You can find clay in a variety of forms -- including air-dry clay and oven-bake polymer clay -- and the drying process is different for each type. Because of evaporation and shrinkage, you always incur a risk that your clay masterpiece will crack as it dries. However, you can minimize this possibility by following best practices.
Just as the name implies, air-dry clay dries to hardness by exposure to air for a certain amount of time. Firing or baking your piece isn't required, but you might need to leave it undisturbed for 24 hours or so.
Air-dry clay is water-based, so you can add water to change the consistency and make it more workable. However, be aware that the risk of cracking increases when the clay is too wet, so resist the urge to add more water than you absolutely need. If you happen to add too much, don't worry -- just leave the wet clay out for a few hours, monitoring the consistency until it reaches the workability you want. The water will evaporate bit by bit, and eventually, your modeling clay will be less likely to crack as your work dries.
Joints and coils are most susceptible to cracking.
Adhere all your joints firmly to each other by first
ensuring both parts of the clay that you are joining are the same consistency and dampness.
Score the clay that's to be joined with a sharp object like a fork, knife or pin. Use the tool to make several crosshatches in the joining clay, and spread slip on both sides.
Attach the joint, press the pieces together, and use more slip to fill and hold the seam in place.
Smaller and thinner pieces are more likely to crack than thicker slabs, so if possible, make every part of your work more than 1/4-inch thick. Keeping a consistent thickness throughout the piece helps prevent cracking.
If you're using an armature to make your sculpture, the wire frame needs to be as flexible as possible. Otherwise, when the clay dries and shrinks slightly, it may pull away from the armature and crack.
Once you've created your piece, keep an eye on it while it dries. Putting it in a cabinet to dry and turning it periodically to expose a different side helps the piece dry more evenly. Remove it for the last few hours of drying. If you see any small cracks developing, repair them immediately by filling the cracks with slip. This prevents them from becoming bigger as the clay dries. Then smooth and blend the slip into your work so the repair won't be visible when dry.
To make slip, mix clay and water to the consistency of soft butter or thick frosting -- not too runny, but wet enough to fill cracks and blend seamlessly into the surrounding clay.
Oven-bake polymer clay starts its drying process in the air like air-dry clay, but it won't be completely cured until you bake it in your oven. Often used for making jewelry and figurines, the bright colors and special formulations provide a lot of creative options. To dry these clays properly without cracking, includng brands such as Sculpey and Fimo, follow the firing instructions as instructed by the manufacturer. Each brand of clay has different temperature and bake time requirements.
Polymer clays are not water-based, so it's difficult to make slip to help hold your joints together and limit risk of cracking. However, it's possible if you add a little distilled water to a small piece of clay and mix it by smearing it repeatedly against your palette with a knife. It takes some patience, but you can do it.
If you see cracks developing during the air-dry process, repair them with some slip or, preferably, straight polymer clay. Then blend the new clay into the piece until it's undetectable. Don't bake the piece until it's already leather-hard or harder to prevent extra evaporation and shrinkage in the oven.
When you remove the piece from the oven, don't try to cool it down too quickly by placing it in the fridge or a cold area. Cooling your clay piece too quickly can also make it crack.
To learn different polymer clay-sculpting techniques and get free instructions for various projects, check out Sculpey University.