Some clays require nothing more than air for drying and hardening. These materials become solid as the moisture of the clay evaporates into the air. They do not require extra heat and can simply be set aside in a dry room to harden. Some air-drying clays include Das Pronto, which is typically used to shape figurines, as well as Marbelex, which is used to sculpt pottery. Air-drying clays are ideal for children and amateur sculptures because no equipment or heat is required to finish the project. However, clay products that harden in this way are more prone to breaking.
Some clay projects should be completed by hardening the material in an oven. These ovens don't need to be made specifically for art projects; most oven-hardening clays will become firm when placed in a properly heated kitchen oven. This method of hardening is easier and more accessible for many artists. Some oven-hardening clays include Super Sculpey, a pink clay often used to make dolls and jewelry, and Sculpey III Granitex, which looks like stone when it dries.
Compared to other clay hardening methods, using a kiln to harden your clay will create the strongest, most water-resistant material. A kiln is a machine with an insulated area that's made especially for drying clay pieces. Temperatures inside the kiln can go higher than 2,500 degrees Fahrenheit. The temperature at which the clay is fired dictates the color of the finished ceramic piece. For example, firing the clay at about 1650 degrees will harden the clay in a cherry red color, while hotter temperatures result in yellow or white materials.
Not all clays are meant to be hardened. There are a variety of non-hardening clays that can be used to create art projects and crafts again and again. Artists often use non-hardening clay when making claymation films, which allows them to slightly move the clay characters as they film. Children also often work with non-hardening clays to experiment with making different shapes before smoothing the pieces and starting over. These materials can dry if left in the air too long; however, they can usually be restored to their original, pliable state by working some water into the clay.