Air-dry clay is a natural clay that comes from the earth and hardens when it dries. Firing or baking is not required to cure this clay, it has minimal shrinkage and clean-up is easy. However, cracking may occur due to water evaporating as it dries.
When you create your clay piece, you can add water or make slip to join pieces together or soften the clay to a more workable consistency. However, if too much water is added, you increase the risk of cracking when the piece dries. To minimize the chance of cracking, allow some of the water to evaporate before you mold the clay. Check it often to see what the consistency is, and use the clay when it reaches the hardness you desire.
If your clay pieces are less than 1/4-inch thick, they will be more fragile than thicker pieces, so don't make your work too thin or delicate if you want it to be durable and withstand cracking.
Make slip by mixing clay and water until you get a consistency that's reminiscent of heavy cream. When you join two pieces of clay together, roughen the surfaces of each piece and apply slip where they'll connect. Then place the two pieces together and press the joint firmly until you get a solid connection. Do not store slip or clay that has had water added -- only keep pure clay in the container.
Repair any cracks as soon as you see them for best results. Fixing cracks in partially-dried clay is easier and more effective than repairing bone-dry clay.
Mix some heavy slip by combing clay and water together until you have the consistency of pudding.
Moisten the area around the crack by spraying it with water.
Apply a small piece of clay to a large crack and work it into the opening. Apply slip over the repair and smooth it into the piece. For smaller cracks, simply use the heavy slip to fill the crack -- be sure to smooth the edges.
Allow the clay to continue drying to its finished state.
For more problematic cracks or broken pieces, put one part toilet paper to three parts clay in a bucket or container, cover it with water and let it soak overnight. Pour the mixture into a blender and mix it up thoroughly. Allow the mixture to dry to the consistency of heavy slip, and use it to repair the broken clay. The paper fiber gives the slip added strength to hold the pieces together.