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.
rhubarbes: Designer Hilla Shamia Fuses Cast Aluminum and Tree Trunks to Create One-of-a-Kind Furniture Piece. (via Designer Hilla Shamia Fuses Cast Aluminum and Tree Trunks to Create One-of-a-Kind Furniture Pieces
These life-size books, made of concrete by Flat Earth Designs, are cast from actual vintage books so you can read the titles on the spines. These are the ideal ornamental garden pieces for the literary gardener.