>We were making small African mask models and we used
>different types of air dry clay and almost all of them
>cracked in half. They were about 1/2 inch thick,
>which may have been too thin? I've also just tried
>using the clay to make animal figurines, with those
>cracking as well.
WHY DOES IT CRACK?
A half inch should be more than thick enough. When either self-hardening or pottery clay cracks during drying, it is the result of uneven drying. When the last parts of the piece dries (shrinks) it cracks because the last parts are shrinking in relation to completely rigid clay.
WAYS TO AVOID CRACKS:
To avoid cracks, dry it slower so that it drys more uniformly. Drying pieces inside a cabinet or group them in some boxes. Experiment and share your findings. After they look totally dry, put them out to finish drying. Place things so that they are exposed to air equally from all directions. Often I turn things over as soon as possible so the bottoms are not drying slower. If is is clay to be fired, do not fire until they are totally dry. Damp pieces are cooler when they touch your cheek.
If the design allows, it helps to make all the parts about the same thickness so they dry more uniformly. Sometimes I have to spray water on parts that are getting dry too fast, but this must be done before it is bone dry. If you add water to dry clay, it will often crack from the expansion caused as water goes into it. A better solution may be to wrap a bit of plastic on the parts of sculpture that tend to dry too soon (this too must be done before it get too rigid).
Some shapes are tricky to dry. I have seen many bowls that crack across the bottom or down from the top. In both instances it is because the rim of the bowl dried first. The when the bottom dried, it caused the top edge to crack or it caused a crack in bottom because of the delayed drying of the bottom. Bowls dry best if turned over as soon as the top can support the weight.
WHICH CLAY TO USE?
As a clay artist, I was asked to study a group of self hardening clays and oven hardening clays to recommend the best one for a kit sold to parents. Of the self-hardening clay, the nicest clay to work with was the Amaco self-hardening clay. It is a bit Dellia Robia was the all round winner of the oven baked clays. You can check on prices (oven baked clay is more expensive), but I expect that the Amaco is cheaper. None of the oven baked or self-hardening clays are as strong as fired clay. I did not test anything for fumes produced when heating or for any safety issues.
LEARNING INVENTION & HOW ARTISTS EXPERIMENT TO PICK MATERIALS
Many kids assume that any shape that they can make from clay is just as appropriate as any other shape. Artists and inventors have learned otherwise. Sometimes I give them a little length of wire and a piece of clay and ask them to experiment a bit and decide which would be best for the legs of a spider or for the tail of mouse? What could leather strips add? What can they think of to add? With self-hardening clay, all kinds of foreign objects can be installed immediately. What if ears are made from manilla folders or plastic cut from plastic milk jugs? Wire or broomcorn whiskers? How many things can kids imagine and invent if you give them an assortment of paper clips? Some string? Clothing made from paper? What could be added with thin copper sheeting or aluminum cans cut with scissors. How could it hang from the wall or from the ceiling? What about self-hardening clay to join other materials in an assemblage. How about papier-mbchi to reinforce areas that need a bit
more strength? What can they invent with articulated (moving parts)?
With clay to be fired, I ask them how they would prepare the piece if they want to add something after firing. I may need to ask them how much bigger the openings need to be to account for drying and firing shrinkage. If they glaze, I need to ask them more questions about this while they are glazing.
I have a few web pages on clay that may also be helpful.