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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