Why do cracks across the base of ware form an S shape?

This cup is being force dried with a heat gun. The speed of the drying is not the problem. I can dry this mug in minutes as long as I apply the heat evenly to all surfaces. But in this case I have dried the side walls first and the base is far behind. The first surprise is where the crack starts: It is actually the meeting of two, they each start at the outside edge (that is where the stress encounters clay stiff enough to crack). Another surprise is when: Near the end of the process the cracks suddenly grow (these photos span only about a minute). The way it which the two cracks find each other at the center produces the characteristic S-crack.

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Articles	Drying Ceramics Without Cracks Anything ceramic ware can be dried if it is done slowly and evenly enough. To dry faster optimize the body recipe, ware cross section, drying process and develop a good test to rate drying performance.
Glossary	Drying Crack During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.

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