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.
LAMPE GAYALUX - The lamp Gayalux concrete is both sculptural and light. This ball is anchored to the mineral soil, it seems levitate with his ray of light passing through. Concrete leaves the architecture for the object, the block is curved like a skin.