Air-hardening modelling clay is a crafting material that cures without the need for additives, heating or any other treatment. This clay is particularly useful for sculptors on a budget or in a hurry. It is also safer for children to work with than kiln-fired or oven-baked clay, which require the use of hot objects.
All air-hardening modelling clay do not require a kiln or heat treatment to cure and gradually harden either through exposure to air or through chemical reactions--such as with some polymer clay. Drying times vary but in general, smaller pieces will harden in a few hours to one day and larger pieces may need a few days.
Some air-hardening modelling clay are made of mineral ingredients. They can be smooth and fine textured with only small particles of clay, or have a fibrous texture because of added ingredients such as nylon fibres that increase the durability of the clay. Smooth, fine clay allow more detail and smoother textures, whereas fibrous clay are better suited for younger crafters.
Sculptures made from water-soluble air-hardening clay need to be protected from water, which breaks down the clay and makes it soft again. These pieces should not spend a lot of time outside where they may be exposed to moisture. They take acrylic paints well, but other kinds of paints--such as poster paints--may require a primer as the clay is very porous.
Wood and Paper Clay
Some air-hardening modelling material are made of paper or wood pulp. These are not technically "clay" as they contain no mineral ingredients. However, they can be worked exactly like clay and are generally referred to as such. Paper or wood-based clay are lighter than mineral clay once dried, and incorporate well into paper-mache projects.
No-Bake Polymer Clay
Air-hardening polymer clay can be used with materials and moulds that cannot be baked in an oven and are more flexible and water-resistant than other polymer clay that require baking. This clay can also be painted with non-water-based paints and varnishes. Examples include Lumina, a translucent no-bake polymer clay, and the lightweight Makin's Clay.