The heat lamp dries the out edge in minutes (this photo makes it appear hotter than it really is). The center section of the disk is protected by the glazed bowl and takes an hour or more to dry. This sets up stresses that cause the disk to crack. The nature and size of the cracks enable establishing a drying factor value for the clay.
In ceramics, drying performance is very important to optimizing production. More plastic clays shrink more and crack more, but they are also better to work with.