Earthen Clay
Molding or modeling clay is a substance made from a combination of liquid and dry ingredients. The particular combination makes a substance that will hold the shape in which it is formed. Some types of molding clay can be shaped and reshaped and will not dry out for a very long time. Other types of molding clay are meant to be shaped and then air-dried or baked for a permanent finish.
The most basic molding clay is made from some kind of earthen clay mixed with enough water to make it moldable. Once the artist has achieved the desired shape, the piece is either glazed or not and then left to air-dry or baked in a kiln until the water dries out and the clay is hardened.
The Basics of Reusable Clay
The most familiar form of reusable molding clay has to be Play-Doh. Homemade "play-dough" is a simple combination of flour, salt and water, sometimes with the addition of cream of tartar and a bit of oil. The mixture is cooked over low heat until it reaches desired thickness, colored with food coloring (if so desired), and then allowed to cool before use. It keeps its "moldability" for a good amount of time as long as it is kept in airtight containers when not being used. Eventually, it will oxidize: The air will pull all the moisture out of the mixture and the play-dough will not be so playful anymore. Keeping it tightly covered when not in use simply slows the oxidation process so the clay remains usable for some amount of time.
The flour and water form the basic dough. The salt bonds around the flour particles and serves two purposes. First, it acts as a preservative and prevents the dough from molding while being kept at room temperature. Second, when the clay-maker decides he has formed the perfect play-dough shape and wants to make it permanent by leaving it out to dry, the salt keeps the flour particles stuck together so the shape holds as the water dries out. The cream of tartar keeps the dough from crystallizing, and the oil creates a smoother texture.
Oil- and Wax-Based Clays
Oil-based molding clays, such as Plasticine, are sold as non-drying. Actually they do dry, but they do so very slowly due to the fact that the liquid used in their chemical make-up is oil and not water. Because this type of clay is oil-based, it is not water soluble and can be warmed up to become more pliable or cooled down to become more "stable." The warming and cooling can be repeated many times, making an oil-based clay vastly reusable.
Wax-based clays are essentially oil-based clays made up of wax, rather than clay, as the main "dough" ingredient.
Newer, Better and More Complicated
Commercial versions of reusable clay, whether with the ability to air-dry or not, follow the same basic principles as the homemade play-dough version described earlier. The differences are that commercial versions are more likely to use lab-produced synthetic materials to create the same effect as the flour/salt/water combination creates at home.
Recent developments in the world of molding clay include adding nylon filaments to the clay mixture to increase flexibility and reduce cracking during the drying process.
