Furniture
Wohnideen mit Beton