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What's a good air dry clay that will not crack when it dries?

 I am looking for a clay that I can use to make small cars with.  Since I am embedding plastic wheels and legos, and other plastic items, I don't think polymer clay will work because of the baking.  So I want to try air dry clay but the ones I've tested crack really bad, maybe I am not using the right brand.

Thanks

I agree with cyberpageman's suggestion to cover outside surface overnight to prevent premature drying of surface. For the inner clay can not move water from center, especially when surface is dry hard. My added suggestion is to use a fibrous material (wood) within the model as a core, to lessen the thickness of clay. A skin of clay verses a glob of clay. It also provides a sponge like affect to hasten drying of inner clay. Over time, slowly the moisture for the fibers will dissipate thru clay normally. Do think of letting filler fiber material reach to clay's surface to aid in internal drying, keeping pace with outside surface. JOAT T
 Generally cracking is due to the surface of the clay drying out before the inside and shrinking, which causes it to crack.  Cover it with a damp cloth or put it in a box so it will dry slowly, over a day or two.  But dry clay is REALLY weak.

Also, if you try and embed something in clay, the clay will probably crack because it shrinks a bit when it dries, and the plastic doesn't. 

I Googled "casting small parts" and found
http://www.rcgroups.com/forums/showthread.php?t=428202
Check it out.